Arthur Eaton TREATISE ON PRIMARY GEOLOGY; BEING AN EXAMINATION, BOTH PRACTICAL AND THEORETICAL, OF THE OLDER FORMATIONS. BY HENRY S. BOASE, M.D. f SECRETARY OF THE ROYAL GEOLOGICAL SOCIETY OF CORNWALL, &C. &C. LONDON: PRINTED FOR LONGMAN, REES, ORME, BROWN, GREEN, & LONGMAN, PATERNOSTER-ROW. 1834. Intellectus humanus in iis quae semel placuerunt, (aut quia recepta sunt et credita, aut quia delectant,) alia etiam omnia trahit ad suffragationem et consen- sum cum illis : et licet major sit instantiarum vis et copia, qua? occurrunt in con- trarium ; tamen eas aut non observat, aut contemnit, aut distinguendo summovet et rejicit. Novum Organum* TO THE REV. ADAM SEDGWICK, F.R. & G.S. PRESIDENT OF THE BRITISH ASSOCIATION FOR THE ADVANCEMENT OF SCIENCE, &c. &c. &c. AS A MARK OF HIS GREAT ADMIRATION AND SINCERE ESTEEM, THIS WORK IS RESPECTFULLY DEDICATED, THE AUTHOR. 690908 CONTENTS. CHAPTER I. INTRODUCTION : Reasons for undertaking this work nature of its con- tents its objects twofold. The study of the primary rocks long neglected beginning to attract greater attention the proposed order of arrangement Page 1 CHAPTER II. DESCRIPTION OF THE GRANITIC ROCKS OF CORNWALL. Definition of the term primary, as here employed. General aspect of primary districts. The present nomenclature of the primary rocks de- fective. The ternary compound of felspar, quartz, and mica, the type of granitic formations; its varieties. Excess of felspar, and the ac- cession of shorl ; the characteristics of the Cornish granitic formation. The different kinds of Cornish granite: shorl-rock, protogine, eurite, felsparite, and the quartzose varieties of these rocks, or quartz-rock. These granitic rocks associated together, as alternating beds, as irregular imbedded patches or masses, as veins, as el vans or dykes - 7 CHAPTER III. THE GRANITIC ROCKS OF OTHER COUNTRIES COMPARED WITH THOSE OF CORNWALL. Descriptions of these rocks neither numerous nor circumstantial. The granite of Aberdeenshire characterised by its hornblende its varieties. The association of granite and porphyry in the mountain Cruachan and of granite and quartz-rock near Glen Tilt. The granitic district of the eastern part of Ireland it abounds in quartz and is cha- racterised by mica. Granitic rocks of the Erzgebirge, at Freyberg, Altenberg, and Zinnwald. Granite of the Hartz mountains also micaceous its nature doubtful interstratified with schistose rocks. Remarks on the binary compounds of quartz with shorl, mica, or talc V - ' *- - - - 24- VI CONTENTS. CHAPTER IV. DESCRIPTION OF THE PRIMARY SCHISTOSE GROUP OF CORNWALL. The division of the primary rocks into stratified and unstratified ob- jectionable. Proposed to divide them into granitic and schistose groups only an artificial arrangement to facilitate description. Primary slates enumerated. Schistose group of Cornwall. Consists of two series, the porphyritic and the calcareous. Definition of these series. The slates next the granite variously named by Geologists. They are of a peculiar nature. Of two kinds one bounding the eastern granite, the other the western. They pass into other kinds of slate. The nature of this transition. Greenstone, actynolite-rock, chlorite-rock, and the magnesian-rocks. Talc-schist, serpentine, and euphotide, described. Remarks on the quartzose varieties of all these rocks, commonly called quartz-rock. Nature and position of the gra- nitic rocks or elvans, in the schistose group of Cornwall - Page 36 CHAPTER V. DESCRIPTION OF THE SCHISTOSE ROCKS ASSOCIATED WITH GRANITE IN VARIOUS COUNTRIES. Mica-slate and clay-slate of the eastern part of Ireland. Their alternations with quartz-rock and hornblende rocks. Their associations with granite. The composition of this rock, and its modes of arrangement. The gneiss formation of the Western Isles of Scotland. Its beds of mica-slate, greenstone, compact-felspar, clay-slate, talc-schist, serpentine, limestone, quartz-rock. The nature of their connection with each other, and with the gneiss. Abounds in bunches and veins of granite. Description of the primary schistose rocks of Norway, gneiss, mica- slate, and clay-slate, with their subordinate beds. They contain im- mense beds of granite, and are interstratified with smaller ones. The primary schistose group of Saxony, gneiss, mica-slate, clay-slate, and shorl-schist. The talcose formations of the Alps, including tal- cose granite or protogine, talc-schist, serpentine, and other magnesian rocks. Of the island of Corsica, composed of granite, eurite, proto- gine, hornblende-rocks, euphotide, talc-schist, serpentine, and analogous rocks - - 58 CHAPTER VI. ON THE STRUCTURE OF THE PRIMARY ROCKS. Two kinds of structure, the internal and concretionary. Both of these also admit a binary subdivision. The concretionary being either simple or compound, and the internal structure either massive or fissile. Structure of granitic rocks developed by the action of the elements can also be ascertained by cleavage. - Macculloch's remarks on the spheroidal structure of granite. This structure not confined to blocks, CONTENTS. vii but common to the whole mass. The general structure dependent on the aggregation of individual concretions, The layers of granite pos- sessed of determinate bearings like strata. Structure of the'primary schists. The nature and disposition of strata. The curvatures and convolutions in the beds of primary rocks. Their angular and sphe- roidal structure, resembling breccia and conglomerates - Page 91 CHAPTER VII. ON THE RELATIVE POSITION, AND ON THE NATURE OF THE GRANITIC AND SCHISTOSE ROCKS AT THEIR JUNCTION. Position of the primary schists next granite. They extend far over this rock, through the transverse valleys in Cornwall. The mica-slate of Ireland similarly situated, capping the granitic mountain of Lugnaquilla. The Cornish strata skirt the granite in an irregularly undulating curve. The same arrangement obtains in the Isle of Arran, in the north of Scotland, and in the Alps. Dip of the strata at the junction, in Arran, in Cornwall, in Galloway, apparently irregular not so dependent on the structure of the primary rocks illustrated by sections. The anticlinal axis of primary districts said to be simple in each geographical range not so in the Ocrynian range, nor in the Pyrenees. Composition of granite and primary schists at their junction in Corn- wa ll } near Cherbourg, in Ireland, in lona and Mull, in Glen Tilt, and near St. Paul de Fenouillet in the Pyrenees - - 119 CHAPTER VIII. ON THE VARIOUS MODES IN WHICH THE GRANITIC AND SCHIS- TOSE ROCKS ARE ASSOCIATED TOGETHER. The occurrence of granitic rocks in the primary schists in the form of beds or courses in irregular bunches or masses in Cornwall, Scotland, Ireland, andNorway. Portions of these schists also con- tained in granite in Cornwall, in various parts of Scotland, and in France. The primary strata traversed by granite-veins in Cornwall, and in Scotland. Summary of the various appearances presented by these veins - 143 CHAPTER IX. ON THE MINERAL AND METALLIFEROUS VEINS IN THE PRIMARY ROCKS. Our knowledge of veins very imperfect and too hypothetical. The definition of veins not correct. The following details on this subject entirely drawn from Cornwall. Description of veins as seen on the sea- shore, in quarries and similar open situations of small or concretionary veins of larger veins, not confined to individual rock-concretions, but traversing one or more layers or strata. These commonly called Vlll CONTENTS. contemporaneous and true veins but are identical, only differing in size. Large quartz-veins intersecting both the granite 'and slate, at St. Michael's Mount, at Mousehole, and at Cape Cornwall. The connection between metalliferous veins and the containing rocks. Veins curved both in their course and underlie, as at Polgooth and Huel Jewel mines. Veins intersect each other both in their length and depth, accompanied by apparent movements, as at Dolcoath, Weeth, Ting Tang, Huel Friendship, South Huel Towan, and Huel Peever mines. -Interference of veins and el van-courses, as at Polgooth and Dolcoath mines. Veins similarly affected without the intervention of veins of any description, as at Balnoon and Ding Dong mines Page 163 CHAPTER X. GENERAL REMARKS ON THE PRIMARY ROCKS. Recapitulation of the principal facts detailed in the preceding chapters. Composition of the granitic rocks. Each granitic formation characte- rised by peculiar series of granite, dependent on the nature of the accessory mineral. This mineral subject to transitions, accompanied by corresponding changes in the series. The different kinds of granitic rocks variously associated together their disposition among the primary schists the nature of this connection. Masses of granite, whether large or small, insulated or interstratified with slates, are not of a different nature. General remarks on the primary schists their composition, modes of union, and frequent passage into each other. No fixed order of succession. Their nature illustrated by a series commencing with micaceous gneiss. The strata of each member of the schistose group composed of slates and compact varieties the latter pass into granitic rocks. This group, in each primary district, charac- terised by same minerals as the adjacent granite. Remarks on the apparent fragmentary composition of some primary rocks, * and on the analogy between rock and mineral veins. - - 193 CHAPTER XL A SKETCH OF THE PREVAILING THEORY CONCERNING THE NATURE OF THE PRIMARY ROCKS. Two classes of the primary rocks, the stratified and unstratified the former of aqueous, the latter of igneous origin. The whole earth ori- ginally igneous the granitic rocks the result of its secular refrigeration. Igneous rocks of three kinds granitic, trappean, and volcanic. Gran- ite a rock of all ages. The crystalline strata associated with granite also belong to various epochs supposed to be altered or metamorphic rocks. The determination of the precise age of an individual mass of granite. De Beaumont's theory concerning the relative ages of mountain-chains not generally received. Granite protruded in a solid as well as a fluid state. The prevailing theory a modification of that of Hutton CONTENTS. ix appears to be objectionable. LyelPs statement of this theory abridged. The nature of the objections about to be advanced, and the order in which they are proposed to be discussed - Page 212 CHAPTER XII. ARE THE PRIMARY SCHISTOSE ROCKS STRATIFIED ? Definitions of the term stratum not satisfactory yet the basis of modern classifications. If primary slates be stratified so are also the granitic rocks. Primary slates and sedimentary deposits said to be analogous proofs thereof advanced by Lyell. This opinion of their common origin combated for, according to this evidence, the igneous rocks are analogous to the stratified which involves a contradiction. Granite said to be occasionally stratified. Whence this contrariety of opinion? Attributable to the structure of rocks common to the individuals of every geological epoch, and every mode of origin. Not dependent on the successive depositions but superinduced during consolidation. This no new doctrine as regards igneous rocks ought to be extended to aqueous deposits. The structure of rocks a variety or mode of crystallisation. This subject demands farther investigation. Conclusion primary slates not stratified, in the usual acceptation of the word ----._ 230 CHAPTER XIII. HAVE THE PRIMARY SCHISTOSE ROCKS BEEN ELEVATED INTO THEIR PRESENT INCLINED POSITIONS BY PLUTONIC AGENCY? Points proposed for discussion the proofs of elevation and the existence of an elevating power. Position of tubulites in strata of Shropshire. Elevated strata of conglomerate in Suffolk, at Helford Harbour at Bad-na-Bae. Inclined fan-shaped strata of Caithness. Depositions on inclined planes. Calcareous grit of Palagonia. Shingle strata at Monte Calvo. Inclined strata of vast extent. The vertical strata of the Isle of Wight. The nature of igneous elevations. Igneous operations now in action earthquakes volcanos elevation craters not generally admitted. Changes in the relative level of sea and land. Temple of Serapis coast of Chili ancient beaches subsidence of the Baltic sea-cliffs and caves inland sub-marine forests. Phenomena of trap rocks their position in the strata of Sicily, Sky, and Pembrokeshire. Protrusion of granite in a fluid and solid state disputed. De Beaumont's theory. Position of strata next granite not always easily determined. The bearings of primary strata not obvious layers of these rocks threefold continuous with those of granite. Conclusions - 257 CONTENTS. CHAPTER XIV. ARE THE PRIMARY SCHISTOSE ROCKS SEDIMENTARY DEPOSITS ALTERED BY THE CONTACT OF IGNEOUS ROCKS ? Passage of primary slates into secondary rocks. Transitions of two kinds mineral and mechanical the latter no criterion of identity. The relative position of strata not easily determined the lias group of the Botzberg. The capability of caloric to alter rocks. Lyell's remarks on this subject. Sir James Hall's experiments on the fusion of rocks. The state of rocks next actual lavas also next trap-rocks in Skye in Anglesea in Ireland. Dolomization. Strata next igneous rocks only partially altered. These changes similar to the effects of caloric. Condition of primary strata next granite indicates a transmutation of one earth into another. The contact of ignited granite and sediment- ary deposits disputed. The arrangement of primary slates in compatible with their being altered deposits. -These slates contain elements not found in sedimentary rocks. The transfusion of alkalies analogous to that of magnesia in dolomite. Objections to such transfusions. The primary slates of Dartmoor and of the Hartz said to be altered greywacke. Those of the Alps, altered oolitic strata. The association of gneiss and limestone in Jungfrau of granite and limestone in Glen Tilt and the Pyrenees. Conclusion Page 294? CHAPTER XV. DO THE PRIMARY ROCKS AFFORD PHYSICAL EVIDENCE THAT THEY HAVE EXPERIENCED FISSURES, DISLOCATIONS, AND OTHER MECHANICAL MOVEMENTS ? The apparent indications of displacements. The curvatures of rocks. Alternations of curved and straight strata. Veins often tortuous in straight beds. The colouring ingredients of rocks arranged in undula- ting and contorted lines. Similar appearances in agates and alabasters -also in igneous rocks. Sir James Hall's explanation of curved strata. The moving power imaginary. The spheroidal structure of rocks. Primary conglomerates not fragmentary. The secondary sometimes mechanical sometimes concretional. Igneous and aqueous rocks also conglomerated. Granite-veins their structure and composition in- compatible with theory analogous to slate-veins and to certain arrangements in crystalline rocks. Mineral veins different kinds of veins of segregation in Cornwall. Definition of true veins objected to. Intersection of veins no criterion of their relative ages. Re- marks on the heaves and other supposed movements of veins. Conclusion - 4 -"?'? ".-, : - 333 CONTENTS. Xl CHAPTER XVI. MAY NOT ALL THE PHENOMENA OF THE PRIMARY ROCKS, BOTH STRATIFIED AND UNSTRATIFIED, BE SATISFACTORILY EXPLAINED ON THE SUPPOSITION THAT THESE ROCKS ARE OF CONTEMPORANEOUS ORIGIN? Recapitulation of the principal facts and of the conclusions deduced therefrom. The primary slates not sedimentary deposits are in their original positions their contortions, fragment-like structure, and veins, no indications of violence not derived from the metamorphoses of fossiliferous strata. The prevailing theory not founded on a correct deduction from facts. Another explanation of the phenomena of the primary rocks offered. Its consistency with the general opinions concerning the nature of the secondary and tertiary formations. Conclusion - - Pae 370 PRIMARY GEOLOGY. CHAPTER I. Introduction : reasons for undertaking this work the nature of its contents its objects twofold. The study of the primary rocks long neglected beginning to attract more attention. The proposed order of arrangement. THE discussion of geological opinions, which are professedly hypothetical, and which can only be expected to enjoy an ephemeral existence, in consequence of the rapidly progres- sive state of the science, may appear to many to be an unpro- fitable occupation of time, which might be better employed in the examination and description of the phenomena of nature. But it will, perhaps, be more generally admitted that such speculations are conducive to the welfare of the science, by promoting and directing various investigations, and by ex- citing the geologist to greater exertions during his toilsome excursions. By such an admission, however, it is not intended to advo- cate the labours of the mere theorist, whose knowledge has been entirely derived from books ; but, to contend for the utility of such theoretical views as have been deduced from practical experience. Having, therefore, observed, during a geological survey of Cornwall, many facts which appeared to.be incompatible with the prevailing theory, I was induced to publish them in the " Cornish Transactions," and to attempt their solution by an- other explanation. B FOR UNDERTAKING THIS WORK. [Ch, I. It has been advanced, in reply, that the granitic formation of Cornwall may not be analogous to that of other countries : it has also been suggested, that I have greatly erred in sup- posing the circumstances of this narrow peninsula to be appli- cable to all granitic districts ; and that a more extended knowledge would lead me to confess, with the peasant Tityrus, " Urbem, quam dicunt Romam, Melibcee, putavi Stultus ego huic nostrae similem." This censure, however, has been expressed in the most court- eous terms ; and certainly the position into which I ventured, as an opponent of generally received principles, fairly exposed me to such reproof: but this would not have been so strongly called for, if the limits of the channel through which my dis- sent was made public had permitted me to enter more fully on the subject. Now, however, no such restraint can operate, and, by freely quoting the works of those geologists who have most carefully examined the primary rocks, I shall endeavour to show that the geological structure of Cornwall is similar to that of other granitic districts. Having failed in my first attempt to induce geologists to enquire into the validity of these objections, which indeed bear on the fundamental principles of the science, I, in the next place, invited the geological section of the British Asso- ciation at Cambridge to discuss this subject, being prepared to maintain my position, not only by reference to Cornwall, but also by arguments derived from the published descrip- tions of other countries. Circumstances, which need not be here mentioned, prevented this proposition from being then entertained ; but it was appointed that the matter should be fully discussed at the succeeding annual meeting, which is to be held at Edinburgh during the present year. In consequence of this resolution, I now publish an ex- position of my objections to the Plutonic theory, with the intent that the members of the Association may clearly under- stand the grounds of dissent, and come prepared to examine Ch. I.] ITS OBJECTS, AND PLAN. 3 into the merits of the question on an open and fair field. By thus freely disclosing to my opponents the plan of attack in- tended to be followed in the approaching discussion, my chance of success is certainly not augmented : but truth, not victory, is the object contemplated; and if my conclusions have been legitimately deduced from the facts brought for- ward, their value cannot be diminished by an exposure to the fullest scrutiny. It may possibly be regarded as a mark of great presump- tion, in thus venturing to attack opinions which are advocated by the highest authorities : and it can only be urged in ex- tenuation of such a prepossession against my labours, that geology is as yet in its infancy ; and that its pursuit does not, like astronomy, and the more perfect mathematical sciences, require a great depth of learning and acumen in order to enable a zealous disciple to promote its advancement. Appealing, therefore, to my long, and I trust not altogether unprofitable, examination of the primary rocks, I rely on the well known candour and liberality of the eminent men from whom I have presumed to differ ; feeling convinced that, if I should substantiate any objection to their theory, they will rejoice that an error has been corrected. In conducting the proposed enquiry, there are two objects which will demand attention : First, the description of the primary rocks both of Cornwall and of other countries, in order to ascertain in what respects they are analogous to each other ; and, in the next place, an examination whether the phenomena exhibited by these rocks are in accordance with the principles of the prevailing theory. On this plan, the following pages will be nearly equally divided between a narration of facts and hypothetical specu- lations : and the former part will lead to such lengthened preliminary details, as to constitute an essay on primary geology ; which, so far from detracting from the interest of the work, may, perhaps, impart to it a more permanent value B 2 4 NEGLECT OF THE STUDY OF THE PRIMARY ROCKS. [Ch. I. than it would otherwise possess. Indeed, without such an introduction, the succeeding theoretical discussion would not be generally understood : for the granitic formations have not been carefully studied by all the cultivators of geology during the last twenty years ; and even now they attract little at- tention, as is clearly demonstrated by the brief and un- satisfactory manner in which they are sketched in the recent, and otherwise excellent, works of De la Beche and Lyell. Dr. Macculloch, indeed, during the early part of the period specified, continued to enrich this branch of the science with a vast body of facts, to which I am indebted for numerous and most important illustrations : but, with this exception, no geologist of note, in this country, has published any minute and descriptive details concerning the primary rocks ; all have been absorbed in the pursuit of the wonderful and fascinating knowledge unfolded by the fossiliferous strata. This predilection in favour of a new study, which gave access to unexplored regions, abounding in valuable productions, is not surprising ; but, now that time and successful researches have somewhat satiated the vehement curiosity at first ex- cited, it may be hoped that the other department of the science will no longer be regarded as uninteresting or un- fashionable ; but that each will equally be the subject of active and patient investigations. There is reason to expect that this desideratum is on the eve of being accomplished ; for Professor Sedgwick, in his address to the Geological Society in 1830, pointed out and advocated the importance of the study of the older rocks. And he has also added example to precept : for some time past he has been, and is still, engaged in exploring the intermediate or transition rocks, the debateable land on the confines of the primary formations. Nor must it be forgotten, that De la Beche has been occupied during the last two years in the examination of Devonshire ; so that the primary rocks must necessarily come under his observation : and that they will receive a careful investigation may be expected, not only from h. I.] ATTENTION RETURNING TO IT. 5 his known character, but also from the estimation in which he holds this department. " The inferior stratified rocks," he observes, " may not at first sight be so attractive as the contemplation of the varied forms of organic life, and the probable conditions under which it may have existed ; but it will, nevertheless, be found equally, if not more, delightful as the enquirer obtains more certain results, from the in- vestigation being conducted through the medium of the exact sciences." * Under the circumstance, therefore, of the returning at- tention of geologists to this long neglected branch of the science, my labours may now meet with a more favour- able reception than they would have some years ago, when the study of the granitic formations was regarded as an un- profitable pursuit, the riches of which had been exhausted. And it is hoped that the details here collected may prove instructive to the student, by conveying a more accurate idea of the nature of the primary rocks than he has, hitherto, been able to obtain : and they may not, perhaps, be altogether unacceptable even to the experienced geologist, as they may serve to call to his recollection, and to place in one point of view, many scattered and insulated facts, which can scarcely fail to appear more luminous by the concentration of their diverging and individually feeble rays into one narrow and well-defined focus. It has always been kept in view as an important object, to preserve these details as free as possible from all hypothesis : but the language of the science is, in many instances, so en- grafted and founded on theoretical speculations that it could not be always accomplished; for, in some cases, the pub- lished descriptions would not admit of translation into less exceptionable language without the risk of perverting the author's meaning, or of incurring the imputation of mis- representing facts. * Geological Manual, 8vo. p. 432. B 3 6 ORDER OF ARRANGEMENT. [Ch. I. In the arrangement of these details the following order has been adopted : 1 . A description of the various kinds of granitic rocks, and the modes in which they are associated together. 2. Of the primary schistose rocks, under the same re- lations. 3. Of the structure of the primary rocks. 4. Of the nature of the primary rocks, both granitic and schistose, at their junction with each other. 5. Of the modes in which these rocks are intermixed and connected together. 6. Of the mineral and metalliferous veins by which these rocks are traversed. Having thus premised the nature of the arrangement, it is now proposed to enter on the subject without farther preface. Ch, II.] DEFINITION OF THE TERM PRIMARY. CHAPTER II. DESCRIPTION OF THE GRANITIC ROCKS OF CORNWALL. Definition of the term primary as here employed. General aspect of primary districts. The present nomenclature of the primary rocks defective. The ternary compound of felspar, quartz, and mica, the type of granitic form- ations : its varieties. Excess of felspar, and the accession of shorl, the cha- racteristics of the granitic formation of Cornwall. The different kinds of Cornish granite : shorl-rock, protogine, eurite, felsparite, and the quartzose varieties of these rocks, or quartz-rock. These granitic rocks associated together, as alternating beds, as irregular-imbedded patches or masses, as veins, as elvans or dykes. BEFORE proceeding to describe the various primary form- ations, it is desirable to define in what sense the word primary is here employed; and this is the more necessary, since it has not only fallen into disrepute both with foreign and British geologists, but attempts have been lately made to erase it altogether from the nomenclature, as conveying an idea incompatible with the prevailing theory. The term primary, in its original acceptation, is certainly very objectionable, inasmuch as it has a theoretical signi- fication, which ought to be avoided in every classification: but the word hypogene, by which Lyell has proposed that it shall be superseded, is liable to the same objection ; and the term crystalline schists, advanced by Boue, though free from this defect, is not more happy, because several igneous and sedimentary rocks have a similar characteristic. For these reasons we have continued to use the word primary ; but, at the same time, regret the want of such a term as would be generally applicable to rocks of this nature without implying any hypothesis. In the mean time, how- ever, it is proposed, in the following pages, to consider as primary rocks the various kinds of granite, and all those B 4- 8 GRANITIC ROCKS OF CORNWALL. [Ch. II. crystalline and non-fossiliferous masses, both compact and schistose, which are usually associated together under different arrangements, and intimately connected by frequent mineral transitions. This proposition may appear to some to be inadmissible, on the ground that the rocks thus brought together belong to several distinct geological epochs : but, admitting this view of the subject, (for, though it will be disputed hereafter, it is immaterial at present,) still these rocks constitute such a natural family, that they may be examined and described as such, independently of all theoretical considerations concerning the nature of their origin. Since this is a point that ought to be satisfactorily established, it may be stated, for the sake of the general reader, that Lyell admits that, " if we investi- gate a large portion of a continent which contains within it a lofty mountain range, we rarely fail to discover another class, very distinct from either the subaqueous deposits or volcanic rocks ; and which we can neither assimilate to de- posits such as are now accumulated in lakes or seas, nor to those generated by ordinary volcanic action." Again, he adds, that " nothing strictly analogous to these ancient form- ations can now be seen in the progress of formation on the habitable surface of the earth ; nothing, at least, within the range of human observation." * Since these remarkable form- ations are so distinct from all other kinds of rocks, no great disadvantage can arise from our treating of them under the denomination of primary rocks ; it being always remembered that this name is not used in a theoretical signification* Having offered this apology for the employment of a term so objectionable, we will now proceed to our task without farther preamble. The primary rocks generally impart to a country a wild and desolate aspect : here and there, indeed, landscapes of great luxuriance and beauty present themselves; but the * Principles of Geology, vol. iii. p. 10, 11. Ch. II.] ASPECT OF PRIMARY DISTRICTS. 9 most common feature is a wide expanse of barren hills, abounding in grand and romantic scenery. The character of such countries is more dependent on the granite than on any other member of the primary class. ei Granite is one of the most universal rocks, forming some of the highest and most remarkable chains of mountains. It is not, however, limited to such high ranges as the Himalaya or the Alps, or even to the much lower ridges of Britain ; since it also occupies many extensive tracts of comparatively level land. This rock has been commonly supposed to be characterised by the pinnacled and serrated form of its moun- tains, such as the well-known summits of the Alps, and of the island of Arran : but this is not the case, for it assumes every variety of outline. The mountains about Loch Etive, in Scotland, have a simple conical form, which is particularly marked in Cruachan ; the extensive ridge which surrounds the sources of the Dee, forming the loftiest tract of land in Britain, presents a series of heavy-rounded elevations ; in Cornwall, in Galloway, and in Sutherland, it offers the same uninteresting aspect ; while, in many parts of Aberdeen shire, it occupies the lowest grounds, presenting large tracts of level surface." * When a granitic country is characterised by elevated and precipitous hills and mountains, its surface is covered with numerous detached rocks; diversified here and there with projecting tors or cairns of various fantastic forms, according to the nature of the rock, and of the corroding influence to which they have been subjected. Such tracts, whether they occupy a whole district or are confined to portions thereof, are generally barren, or, at best, they only afford a scanty pasture for cattle during the summer months, and supply the cottager with wild plants, and turf for fuel ; but, wherever a granitic country is composed of undulating hills, with occa- sional level spots, though the latter are commonly marshy, * Macculloch's System of Geology, vol. i. p. 54. 10 GRANITIC ROCKS OF CORNWALL. r Ch. II. the former sometimes possess a good soil, adapted either for tillage or pasture. The primary schistose districts which surround the granitic rocks partake of the same characters : they are, however, in general, more capable of cultivation ; and their valleys find a better drainage in the rivers, by which they are frequently traversed. When we take a bird's eye view of a primary country, the surface very commonly exhibits a system of valleys, which run parallel with the central ridge of granite ; and these longitudinal valleys are intersected by others which cross them at right angles ; and as the intermediate hills are more or less rounded, the surface of the country has an undulating appearance in both directions, which has been often and aptly compared to the waves of the sea ; and the simile is farther appropriate, inasmuch as, when the curves of the hills are regular and gentle, or variously contorted and abrupt, they resemble the sea when agitated by the wind with different degrees of violence. On the form of these two systems of valleys many features of the primary districts depend. Through them the rivers flow, seeking an outlet into the sea by the nearest continuous descent : sometimes they effect this along the longitudinal, at others, through the cross valleys, receiving tributary streams on either hand from the lateral valleys, or curved hollows, which they intersect. But very often the course of a river is diverted more than once from its original valley, in consequence of the concavity of a cross curve rising above its level, or that of the other system descending below this point, by which irregularity the stream flows along a more favourable drainage ; and, not unfrequently, when both systems of valleys oppose like obstructions, the rivers, thus dammed up, are converted into lakes ; which again obtain an outlet, or not, according to the positions of the adjacent concave curvatures of the intersecting ranges. In this manner, the course of a river, whilst within a primary region, is determined by the Ch. II.] DEFECTIVE NOMENCLATURE OF PRIMARY ROCKS. 1 1 form of the surface ; and thus it is that, whilst some streams are short and rapid, others, which at first run in parallel and adjacent valleys, are conducted by a long circuitous route, exhibiting rapid falls, or gliding slowly along, according to the nature of their channel. The longitudinal valleys are the most extended, and often present a more gentle outline ; whilst the transverse commonly form narrow and abrupt hollows, or gorges, in which the rock is exposed to view, affording to geologists opportunities for examination as favourable as any in Alpine regions. Such is the external appearance of the primary rocks ; and we now proceed to describe their mineral characters, dividing these rocks into two artificial classes, the granitic and schistose, in order to facilitate their description. Under the head of granitic rocks it is proposed to comprise all those various compounds which are usually associated with common granite, being continuous therewith, and forming an integrant part of the same unstratified mass. On account of this intimate association, it has become a pretty general practice to designate all these varieties of rock by the name of granite. They are certainly all geologically identical when they occur in the same mass ; but, as will be seen hereafter, they are not confined to this position, and therefore their nomenclature ought not to be founded on a geological basis. This practice is also objectionable, inasmuch as it prevents the student from taking practical lessons in the book of nature until he has completed the course of theo- retical instructions ; and then it too frequently happens, that he can only make his observations according to the prescribed rules, and under a particular bias, prejudicial to impartial observations. If a rock be designated according to its geological relations, its name must be liable to change, because the principles of the science are not as yet permanently established ; besides, on this plan, in the examination of countries previously unex- plored, too much depends on the correct judgment of the 1 2 GRANITIC ROCKS OF CORNWALL. [Ch. II. observer : but when a rock acquires a name from the nature of its composition, whether purely mineralogical or blended with organic remains, then this part of the science becomes perfectly descriptive, like the other branches of natural history; and much curious and accurate information may then be ex- pected from travellers who are not accomplished geologists. This subject of nomenclature will be entered on more fully hereafter, when the general reader will be better pre- pared to appreciate the importance of mineralogical distinc- tions among the primary rocks ; and it is hoped that the geologist will admit that they are not frivolous, when it is shown that by their means some arrangements in the structure of the earth will be developed which otherwise could not have been detected. In most of the lately published geological accounts of coun- tries we look in vain for details concerning primary rocks : we sometimes, indeed, learn that such and such a district consists of granite, but cannot collect any information con- cerning its composition, or the manner in which its varieties are associated together. We are therefore left quite in the dark as to the nature of this rock ; for the various kinds which may come under the designation of granite are not only exceedingly numerous, but have very frequently no resem- blance whatever to the common variety so universally known by the name of granite. This ought not to be. And Dr. Macculloch*, although he has so strongly advocated this method, admits, in his directions for conducting geological observations, that " a correct description of its mineral characters is necessary, as the Varieties of this rock are highly interesting, especially under its passages into trap." Now, this trap is only a variety of that kind of granite which abounds with hornblende, the characteristic mineral of this rock in many parts of Scotland : and if the different forms of granite are deserving of remark when hornblende is present, surely they ought also to be * System of Geology, vol. ii. p. 473. Ch. II.] COMPOSITION OF GRANITE. 13 noted when shorl characterises the rock, as in Cornwall ; or when talc prevails, as in the Alps, in Corsica, and in some parts of Norway. In each of these instances, the suite of rocks of which the granitic range is composed is very different ; and this variation is distinctly connected with the nature of the characteristic mineral. This circumstance alone, therefore, independent of facts to be hereafter adduced, is sufficient to show, that geologists ought not to rest satisfied with stating that a country is com- posed of granite : it is, certainly, in descriptions very con- venient to be able to denote in general terms that a country consists of granitic rocks ; but this ought not to obviate the necessity of specifying the nature of the individuals of which this group is constituted. These preliminary remarks may, perhaps, induce attention to the contents of this Chapter, which otherwise might be passed over as tedious details by those who have not interested themselves in the examination of primary rocks ; but they will be found a necessary introduction to the right under- standing of the phenomena to be hereafter discussed, and may enable the student to comprehend " those almost infinite vari- ations in the composition of these rocks, which appear to set classification at defiance ; but which, however perplexing at first sight, will, it is presumed, ultimately lead to an accurate knowledge of the nature of the granitic rocks." Granite, in the common and original acceptation of the term, denotes a rock composed of felspar, quartz, and mica. It oftentimes contains, in addition to these, some other minerals ; but those just enumerated are considered, in the following pages, as essential to true granite ; and if either of them is wanting, the compound may then receive a distinct denomi- nation. These component minerals of granite, both essential and accidental, are united together by a confused crystallisation, not only mutually penetrating and interfering with each other, but sometimes the small crystals of one ape completely enve- 14 GRANITIC ROCKS OF CORNWALL. [Ch. II. loped in the large crystals of a different kind of mineral. And it is a very common occurrence for one, or even more, of these minerals to be developed in large crystals, in a granular basis of the whole, so as to constitute a porphyritic granite. This character is generally imparted by the felspar, and rarely by the quartz or mica. When the crystalline minerals are united in moderate- sized granules, the rock may be called common granite, since it occurs more universally than any other kind : indeed there is, perhaps, no primitive range of any considerable extent in which it is altogether wanting. In the characteristic speci- mens of this variety the felspar is most abuncjant, and the quartz exceeds the mica : the respective proportions of these, however, are liable to great fluctuations. Other varieties of this rock, depending on the size of the component parts, may be noticed; which, relatively to the common, may be termed small and large grained : the former exhibits every degree of diminution, until the ingredients can scarcely be distinguished by the naked eye ; and this kind, like the common, is of too frequent occurrence to need any particular references. The latter, or large-grained variety, however, when its minerals extend to any great size, is more rare : it is not to be found in Cornwall, but near Aberdeen in Scotland, and in Siberia. Each constituent attains a very large size, and is very crystal- line, more particularly the mica, in Siberia, where it occurs in large hexagonal tables, easily separating into transparent laminae or plates, which have been applied to economical purposes. When either of these kinds of granite contains, in addition to the essential minerals, any other substance in such quantity as to alter the appearance of the rock, then it may be regarded as a distinct variety; but when the proportion is very small, the presence of the adventitious mineral may be overlooked in the classification : though this circumstance ought to be noticed, because, in all probability, it will be Ch. II.] CHARACTERISTICS OF CORNISH GRANITE. 15 found to increase in importance in other parts of the same range. Such are the ordinary varieties of true granite, rocks which are of such frequent occurrence, and for the most part in such general use, that they are familiar to the most cursory observer. So far, then, the nature of these granitic rocks cannot be mistaken: those that remain to be described are far more numerous, and more partially distributed, and can therefore only be found in certain localities. On this account it is pro- posed to describe the granitic rocks of several districts, by which means their history will be better developed, and the details be rendered more interesting. In Cornwall the granitic rocks occur at the surface, in the form of eight insulated masses, four of which are of much greater extent than the others ; each of these masses presents some peculiarities, but all of them contain several varieties of true granite. The most general feature of the Cornish granite is the abundance of its felspar, which not only forms the greater part of this rock in the ordinary granular varieties, but is frequently superadded in the form of large porphyritic crystals, constituting no inconsiderable proportion of the whole mass. Shorl, however, is the characteristic mineral of this district; indeed it is seldom altogether absent from these granitic rocks for any extent, though it is often in such minute particles as to require a magnifying glass for its detection. When shorl forms a portion of true granite, the rock may be sufficiently distinguished by the term shorlaceous granite ; but when the shorl usurps the place of mica, or is combined only with quartz, then the compound requires a peculiar name: that of shorl-rock has been applied to some of its varieties, and it is proposed to use it here in a more extended sense. Shorl-rock is sometimes composed of a crystalline granular mixture of felspar, quartz, and shorl ; and this granitic shorl- rock, assuming various appearances, according to the size and 16 GRANITIC ROCKS OF CORNWALL. [Ch. II. proportion of its minerals, is generally associated with, and gradually passes into, the different kinds of true granite. On the other hand, it frequently, by the disappearance of the felspar, becomes perfect shorl-rock, a crystalline compound of quartz and shorl, which, like granite, exhibits many varieties, according to the size or predominance of either of its constituent parts. One form of this rock deserves very particular notice, not only on account of its association with its congeners, but also because it is of frequent occurrence amongst all the other kinds of granitic rocks. It consists of an intimate union of quartz and shorl, forming a compact shorl-rock, which sometimes has the external appearance of a hornblende-rock, but may be distinguished therefrom by its great brittleness. In the compact shorl-rock the quartz generally predominates, so much so that the shorl often seems only to act the part of a colouring mineral ; and this rock is often seen accompanying quartz in alternating stripes of various sizes, from that of a line to a foot or more in thick- ness. These are sometimes straight, and sometimes curved, or even contorted, in as intricate figures as the agates ; and not unfrequently the shorl, as the colouring ingredient, is so dis- posed, both in rounded and angular forms, as to impart to the mass the resemblance of a fragmentary rock. The granitic rocks of Cornwall also present other kinds : the most important of these is that one which contains talc instead of mica ; for its granitic variety is the source of the china-stone and china-clay, more than twelve thousand tons of which are annually exported for the use of the potteries. This rock is a species of protogine : its talc is commonly in the state of small scales, exhibiting various tints of yellowish green. It assumes most of the forms which are common to the shorl-rock : all of them do not probably exist in Cornwall, but they occur in countries where this rock is more exten- sively developed. The most common kinds of Cornish protogine depend on the size and proportion of their felspar, quartz, and talc : but Ch. II.] VARIETIES OF CORNISH GRANITE. 17 sometimes this rock becomes quartzose, by the gradual loss of the felspar, analogous to the perfect shorl-rock, and in this state it forms the only protruding masses ; for the granitic kinds are so prone to decay, that they are perfectly disin- tegrated for more than twenty feet beneath the surface. This quartzose protogine is, however, more commonly seen as loose boulders on the hills ; and although now seldom found, they were probably more abundant formerly, as this stone enters pretty largely into the construction of churches and other ancient buildings in the neighbourhood. It has already been stated, that the granular mixture of the true granite sometimes becomes so minute, that its consti- tuents can scarcely be discerned by the naked eye. This variety frequently passes into such a fine granular homo- geneous rock, that, even with the magnifying lens, we cannot detect the usual granitic minerals : it appears, indeed, to have passed, by the intimate blending of the quartz and felspar, into a variety of compact felspar, to which the name of eurite may be appropriately applied. This substance, however, is seldom perfectly pure, but forms a basis in which small granules of quartz and minute scales of mica are imbedded. It some- times becomes porphyritic, by the presence of distinct crystals of felspar ; in some places it also contains shorl or hornblende ; and it not unfrequently happens that, towards the centre of a bed of eurite (by the increased size of the concretions of quartz, mica, and felspar), the basis disappears, and the rock becomes a perfect fine-grained granite. This euritic suite of granitic rocks prevails in certain parts of Cornwall, while in other parts a similar series occurs, only the basis, instead of being fine, granular, and rather soft, as in the eurite, is a hard compact felspar ; so that, although the composition of these rocks appears to be the same, their appearance is very dissimilar. It is proposed to call this genus felsparite, in order to distinguish it from the other granitic rocks. The species of felsparite arise from the nature and disposition of various minerals in the basis of this genus, 18 GRANITIC ROCKS OF CORNWALL. [Ch. II. as in the case of eurite. Of these, the porphyritic variety, arising from the presence of crystals of felspar, is of common occurrence, and very characteristic of this rock : it is the felspar porphyry of Macculloch and other geologists ; but, as in this work the word porphyry is not used in a generic but a specific sense, it will be called porphyritic felsparite. This series of rocks not only passes at its centre into a fine-grained granite, like eurite, but also into a large-grained, and even a prophyritic granite. But the most interesting, and indeed the most conspicuous, circumstance to be noted concerning felsparite is the constant change in the nature of its basis, caused by the fluctuations in the proportions of the felspar and quartz which enter into its composition ; for it assumes different characters according as one or other of these minerals predominates, exhibiting all the intermediate shades between a crystalline compact felspar, jasper, hornstone, iron- flint, and even quartz itself. The description of the various kinds of Cornish granite must not be terminated without a few remarks on those varieties which are very quartzose, passing, indeed, very com- monly into perfect quartz. These are confined, in a great measure, to the immediate vicinity of quartz-veins, which are sometimes metalliferous ; and, in these instances, if the vein be of moderate dimensions, they would attract little attention ; but when the veins are of a large size, or when an entire layer of granite is quartzose, then they would be pro- nounced by most geologists to be kinds of quartz-rock. When these quartzose varieties have been observed in other granitic countries, they have been regarded as superficial remains of strata which once covered the granite : but, as they are clearly not of this nature in Cornwall, it is desirable to bring them into one point of view, in order to justify our considering them, not as stratified rocks, but merely as varieties of granite. In the case of common granite, the mass sometimes be- comes so exceedingly quartzose, at the same time retaining its Ch. II.] QUARTZOSE VARIETIES. 1 9 granitic aspect, that the rock is undoubtedly entitled to the term quartzose; as at St. Michael's Mount, at Kerris in Paul, and elsewhere. In some cases, however, the felspar gradually disappears ; so that the rock consists only of quartz and mica (the latter, in some parts, bearing no inconsiderable proportion to the quartz), and has a perfectly granular cha- racter, like the hyalomicte of the French, the greizen of the Germans. One notable example of this occurs near Pen- zance, at Ludgvan : it has furnished an excellent building material, which is very common in the old houses of the town ; but, unfortunately, the place whence it was obtained is no longer exposed to view. Its quartz is grey, and rather large- grained ; its mica abundant, in small silvery scales. Those varieties in which the mica is more sparing, and more intimately blended with the quartz, may be found on most of the elevated hills ; more particularly at Roughtor, and Kitt Hill. In like manner, the shorlaceous granites of the central and Land's End districts abound in quartzose varieties of this rock. When the felspar is wanting, shorl-rock, the equiva- lent of the micaceous hyalomicte, makes its appearance ; and the shorl-rock is either granular or compact, according to the mode in which the component minerals are aggregated. The compact, either simply of a dark colour, or striped with quartz, is the most prevalent, and forms large courses, layers, or veins in the granite. The talcose granite, or protogine, also possesses its quartzose varieties under similar circumstances as the true or micaceous granite, and more particularly that kind which results from the disappearance of the felspar; and is a granular compound, of a greenish colour. It is common in the china-stone quarries in the central district, where, however, it is generally associated with shorl ; and it projects on the side of Tregonning Hill, in the form of tors, or per- haps, rather, of boulders or transported masses. It is well adapted for building, like the Ludgvan stone, cleaving well, c 2 20 GRANITIC ROCKS OF CORNWALL. [Ch. II. and capable of being wrought with a chisel, as shown by its having been found to form the arches of the old chapel of Penzance, when it was lately pulled down ; and the same stone may be also seen in the mother church at Madron, which is much more ancient. It is, therefore, very probable (since the granite, a few miles north of Penzance, contains protogine), that this talcose quartz-rock was obtained from erratic blocks, which have long since been consumed for building purposes. The quartzose varieties of felsparite and eurite have been already noticed, as arising from the gradual predominance of silica over their felspathic ingredient; giving rise to horn- stone, iron-flint, and many kinds of compact quartz-rock, known to the miner under the name of capel. This subject will be again reverted to, when it will be seen, that these, and other binary compounds, are of sufficient importance to require distinct appellations : for the present, we shall only farther observe, that, besides the evidence afforded by the cliff-sections, mining operations have also shown, that these quartzose compounds are not superficial, but are often per- sistent to great depths. Thus we learn, that the granitic masses of Cornwall are not, as some have supposed them to be, composed of one kind of rock, uniform in its constitution, and uninteresting in its varieties, but are as complex in their composition as the stratified rocks; and, in their mineral transition into each other, afford as curious and instructive subjects for investi- gation : on this account, the utility of mineralogical dis- tinctions is obvious; and it will be still more apparent, when we consider these granitic rocks hereafter, not only in their relations towards each other, but also towards the crystalline schists with which they are associated. The granitic rocks are variously arranged among them- selves. Sometimes each kind occupies a considerable space ; or they alternate together in smaller masses, which are dis- posed in distinct and regular beds, highly inclined, and main- Ch. II.] ASSOCIATION IN ALTERNATING BEDS. 21 taining a parallel course. The nature of granitic countries will not admit of this arrangement being traced to any great extent ; which, indeed, is likewise the case with the schistose rocks; for even their continuation is more frequently pre- sumed than actually ascertained. Even, however, when the characters of these rocks are well preserved over a given tract (that is, when they are distinctly granite, protogine, or felsparite, as above defined), yet a nar- row inspection of any individual bed or layer will exhibit other kinds, in the form of veins, or insulated masses of various forms and dimensions. The occurrence of a different granitic rock within another, arranged in patches or clusters around, as it were, certain centres of attraction, is of such common occurrence that it seldom arrests the attention ; even when it does not consist of precisely the same ingredients as the main mass, differ- ently combined as to proportions, or to the size of the com- ponent particles. But when these insulated compounds assume an elongated form, of greater or less regularity, then, under the name of veins, they have been considered more interesting, and have given occasion to much speculation concerning their origin. These granite-veins within granite, as they have been called, are very numerous in the Land's End district ; and are beau- tifully displayed in some parts of the cliffs near the Logan Rock and Lamorna Cove, more especially on the shore, where the rock is polished by the action of the waves. Mr. Carne has enumerated, in the " Cornish Transactions," some instances of these veins in this locality, stating that they are of three kinds : first, those which are of the same composition as the containing rock, but rather decomposed, and do not possess regular walls; secondly, those which only differ in containing large red-coloured crystals of felspar ; and, thirdly, those veins which are com- pact and fine-grained, very different from the contiguous c 3 22 GRANITIC ROCKS OF CORNWALL. t Ch - IL granite : they are numerous, and, when they meet, they do not traverse each other, but unite.* Mr. Majendie, in noticing the veins and concretions of fine-grained granite and shorl-rock, in the porphyritic granite of the Logan Rock, remarks, that these, which on a slight view might be taken for fragments, are often penetrated by large crystals of felspar proceeding from the granite mass, f Some examples of these granite-veins, on the same coast, have been described by Oeynhausen and Dechen. The granite of Tol-Pedn-Penwith, they observe, is of the common kind, with porphyritic twin crystals of felspar with shorl and pinite. Near the village of Sawah the fine- grained granite is like that of veins, consisting of quartz and red felspar, with a little mica, but a larger quantity of shorl : its position, however, is not that of a vein. The mass of the fine-grained rock continues of a fine texture to the distance of twenty feet from its junction with the large-grained ; but, farther off, the constituent parts become larger and larger, and porphyritic crystals of felspar appear here and there ; so that this rock gradually passes into the large-grained variety, both sorts of granite only differing from each other by their texture, and different state of crystallisation. Several granite- veins, exactly of the same composition as the fine-grained just described, abound in the cliff: one of these, in a little cove near the signal-station, is heaved nearly two feet by a quartz vein, as is also a shorl vein; but the latter traverses the granite-vein, without producing any heave. This granite- vein continues for a considerable distance into the sea ; it is here divided into two branches. The large twin crystals of felspar they meet with are intersected by them, and heaved about half an inch.:}: When these granite-veins are of a large size they are termed elvan-courses : indeed, this is the only distinction be- Geol. Trans. Cornwall, vol. ii. p. 54. f Idem, vol. i. p. 29. Phil. Mag. and Annals, vol. v. p. 224. Ch. II.] ELVAN-COURSES. <23 tween these two forms of elongated masses of granitic rocks. In composition, these elvans are either shorl-rock, eurite, fel- sparite, or even varieties of fine-grained granite. They are particularly abundant in the Land's End district : and their presence is generally indicated by the abundance of tabular and very angular stones in the hedges and farm-buildings. These courses sometimes correspond or are parallel with the layers of common granite in which they are situated ; but at other times they intersect these layers or beds, after the man- ner of the elvans in the schistose rocks. Notwithstanding, however, this apparent irregularity of arrangement, the elvans are connected on either side with the granite by the most intimate mineral gradations, or contain irregular portions or masses of the common granite, with which they also coalesce. Other marks of the close connection which subsists may be enumerated ; such as the penetration of both rocks by felspar crystals, and similar appearances observed in granite veins : but one of the most important was described lately by the Rev. George Pigott, in a paper read at the last annual meet- ing of the Cornish Geological Society ; viz. the continu- ation of the small veins or stripes of shorl through both granite and el van; and also the extension of one of the parallel layers of granite itself through the elvan, at Pedn- merer-mere, near the Logan Rock. The alternation of soft and hard granite, so common in several parts of Cornwall, is of the same nature. The former frequently contains parallel contemporaneous veins of quartz and shorl, which abound in tin ; and, when this is the case, the layer of granite containing the ore is considered by the miner as the lode. Sometimes, indeed, these veins are so numerous that it is necessary to excavate the whole of the rock, like the stockworks of the Germans ; as at Carclaze mine, and the old workings of Beam mine, near St. Austle. 24 GRANITIC ROCKS OF OTHER COUNTRIES [Ch. III. CHAPTER III. THE GRANITIC ROCKS OF OTHER COUNTRIES COMPARED WITH THOSE OF CORNWALL. Descriptions of these rocks neither numerous nor circumstantial. The granite of Aberdeenshire characterised by its hornblende its varieties. The association of granite and porphyry in the mountain Cruachan, and of granite and quartz-rock near Glen Tilt. The granitic district of the eastern part of Ireland, it abounds in quartz, and is characterised by mica. Granitic rocks of the Erzgebirge, at Freyberg, Altenberg, and Zinn- wald. Granite of the Hartz mountains also micaceous, its nature doubt- ful, interstratified with schistose rocks. Remarks on the binary compounds of quartz with shorl, mica, or talc. HAVING examined the granitic rocks of Cornwall, it is now proposed to turn our attention to those of other countries, in order to ascertain whether their constituent minerals are similarly aggregated together ; and whether the masses re- sulting therefrom are subject to the same arrangements. The details, however, on this head are not so circumstantial as might be desired : this deficiency is attributable to two causes ; first, to the indisposition of geologists to enter into mineralogical minutiae, which they have hitherto con- sidered in this country to be comparatively unimportant; and, secondly, to the want of such favourable opportunities for investigation as occur in the cliffs of Cornwall. Notwithstanding the paucity of such descriptions, yet suffi- cient data may be gleaned for our purpose, which is to show, that the granitic rocks of other countries exhibit similar variations in their mineral composition, and similar associations as those of Cornwall. We learn, from the excellent descriptions of the primary rocks of Scotland by Dr. Macculloch, that the granitic masses are analogous to those of Cornwall : they have not, indeed, the same composition, for hornblende, not shorl, is their Ch. III.] COMPARED WITH THOSE OF CORNWALL. 25 characteristic mineral ; but, with this difference, the resem- blance will be found very great. For example, if we examine the granitic range of Aberdeen- shire, common or perfect granite will be found in the moun- tains at the sources of the Dee; as also the coarse-grained, fine-grained, and porphyritic varieties of the same rock : but hornblende is generally present, forming hornblendic granite. This mineral often takes the place of the mica altogether, giving rise to a perfect syenite, which exhibits numerous varieties, according to the size of the felspar, quartz, and hornblende, and the proportion in which these substances are united. As in the case of shorl-rock in Cornwall, so in this syenite, one of the minerals sometimes disappears : in the former, this is the felspar ; but in the latter, the quartz. And when this happens, the compound cannot be distinguished from some greenstones or traps, which are associated with the stratified rocks. The geologist just mentioned thus describes * the different kinds of this hornblende-rock : " These rocks are fundamentally composed of felspar and hornblende ; and ac- cording to the magnitude of their parts, and the relative proportions of these ingredients, the appearances of the specimens vary. In some rare instances, the crystals of hornblende are so large as to attain half an inch in length, although they are not defined in form ; and as the felspar is commonly white, this variety forms beautiful specimens. From this size, the portions of each mineral vary in grada- tion ; forming compounds, which resemble the coarser and finer greenstones. The hornblende is invariably black, but it is not always intermixed in an uniform manner with the felspar ; some instances occurring in which, to the general indiscriminate mixture, are superadded large and distinct patches of irregular crystals, producing that appearance which, when it takes place in ordinary granite from a similar * Quarterly Journal of Science and the Arts, vol. x. p. 36. et seq. 26 GRANITIC ROCKS OF OTHER COUNTRIES [Ch. III. disposition in the felspar, has been called porphyritic. In general the felspar is white, and of the common kind ; but, in the minuter states of intermixture, it has often a greenish hue, and so far loses its crystalline appearance as to resemble the ordinary compact felspar, which is more common in the greenstones of the trap family : this, however, is not easily determined, but I am inclined to believe that it does occur. When the mixture of the two minerals becomes minute, the rock is no longer distinguishable from ordinary basalt : and in some specimens, it even appears that the felspar is at length excluded ; so that there remains nothing but that compact, yet minutely granular aggregation of hornblende, which, according to some mineralogists, constitutes the only genuine basalt. And it must also be observed that, among rocks of this character, some specimens cannot be distinguished from the black clay-stones, in which the peculiar lustre of horn- blende is absent ; and they are soft, with an earthy fracture." From this statement we learn, that hornblende acts as im- portant a part as shorl, giving rise to as many varieties of granitic rocks : and this is not the only analogy between the granite of Scotland and Cornwall, for other kinds of primary unstratified rocks are to be found in the former country. Of these none are more extensively diffused than the different species of porphyry, which are subject to the same mutations as those of Cornwall already described, and appear to have a similar composition, more especially their bases, which are only varieties of compact felspar assuming different charac- ters, as its siliceous element varies in proportion. The vicinity of Loch Etive also particularly abounds in porphyry, according to Macculloch; and its nature is well exemplified in the following extract from the same author : " The northern side of Cruachan * presents a range of nearly perpendicular precipices, extending many hundred feet down the mountain. This section shows that the mass of the * Geol. Trans., vol. iv. p. 121. et seq. Ch. III.] COMPARED WITH THOSE OF CORNWALL. 27 mountain consists of granite, which is composed of a very equal mixture of reddish felspar and white quartz, with very little mica, nearly resembling the granite of Cairn Gorm. The granite is traversed by veins or courses of porphyry, varying from three or four feet to more than fifty in breadth : these are all very erect, and, in a general view, appear to be perpendicular ; and are so numerous, that they form a con- siderable portion of the mountain. This porphyry is of various colours and composition ; its basis consists of that sub- stance called compact felspar, and its imbedded minerals are commonly felspar and hornblende; and, according to the proportion and the manner in which these are severally united, the porphyry assumes a great variety of aspects. Thus, by an uniform mixture of the basis and hornblende, the transition from perfect porphyry into greenstone is effected ; and this greenstone sometimes resembles well-cha- racterised basalt. Specimens of the junction between granite and this kind of basalt may be here obtained in great variety and abundance: the line of junction is, in all cases, clear and well defined ; but does not admit of ready separation, even after long exposure to weather." Thus we learn that, in Scotland as in Cornwall, the granitic rocks traverse the main mass of granite, in regular beds, veins, or courses, which only differ from each other in size : and that these pass into each other, and into the characteristic granite of the district, by the most gradual mineral transitions, so that it is not easy to detach one from the other. In some parts of the granitic ranges of Scotland, quartz so predominates in the composition of the rock, that it deserves particular notice. When this is not accompanied by the displacement of either of the other ingredients, it may be considered merely as a quartzose variety of granite; but very often it is only asso- ciated with felspar or mica, and in many instances both of these minerals are wanting. These forms of granite have received distinct names from 28 GRANITIC ROCKS OF OTHER COUNTRIES [Ch. III. the French geologists ; but Dr. Macculloch regards them all as quartz-rock. Thus, in his account of Glen Tilt, he observes, that the quartz-rock associated with the granite is of a compact and somewhat transparent quartz, containing irregular grains of felspar : it breaks in a flaky manner, and appears to consist of beds which extend along the granitic ridge to Grianan, in a north-easterly direction, and dipping to the westward of north. The granite in this situation is grey, and shows a slight tendency to a foliated structure in the vicinity of quartz-rock ; so that these rocks appear to pass into each other. These beds of quartz-rock occur in many parts of the granitic range, so that it has the appearance of alternating with granite.* Macculloch thinks that these beds of quartz-rock are only superficial and detached portions of the stratified rocks super- imposed on the granite ; but it is more probable that they are members of the granitic rocks : for analogous rocks are de- cidedly so in Cornwall ; and in Ireland they appear to be of the same nature, as will be immediately seen in Mr. Weaver's description of the granitic rocks of the eastern part of that country. We are informed by this experienced geologist, that the large tract of granite stretching from the south side of the Bay of Dublin to Blackstairs and Brandon, about twenty miles north of Waterford, is full fifty-nine miles in length, and is in general remarkably pure, and free from minerals not essential to the composition of this rock : its felspar is commonly of a clear beautiful white, or slightly tinged with yellow or grey, very rarely flesh-coloured ; its quartz is mostly grey, and its mica greyish-white, inclining to brown or black. This granite varies much in the size of its grain. The finest- grained variety occurs on the northern foot of Cadeen : it is remarkably close and firm in texture, appearing almost com- pact : it is accompanied, however, with a coarser-grained kind, with which it appears to alternate in layers, as may be * Geol. Trans., vol. in. p. 296. et seg. Ch. III.] COMPARED WITH THOSE OF CORNWALL. 29 seen in the brook which flows down the northern face of the mountain. This rock is not unfrequently porphyritic ; in almost every glen the felspar crystals may be seen, two or three inches in length, inlaid in a small-grained base. But there is another variety of granite, which occurs in many parts of this tract, and requires more particular attention. " The eastern side of Eagle Hill, for example, corresponds with the usual granite of the country in the size of its grain and proportion of its ingredients ; but on its summit, which forms an abrupt face to the north, quartz appears as the prin- cipal constituent, affecting a considerable variety of aspect. On the western side are great masses and blocks of pure white compact quartz, and lower down on that side is the quartz rock itself: on the south, again, the face of the rock is bared, and the granite here appears to be mostly composed of quartz ; the prevalence of which becomes still more striking in the loose blocks, the felspar having decayed, and left a singularly connected tissue of quartz, exhibiting amorphous masses, veins, and ramifications."* " In the southern part of this granitic district the granite is not immediately surrounded by mica-slate, but clay-slate; and the granite of the hills, extending from Conna Mountain to Crogan Kinshela, of the eastern bank of the Avonmore, of West Aston, of Kilmancanna Hill, and of the low rocky ridge called Carrigmore, which proceeds eastward, and is connected with the Dunganstown range, possesses characters different from those of the granite of other parts of this tract. Quartz, comparatively speaking, seldom appears in it ; felspar and mica are the prevailing ingredients, sometimes the one and sometimes the other predominating. The felspar is usually of a yellowish or greyish white, unless when coloured greenish by mica ; which mineral approaches sometimes to chlorite on the one hand, or to hornblende on the other. The felspar and mica are sometimes so intimately incor- * Geol. Trans., vol. v. p. 132. et seq. 30 GRANITIC ROCKS OF OTHER COUNTRIES [Ch. 111. porated as to constitute an apparently homogeneous mass, much resembling some varieties of trap ; and in other places it verges towards clay-slate in aspect and texture, as at West Aston and Rockstown. This granite acquires also, in some instances, a syenitic character, containing small crystals of hornblende, as in Carrigmore ; and, in the western end of the Dunganstown range, it passes into true felspar porphyry, exhibiting a fine granular or compact ash-grey base of felspar, in which are inlaid small brilliant crystals of hornblende and glassy felspar.* There are few parts of this granitic region in which shorl, tourmaline, and garnets may not be casually found, but it is only casually, and in such small proportion that they must be considered as merely adventitious." Thus we learn, that mica is the characteristic mineral of this part of Ireland ; so that the granitic rocks are, in a great measure, constituted of varieties of true granite. The peculiar kind of rock accompanying these, in the southern portion of this district, appears to be a kind of eurite ; for the descriptions of Mr. Weaver very accurately correspond with the Cornish species of this rock. The association of these varieties in irregular and alter- nating beds, is also another analogy between this mass of granite and that of Cornwall ; and we must not omit to men- tion another circumstance, which furnishes an additional feature of resemblance. " The granite, in the east of Ireland," says Mr. Weaver, " abounds in contemporaneous veins of granite, and also of quartz ; which, however, are not quite so frequent. The former vary from the smallest to a very large grain, and in width from that of a thread to two feet. There is no glen, in which the rock is at all denuded, in which they may not be studied to advantage, and nowhere more so than among the sublime scenery of Glendalough. At the head of the glen the granite forms mural precipices, in which may be seen numerous veins of granite and quartz, several of which * Geol. Trans., vol. v. pp. 168, 169. Ch. III.] COMPARED WITH THOSE OF CORNWALL. 31 range parallel to each other in a north and south direction ; while these are frequently crossed under various angles by others, which sometimes produce a heave or throw in the tra- versed veins. They sometimes occur so numerously in a narrow compass as almost to resemble a kind of net- work : the position of these veins is generally vertical." * The granitic rocks of Saxony cannot fail to interest the geologist, as the field in which Werner obtained no incon- siderable portion of his knowledge on this subject ; but these rocks cannot be satisfactorily described independent of the primary schists with which they are associated. Indeed, this is the case with all the patches of granite, not only in the north of Europe, but in every other country. In conformity, however, with the plan of this work, and with the general opinion that the granitic and schistose rocks are of a very different nature, a brief description of the granite of the Erzgebirge will be attempted. In this mountain chain the granite is not of great extent, only occurring in small patches near Freyberg and Altenberg, on the eastern side ; and on the west, near Schwartzenberg, it forms small hills, the summits of which are rounded, and of little elevation. At Freyberg this rock is fine-grained, and composed of grey or yellowish felspar, greyish quartz, and brown mica, in nearly equal proportions, and without the admixture of any other mineral. Near Altenberg the granite is similar, but, in some parts, contains large crystals of felspar. In the district of Schneeberg the granite is large-grained, and assumes various appearances, according to the size of the felspar crystals, which oftentimes impart a porphyritic cha- racter ; in some places it decomposes very readily into por- celain-clay, and in others it contains much black shorl, crystallised or in nodules, and is traversed by granite veins, which are fine-grained, and, near Schorlau, are metalliferous, containing arsenical pyrites and sulphuret of molybdena. At * Geol. Trans., vol. v. p. 136. 32 GRANITIC ROCKS OF OTHER COUNTRIES [Ch. III. Zinnwald the entire hill is composed of granite and quartz- rock, disposed in regular alternating beds. The granite is fine-grained, and consists of felspar, quartz, and mica, all of which are white : it is, however, rarely found in a perfect, unaltered state, and that only near the centre of its beds: more commonly the mica passes into talc, and its felspar is decomposed into kaolin. These granitic beds vary from three to ten fathoms in thickness, and do not contain a trace of tin ore. The quartz-rock is of two kinds : the one is nearly pure quartz, grey, crystalline, with a greasy aspect, containing dis- seminated scales of mica, and crystals of wolfram and oxide of tin ; the other, the greisen of the Germans, and hyalomicte of the French, is a granitic mixture of grey, crystalline quartz and argentine mica in large scales. Its beds are as thick as those of the granite : they contain disseminated masses of quartz and of granite, and abound in oxide of tin. The alter- nating beds of these rocks are inclined at angles varying from 15 to 20. The beds of compact quartz-rock are commonly enveloped in those of the granitic variety, and thus alternate with the beds of granite: on the one hand, the granitic quartz-rock passes into the compact, by an augmentation in the size of the quartz grains, and by the coalescence of the mica into larger scales, and its diminution in quantity ; and, on the other hand, it becomes finer grained, and graduates into the granite.* This short sketch shows that the granite in the Erzgebirge, as in Cornwall, is disposed in detached and insulated masses, and that these likewise do not possess the same mineral com- position. In the next place we learn, that the different rocks of which these masses are composed also affect a similar arrangement in beds which sometimes alternate ; and it is worthy of remark, that in the granite of Zinnwald, which decomposes into china-clay, talc, not mica, is the characteristic mineral, which is precisely the condition of the granite in Cornwall wherever this substance abounds. The nature of * Annales des Mines, tomes 8. et 9. Ch. III.] COMPARED WITH THOSE OF CORNWALL. 33 the primary slates in this part of Saxony, and the manner in which they are connected with the granitic rocks, will be con- sidered in another Chapter. In the most extensive primary districts of Europe, how- ever, the granite is not disposed in such rounded and insulated masses, but is more or less elongated, so that, when viewed on the large scale, it has the appearance of immense beds, interstratified with the schistose rocks ; under these circum- stances, it is now generally regarded, not as true granite, but as granitic varieties of the strata with which they are asso- ciated. This point will be discussed hereafter ; we shall only now observe, that the most eminent geologists have differed on this subject ; some contending, that these granitic beds, having the same physical and mineral characters as granite, are perfect and veritable granite, whilst others have main- tained the view just given ; thus, the immense granitic masses of Sweden and Norway have been by some pronounced to be true granite, whilst by others they have been denominated granitic gneiss, because smaller layers of the same rock are interstratified with, or completely enveloped in, the gneiss. For similar reasons, most geologists assert that the granitic rocks of the Hartz mountains are members of the stratified rocks, or intruded masses of a more recent origin, because they, in some cases, alternate together, and also with crys- talline schists, which gradually pass into the greywacke formation : we will, therefore, select this example for a de- scription of this kind of granite, waiving all theoretical con- siderations for the present. Several of the principal summits of the Hartz are crowned with granite, which exhibits the ordinary appearance of this rock, being traversed by three systems of fissures, one parallel to the horizon, and the others perpendicular, so as to divide the mass into cuboidal blocks, the solid angles of which have been rounded by the action of atmospheric agents. The granite is composed of the common ingredients, felspar, quartz, and mica, sometimes the felspar, and at others the D 34 GRANITIC ROCKS OF OTHER COUNTRIES [Oh. III. quartz predominating ; and it assumes various aspects, ac- cording to the size of its crystalline concretions. In ascend- ing the valley of the Use, about a league from Ilsenburg, bare escarpments of granite present themselves, which appear to belong to beds of granite situated in the midst of a different kind of rock, which, being less durable, has been removed by the eroding influence of existing causes. At Rosstrapp, on the eastern extremity of the Hartz, the granite is disposed in regular beds, with a general inclination towards the east; and these beds appear to alternate with quartz -rock, greenstone, schist, and a variety of mica-slate. On the northern side of the Hartz, at Adenberg, M. Bonnard saw a bed of granite distinctly enclosed in strata of quartz- rock and schistose jasper, of which the mountain is composed. And on the north-western side of the Brocken, several beds of granite alternate with greenstone : the latter rock is not perfectly stratified, but the beds of the former clearly run north and south, dipping towards the east.* It is difficult, however, to separate the description of the granite from that of the schistose rocks by which it is accompanied, when they are associated in alternating beds : the details concerning the granite of Norway and the Alps will be found in another Chapter. In concluding these brief details concerning the granitic rocks, we will only observe, in addition to the remarks which have been incidentally made, that a comparison of the granitic formation of Cornwall with that of other countries shows, that some compounds that are of rare and limited occurrence in the former, are developed elsewhere into extensive masses ; as in the instances of the binary compounds of quartz and mica, and quartz and talc : and, vice versa., the porphyritic granites and porcelainous protogine, sparingly scattered in other coun- tries, are abundant in Cornwall, as are likewise, in a still greater degree, the shorlaceous varieties of granite, and the * Annales des Mines, torn. vii. p. 44. el seq. Ch. III.] COMPARED WITH THOSE OF CORNWALL. 35 binary combination of quartz and shorl ; facts which point out to us that all well-marked compounds of distinct minerals ought to be distinguished by appropriate names, although they may be only known to exist in small quantities. The above specified binary compounds are as much entitled to dis- tinct names as shorl-rock : if they be not required in the one case, neither is it necessary in the other ; so that the terms shorl-rock, hornblende-rock, and others, should be expunged from the geological nomenclature, if this view be persisted in. If the knowledge of the primary rocks be a study worth pur- suing, a mineralogical classification must be ultimately adopted; for, accurate details concerning their transitions into each other, and their modes of association, cannot be given on the present system. This topic will be reverted to ; but in passing we cannot help seizing every opportunity to urge the necessity of this reformation, being firmly convinced that no measure will tend more to promote the progress of this department of the science. 36 DESCRIPTION OF THE PRIMARY fCh. IV. CHAPTER IV. DESCRIPTION OF THE PTIIMARY SCHISTOSE GROUP OF CORNWALL. The division of the primary rocks into stratified and unstratified objectionable. Proposed to divide them into granitic and schistose groups, only an artificial arrangement, to facilitate description. Primary slates enumerated. Schistose group of Cornwall. Consists of two series, the porphyritic and the calcareous. Definition of these series. The slates next the granite variously named by Geologists. They are of a peculiar nature. Of two kinds, one bounding the eastern, the other the western granite. They pass into other kinds of slate. The nature of this transition. Greenstone, Actynolite- rock, Chlorite-rock, and the Magncsian rocks, Talc-schist, Serpentine, and Euphotide, described. Remarks on the quartzose varieties of all these rocks, commonly called quartz-rock. Nature and position of the granitic rocks or elvans, in the schistose group of Cornwall. THE primary rocks have been divided into stratified and un- stratified; the latter comprising the various granitic rocks, which generally occur in large insulated clusters, and have a compact and massive structure ; the former denoting those non-fossiliferous rocks which surround, and are intimately connected with the granite, and are very commonly distin- guished by a slaty or schistose texture. It will be found, however, that this division is perfectly arbitrary; for both stratified and unstratified rocks are so intermixed in their associations, that it is sometimes impossible to decide to which of these classes a rock, under examination, belongs : in order, therefore, in some measure, to avoid this confusion, the primary rocks will, in the following pages, be supposed to consist of granitic and schistose groups; only understanding by this division, that in the one granites, and in the other slates predominate ; and it must not be forgotten that this is an artificial, not a natural arrangement, merely to facilitate description. Ch. IV.] SCHISTOSE GROUP OF CORNWALL. 37 The rocks which immediately surround the insulated masses and ranges of granite, are very numerous; and although they are generally schistose, they are sometimes massive, and in their structure very similar to the granitic. These circum- stances, however, will come under consideration hereafter ; at present, our object is to obtain a knowledge of the composi- tion and internal appearance of the individuals of the schis- tose group. The primary slates, which have been longest known, and most frequently described, are gneiss, mica-slate, and clay- slate ; in many places they follow each other in this order ; and thus they were observed by Werner in Saxony and Bo- hemia ; and it was therefore for a long time considered as the only and true order of succession. A more extended experience, however, has not only shown that granite may be in immediate contact with either of these rocks, but also that there are many other primary slates, which, in like manner, are not subject to any fixed laws in the man- ner of their association. For our knowledge on this subject we are more particularly indebted to Macculloch, and the French geologists. The following are the principal additions which have been made to the primary slates of Werner : viz., quartz-rock, actynolite, hornblende, shorl, chlorite, and talc schists, and steaschist ; and in the 4th volume of the Cornish Geological Transactions some others have been proposed ; so that it may be safely predicted, that as our knowledge increases, this cata- logue will be extended. In describing the various schistose groups, that of Cornwall will in the first place demand our attention. It may be divided into two series, the porphyritic and the calcareous ; the former including those rocks which occur next the granite, and contain porphyries and other granitic rocks in the form of regular beds or elvan-courses, and which abound in veins of tin and copper ores ; the latter, comprising those rocks which are more or less remote from the granite, contain no elvans, D 3 38 DESCRIPTION OF THE PRIMARY [Ch. IV. but abound much more in greenstone, especially its obscurer varieties, and in dark-coloured limestones ; sparingly metal- liferous, containing no tin, but productive of lead and anti- mony ; and lastly, possessing occasionally organic remains. Most of the rocks of the calcareous series appear to be referrible to the older portion of that class which is interme- diate between the primary and the secondary, commonly known by the name of transition ; a class of Werner's system which for many years had fallen into disuse, but has been lately revived on account of its convenience. The individuals of this calcareous series will not be treated of in this place, with the exception of the magnesian rocks, serpentine, euphotide, and talc-schist, which immediately follow the porphyritic series. The rock in contact with granite in Cornwall has been usually called argillaceous schist, or clay-slate. Dr. Berger, and after him many other geologists, have termed it greywacke; but, as Professor Mohs has very justly observed, it has n resemblance to this rock : some have adopted the word killai from the miners, to denote this kind of slate, but have used it more vaguely than even it is done provincially ; for even the miners acknowledge that some important varieties of this rock are not true killas, but a kind of elvany killas. The remarks of the late Rev. J. J. Conybeare are very appropriate: " The common killas," he observes, " after much question as to its being a variety of greywacke, which, if that term has any definite meaning, it unquestionably is not, has been at last admitted on all hands to be a genuine clay-slate; but this appellation, perhaps, after all, does not convey a much clearer notion of the real nature and constitution of the rocks included under it, than the repudiated greywacke." In fact, no term has been more misapplied than that of clay-slate ; and its application has been general to all fine slaty rocks, no matter to what member of the primary slates they belong, or, indeed, whether they occur in the transition or secondary classes. In Cornwall, for instance, there are at least a dozen kinds of Ch.flV.] SCHISTOSe GROUP OF CORNWALL. 39 rocks that are very fissile, all of which have been indiscrimi- nately called clay-slate, notwithstanding they sensibly differ from each other in their external and physical characters, and are respectively associated with distinct suites of rocks. These differences of opinion have arisen from geologists having examined the Cornish slates in too cursory a mariner. A more careful scrutiny would have shown that, the rock adjoining the granite is neither greywacke nor clay-slate, but a rock sui generis. It is not meant to assert that this rock is not analogous to any of the innumerable varieties of rocks in other countries, which have been termed clay-slate ; but that it does not correspond with clay-slate, in the ordinary acceptation of this term. The rock adjacent to the granite in the western part of Cornwall, in which most of the productive mines are situated, is of a different nature from that similarly placed in the eastern part of the county ; and it is worthy of remark, that the latter not only abounds less in metals, but is also associated with rocks different from those which occur in the former : the distinction of these is therefore important in an economical, as well as a scientific point of view. The former slate appears to be nearly allied to the thonschiefer of Johanngeorgen- stadt : the latter, to the clay-slate described by Mr. Weaver, as occurring in the southern part of .the granitic range in the east of Ireland. Before attempting to show the composition of the Cornish slates, now under consideration, it will be well to describe their respective appearance : and, until geologists have de- termined their true nature, and agreed on distinctive appel- lations, the terms cornubianite and proteolite, proposed in the Cornish Transactions, may, for the present, be admitted, in order to avoid needless repetitions. Cornubianite, the rock most abundant in the western part of Cornwall, exhibits various shades of dark blue and purple, sometimes of an uniform colour, but occasionally with dark stripes, spots, or patches, on a light blue base : it varies from D 4 40 DESCRIPTION OF THE PRIMARY [Ch. IV. hard to very hard ; and breaks, though not easily, into thick plates full of joints; its surface is often spangled with a micaceous mineral, the parallel arrangement of which imparts to the mass a laminated structure : on decomposition, it be- comes much more schistose, and of a dirty yellowish white colour. Proteolite, as its name is intended to indicate, assumes a great variety of forms. In colour it does not much differ from the rock just described, but its tints are generally much lighter ; indeed, its original colour is not often met with, for, owing to the tendency of this rock to undergo a partial decomposition, becoming earthy and argillaceous, it is ge- nerally found of a yellowish or very pale brown colour. It is much softer than cornubianite, and its hardest kind, instead of possessing the fine compact texture of that rock, is always more or less arenaceous. Its species are much more schistose ; its composition is, for the most part, pretty uniform ; some- times, however, a shining mineral is present, in minute spots and scales, and in one instance the whole basis is shining and glossy, approaching very near to mica-slate. Both of these slates appear to be principally composed of compact felspar, which is probably a compound, as already stated, of felspar and quartz in various proportions : in cornubianite, this substance has an uniform massive texture; in proteolite it is finely granular. And, as the felspar or quartz predominates in the compact felspar basis, so these rocks exhibit different degrees of hardness, and other pro- perties characteristic of the prevailing constituent By these fluctuations in the composition of the basis, we can easily comprehend how these rocks sometimes contain beds of pure compact felspar, at others, beds which are entirely quartzose. The colouring or accessary mineral of these slates is not always obvious, but it appears to be either mica or shorl, or some intermediate substance. It may be enquired, how can this be ascertained ? The nature of the colouring mineral can only be proved in this Ch. IV.] SCHISTOSE GROUP OF CORNWALL. 41 manner. The slates hi question, and indeed all the primary slates of Cornwall, form distinct suites or genera : some of the members or beds of each genus are schistose, and others compact ; in the latter the basis is very frequently pure, and the colouring mineral is aggregated into either particles, scales, crystals, or such a body that its nature can be detected ; and from these compact or crystalline beds, there is a gradual and almost imperceptible passage into the perfect slate, so that it appears to be a justifiable conclusion, that both the compact and schistose beds have the same composition. Thus nature, as it were, proffers her own analysis to assist us in our researches, by giving us an insight into the con- stitution of rocks, in which the elementary minerals are so blended as to present only a homogeneous mass. If this view of the subject be correct, we learn that cornubianite and proteolite are both composed of felspar, quartz, and mica, shorl, or an analogous mineral ; that is, of precisely the same ingredients as granite. Mr. Hawkins, many years ago, appears to have entertained a similar opinion, for he suggested " that the common killas is an intimate mixture of quartz with mica, talc, chlorite, and perhaps, in some instances, with felspar." There can, however, be little doubt of the presence of the last-mentioned mineral ; for, as in the case of granite, it generally constitutes by far the greater part of the mass. After what has been now said, the nature of the remaining schistose rocks of the porphyritic series will be easily under- stood, since they have all a basis of compact felspar, united with hornblende, actynolite, or chlorite. All these rocks appear to pass gradually into each other; and these transitions are not only effected by the variations in the composition of the compact felspar basis, but the accessary minerals themselves appear to be subject to similar changes, mutually passing into each other, giving rise to intermediate substances, which, if they be definite compounds, have not been hitherto described : it is more probable, how- 42 DESCRIPTION OF THE PRIMARY [Ch. IV. ever, that they are intimate mixtures of one or more of these minerals. Several instances will be adduced hereafter, which show that mica, shorl, hornblende, and these analogous minerals, do graduate into each other ; and as this is a subject of great importance, and is indispensable to a right understanding of the primary rocks, it ought to be particularly impressed on the mind of the student. " To the geologist," observes the Rev. J. J. Coiiybeare, " who seeks the aid of mineralogy and chemistry, examples of this intimate penetration of one simple mineral by another (so as, in many cases, to alter very considerably the external and empirical characters of that which yet predominates), must be familiar. Many subordinate beds of the earlier greenstone formation exhibit every stage of a similar phenomenon; and an accurate examination would probably show, that most of the substances named petrosilex, corneene, saussurite, jade, and even flinty slate, are, in fact, only admixtures of this nature, in which felspar, varying from its more compact and semitransparent, to its earthy and granular form, is uniformly and intimately penetrated by some variety or other of hornblende, of diallage, and occa- sionally, perhaps, of other minerals, which (as hypersthene) enter more sparingly into the composition of rock masses. Such admixtures can be properly studied only in those endless suites of specimens which Nature herself preserves, and pre- sents in situ. The subject is an interesting one, and well deserves closer attention and investigation than it has yet met with." * It is a very common opinion among geologists, that the almost endless varieties of primary rocks, produced by these mineral transitions, embarrass the investigation of this de- partment of the science, and set classification utterly at defiance. But a more enlarged knowledge of this subject will probably prove, that this frequent passage of one rock * Annals of Philosophy. New Series, vol. vi. Ch. IV.] SCHISTOSE GROUP OF CORNWALL. ^ into another, so far from creating confusion, may form the groundwork of a systematic arrangement. We are some- times apt to forget, that the deviations from our most ap- proved systems are not indications of irregularities in the works of nature, but proofs of our imperfect knowledge ; and, instead, therefore, of attempting to reduce all things within the bounds of our limited views, we ought rather to enlarge, or even alter our opinions, so as to adapt them to the phe- nomena of the Creation. Such a line of conduct has been often and very successfully followed ; and, in the case now under consideration, it would probably be found advantageous, though it could not be expected that the application of this principle would, in the first instance, be rewarded with com- plete success. In the preceding remarks on the composition of the Cornish rocks, it has been seen that there are, here and there, certain distinct and characteristic rocks : each of these, by gradual transitions through a series of layers or beds (which are sometimes compact, sometimes schistose), passes on either hand into those adjoining. These series may therefore be regarded as constituting genera, of which the central or cha- racteristic varieties, commonly massive, and oftentimes crys- talline rocks, are the types. If this view of the subject be correct, the primary rocks do not occur in a state of con- fusion, but are arranged side by side, according to their affinities, after the manner of a well-regulated museum. To return to the description of the rocks of the porphyritic series : the hornblende rocks are, in a great measure, com- posed of compact and schistose greenstones, which repeatedly pass into each other, so that, in traversing them from the granite, they appear to alternate ; this, however, does not take place to any great extent in the length of their beds, for sometimes the massive rocks are insulated in the body of the slate, and vice versa. On approaching towards the granite, the greenstone is generally found to be harder, and becomes more and more siliceous, with quartz veins, which sometimes project 44 DESCRIPTION OF THE PRIMARY [CJh. IV. on the side of the hills in blocks three or four feet in breadth : its hornblende gradually loses its characters, until at length it cannot be recognised as such, having passed into shorl, and thus the transition is effected into proteolite, the genus last described. In like manner, proceeding over the greenstone in a con- trary direction, towards the shores of Mount's Bay, the acces- sary mineral, hornblende, becomes gradually changed into actynolite, as is evident in those rocks which have a compact and crystalline structure. This genus is not of very common occurrence, and may therefore be noticed a little more in detail. The most abundant species of actynolite-rock is hard and compact, known provincially by the names of blue-stone or blue elvan. This variety is accompanied by, and gradually passes into a blue slate, which is fissile in various degrees, and may also be easily separated into small rhomboidal pieces. The massive kind occurs in elevated ridges on the coast, and is very durable, whilst the slate is decayed to a considerable depth, still retaining its form, but progressively diminishing in tenacity from the perfect rock to the surface, where it is sometimes nearly as soft as clay : the blue colour of the slate has disappeared in the decomposed rock, which is ash- white, with ochreous stains, or of an uniform pale fawn colour ; this property readily distinguishes it from slaty greenstone, with which it is nearly allied. Some of the compact kinds of this genus are very beautiful, having a light violet-coloured basis, which is variegated with stripes, and marbled markings of the dark blue species. The violet colour appears to be occasioned by the intimate union of axinite with the compact felspar basis of this rock ; for axinite, in crystals, is often present, in the state of patches and small veins, traversing the rock after the manner of calcareous spar in limestone. At first sight, these rocks appear to be well calculated for ornamental pur- poses, but unfortunately they are traversed by numerous seams or joints, which become discoloured by exposure to the Ch. IV.] SCHISTOSE GROUP OF CORNWALL. 4-5 air, in consequence of the decomposition of iron pyrites. This mineral not only coats the joints of this rock, but is also some- times disseminated throughout its substance in small amyg- daloidal granules. Each of the preceding schistose rocks, but more particu- larly the greenstone, gradually passes into thick lamellar slates, which exhibit various shades of blue and green, are soft, and abound in metalliferous veins, particularly of copper, the matrix of which is composed of quartz and dark green crystalline chlorite : they also contain layers or beds, which are compact and earthy, differing only from the slate in wanting the fissile structure. Thus we find, that the rocks of the schistose group imme- diately surrounding the granite of Cornwall, may be referred to five distinct genera, viz. cornubianite, proteolite, green- stone, actynolite-rock, and chlorite-rock. To these it is pro- posed to subjoin a description of the magnesian rocks, the true position of which has perhaps not been decidedly ascer- tained, though it appears in Cornwall to be in the lowest part of the calcareous group, the upper part of which contains organic remains, and therefore it may possibly belong to the intermediate or transition class. The magnesian rocks of this county may be divided . into three genera : diallage-rock or euphotide, serpentine, and talc-schist ; the last is generally received as a distinct rock, and, indeed, in other countries, it is extended over large tracts, with different associations ; otherwise it might in Cornwall be regarded merely as a schistose species of ser- pentine. The euphotide consists of felspar and diallage, both of which are very crystalline, and are generally very distinct, in the form of various sized crystals aggregated together, and mutually penetrating each other, after the manner of granite : its felspar does not appear to be identical with that of granite, which may be owing to the presence of magnesia, just as 46 DESCRIPTION OF THE PRIMARY [Ch. IV. diallage differs from hornblende in containing a large pro- portion of this earih. The constituents of euphotide, in some places, gradually become so small, that the rock is at length perfectly homo- geneous ; and where this is interlaminated with small scales of shining diallage, the rock acquires a schistose structure. This schistose euphotide has been commonly called hornblende slate ; but it is so different from all the varieties of the latter, and has such a characteristic appearance, that it ought to be distinguished. It appears, sometimes, to be imbedded both in the euphotide and in the serpentine ; and the latter rock, even in some instances, seems to be subordinate to the schist. The fact is, that the schist often forms the connecting link by which euphotide and serpentine graduate into each other ; and though these rocks alternate on a given line, like the massive and schistose greenstones, yet they do not preserve a continuous course for any considerable distance ; but mutually abut against, or are dovetailed into, each other ; sometimes the one, and sometimes the other prevailing. The serpentine of Cornwall is proved to be a compound of diallage and felspar ; or perhaps, rather, of compact felspar, by its frequent transitions into euphotide. This rock ex- hibits a great many varieties ; some of which are hard, whilst others are so soft as to yield to the nail. This difference ap- pears to depend on the felspar base, which undergoes several modifications, between a crystalline, compact, and granular state, as seen in the precious, common, steatitic, and ollareous serpentines ; in the same manner as the rocks of the porphy- ritic group assume various aspects, according to the composi- tion of the compact felspar base ; with this difference, however, that in these, the proportion of the silica modifies the com- pound, whereas, in the serpentine, the changes are attributable to the relative quantity of magnesia. The accessary mineral, diallage, also, imparts characters to the serpentine, according as it is intimately combined with the base, or is disposed in distinct forms. And, lastly, both of the component minerals, Ch. IV.] SCHISTOSE GROUP OF CORNWALL. 47 in all their modifications, traverse the serpentine in irregular strings, veins, and layers. The talc-schist of Cornwall is not a compound of quartz and talc, as described in systems of geology, but of the same substance as forms the basis of the serpentine, intimately blended and laminated with talc ; which mineral may be seen in distinct scales, in the quartz-veins and nodules which occur in this slate. It appears to be produced by the transi- tion of the diallage into talc, in the same manner as horn- blende passes into mica. It is worthy of remark, that the ollareous serpentine and talc-schist of Pollaphant, is accom- panied by a bed of crystalline limestone; which is glossy and variegated by the intermixture of talc, and abounds in tabular spar and hornstone. This single instance is curious, as we find this association extensively developed in other countries. In the foregoing enumeration of the Cornish primary strata, it may have been observed, that no mention has been made of the quartz-rock of Macculloch. This omission has been intentional, because nearly all the varieties of this rock do occur in Cornwall, and some of them occupy extended tracts, the boundaries of which are distinctly marked by barren downs : but the quartz is generally combined with the component minerals, or the substance itself of the adjoin- ing slate ; so that it appears to be a more natural arrange- ment, to consider these rocks as quartzose varieties of the slates into which they graduate. It has already been stated, that the rocks of the schistose group are composed of compact felspar, united in various pro- portions with mica, shorl, hornblende, or some other accessary mineral ; and that some species of these rocks are produced by the relative quantity of the component parts of the compact felspar base : thus, when silica predominates, the species be- comes quartzose ; and if it prevails, to the exclusion of all other substances, then it gives rise to masses, layers, or veins of quartz, which may be regarded as subordinate to the rock 4-8 DESCRIPTION OF THE PRIMARY [Ch. IV. in which it occurs. This mode of classification appears to be more simple, and more in accordance with nature, when the quartz is blended with the substance of the adjacent strata ; but when the quartz is pure, or combined with crystalline minerals, then it becomes a true quartz-rock ; often graduat- ing into hyalomicte, pegmatite, and analogous rocks, and must then be regarded as the equivalents of those granitic beds which are interstratified with the primary schistose rocks. The same genus does not always contain a quartzose species of the same extent in every locality ; this, like all the cir- cumstances relating to the primary rocks, cannot as yet be reduced to any fixed laws : but it may be remarked, that the quartzose species very often form the connecting link, through which two adjoining genera pass into each other. And it may also be here observed, that these quartzose rocks very often assume a brecciated or fragmentary appearance, evidently arising from some portions of the quartz being free from the colouring or slaty matter with which the greater part is intimately combined. The variegated and agate-like figures which these rocks assume, cannot be easily described ; sometimes the pure, and sometimes the coloured quartz, pre- dominates ; and not unfrequently the latter resembles frag- ments of slate, the nearest sides of which, though sometimes many inches asunder, so correspond, as if they had been at one time united : but a careful scrutiny of this rock will con- vince every one, that this fragmentary appearance is only a curious coincidence ; for if one part of it seems to support such an opinion, a thousand others will show that the rock could not have been formed by the aggregation or cement- ation of fragments. These curious fragmentary appearances are not peculiar to Cornwall : we shall, therefore, have occa- sion to revert to this subject, when treating of the structure of the primary rocks. In order to complete the sketch of the schistose group of Cornwall, it remains to state, that granitic rocks often occur therein as large beds or dykes, called by the miners elvan- Ch. IV.] SCHISTOSE GROUP OF CORNWALL. 49 courses. These rocks might have been treated of in a distinct chapter, or in connection with granite, but the plan here adopted is preferred, in order to present a faithful descrip- tion, as free as possible from theoretical opinions concerning their nature. By the term granitic rocks is understood such compounds as have already been described as forming portions of the central masses of granite. They seldom resemble true granite, but exhibit most of the species of shorl-rock, proto- gine, eurite, and felsparite, more particularly of those kinds which are porphyritic. These granitic rocks are most abun- dant in the vicinity of the granite ; and very commonly, though not invariably, are found to underlie towards the main mass of granite at very considerable angles, averaging about 70 or 75 : that is, their dip generally differs from the inclination of the laminae of the schistose rocks, which usually slope from the granite. The direction of these granitic rocks is, in general, from N.E. to S.W. ; corresponding, in this respect, -with the greater portion of the slates, and with the principal metalliferous veins : and like these, also, their course is not in an undeviating straight line, but is variously curved ; and this character is also detected in their underlie. The great elvans of Polgooth mine may be referred to, as affording illustrations of this circumstance. Concerning the kinds of slate, with which these granitic courses are connected, and the relations which they respect- ively bear to each other, it will be necessary to adduce some examples in detail; for this important subject will furnish some leading arguments in the subsequent examination of the prevailing theory. The varieties of felsparite, described as one of the Cornish granitic rocks, are more frequently met with amongst those slates which have a similar hard compact felspar basis, as cOrnubianite, actynolite rock, and some of the species of greenstone. Examples of this association may be seen in Dolcoath, and other mines situated near the junction of the 50 DESCRIPTION OF THE PRIMARY [Ch. IV. granite and the slate ; but it is much better exhibited on the shores of Mount's Bay, where several beds of felsparite traverse compact and schistose species of cornubianite and actynolite rock, on the beach below high-water mark. One of these, near Penzance, is particularly deserving of attention; as a considerable portion of it is exposed to view, and is easily accessible. Immediately at the back of the pier, it may be seen stretching N. E. and S. W., and inclining at a great angle northward. It is a porphyritic felsparite (felspar por- phyry), the imbedded crystals being small and v/hite : these gradually disappear towards the sides of the bed, till at last this rock is a plain compact felsparite, very hard, apparently in consequence of an excess of silica in its composition ; and in this part, veins of quartz are frequent, as is also the case in the adjoining stratum of compact actynolite-rock, at the point of their junction, where these rocks can only be dis- tinguished from each other by the blue colour of the latter. This granitic bed is lost eastward, under the sea, and west- ward, under houses and the little neck of land which runs towards the battery ; but on the other side of this small pro- montory, it again makes its appearance, and may be examined for nearly a thousand feet in length. It first emerges from under the sand-bank near the chimney rock, a prominent part of this bed ; the highest part of which consists of a single prismatic block. The ground plan (fig. 1.) will serve to illustrate its position, and its mode of connection with the adjoining rock. The part figured is situated between the chimney rock and the eastern extremity next the sand-bank, and is about two hundred feet in length : in some points the line of junction between the el van and slate bears N. E. by E., or even N. E., as at the back of the pier ; the general direction of the whole course, however, is E. by N, and W. by S., and the occasional departures from this are apparent in the plan. On the southern side the junction may be traced nearly two hundred feet farther; on the northern side, with little interruption, Ch. IV.J SCHISTOSE GROUP OF CORNWALL. Fig. 1. 51 Elvan Course, near Penzance. (Ground Plan.) for seven hundred feet : and although the course is very irre^- gular in the part selected, the remainder of it, on the side exposed, exhibits a line very slightly undulated. As regards the dip of this el van there seems to be a difference of opinion, Mr. Hawkins having stated that it underlies, in the Wherry mine, towards the S. W. one foot and a half in a fathom *, whilst Mr. Carne says that it dips north, f The appearance of the dip is very various : at A, it is S. by E. at 45 ; at B, N. W. at 70 ; at C, it underlies the slate towards the S. E. very gently, and in a curved line, which, at a distance of ten feet, attains rather a higher level ; at D, it is nearly perpendicular ; on the northern side, at E, F, and G, the dip is uniformly to the N. W. at angles fluctuating between 60 and 70. Now the more extended surfaces of the large con- cretions, in the middle of the course, generally dip at low angles to the southward, either E. or W. of this point, according to the curvature of the elvan ; whilst the lesser surfaces de- cline in an opposite direction at elevated angles ; and we therefore consider these to be the true bearings : the latter * Gcol. Trans, of Cornwall, vol. i. p. 140. E 2 f* Idem, vol. ii. p. 82, 52 DESCRIPTION OF THE PRIMARY [Ch. IV. accords with the dip observed in the mine, and we consider the northern surface underlying the slate to be the termi- nation of the elvan in width, for what is only seen here in part probably entirely obtains at both extremities, since they are lost under the schistose rocks ; and we also think that some of the irregularities in the breadth of the elvan may be traced to the persistence or abrasion of the slate, more par- ticularly the latter, as at A and G. The elvan is traversed by lines or joints, which cross it in the direction of S. by E. and N. by W. : these lines are very close together at G, and there the porphyry is much stained with oxide of iron, and is in a decomposing state; and the rock adjoining is similarly circumstanced. Where the elvan is a well characterised por- phyry, the lines are farther apart, that is, the elvan is composed of larger concretions. But the most important circumstance concerning these lines is, that they, in innumer- able places, continue their course uninterruptedly into the slate, as may be well seen even at A and F, where the elvan is seventy-five and ninety feet in width. The same observ- ation equally applies to the longitudinal joints, where they meet with the protruding slate. Lastly, it may be noticed, that at H there are several bunches and elongated portions of rock perfectly resembling the slate at its junction with the porphyry ; and that at fifty feet north of E, the laminae of the slate are very flat and undulating, sometimes rather inclining S., but oftener to the N. Beds of eurite are of frequent occurrence in the Cornish rocks, which it has been proposed to call proteolite; and in this case also we find a similar correspondence in the nature of their respective bases, both consisting of a soft and fine granular compact felspar. Several beds of eurite occur between Camelford and Bod- min. At Tremagenna, it is a white rock, composed of com- pact felspar, in which are imbedded limpid grains of quartz, and scales of mica ; it is also in some parts stained with rust coloured spots, produced by the decomposition of some ad- ventitious mineral, probably of hornblende ; and small irre- Ch. IV.] SCHISTOSE GROUP OF CORNWALL. 53 gular crystals of felspar may be observed here and there, but they are of rare occurrence : its junction with the slate is not exposed. At Kernick, about three miles from Bodmin, there are several beds of granitic rocks: the mica is in larger scales than in the last, and small portions of hornblende are dis- tinctly developed ; and here also their junction with the slate on either side is concealed, so that their size and bearings cannot be ascertained. About a mile and a half from Bodmin, on the road to Truro, an interesting kind of eurite occurs, which we have termed porcelainous eurite, as indicating its nature. It is of a pale greenish yellow colour, rather uniform and compact, but containing grains of limpid quartz : it decomposes to a considerable extent, and then resembles china-clay, which has been already described as a disintegrated protogine, a rock distinguished from granite by containing talc instead of mica, and which abounds in the mass of granite about three miles off. If this bed of porcelainous eurite were well ex- posed, its constituent minerals would probably be distinctly exhibited, as is generally the case in the middle of these elvan-courses, where the rock becomes porphyritic and even granitic. Granular and compact shorl rocks also occur in this part of Cornwall imbedded in proteolite, and although at first sight these rocks appear to have a very dissimilar composition, yet a careful examination will show that they are nearly related. The proteolite is frequently coloured by or even intermixed with shorl, in various states ; and when the compact felspar basis becomes siliceous, the shorl is present in greater abundance, till at last it graduates into compact shorl-rock, and the constituents becoming distinctly crystallised, die granular shorl-rock is produced. The celebrated Roach rock is composed of the last-mentioned variety : it rises out of the slate in the form of an immense tor, about a mile distant from the granite. E 3 54- DESCRIPTION OF THE PRIMARY ^Ch. ^. The granitic el vans (species of eurite) are very abundant between St. Agnes and Cligga Point, on the north coast, where they may be seen running in regular beds, alternating with the schistose rocks into which they gradually pass : some parts of these el vans are very shorlaceous and quartzose ; and being on this account more durable than the felspathic kinds and the slate, they project on the shores in very abrupt ridges. On the shore at Swan-pool, near Falmouth, there is a large elvan-course, the upper part of which in the cliff is ex- tensively decomposed into a state of clay : its course on the shore may be traced for several fathoms in length ; but what is more particularly worthy of remark is, that near low-water mark it abruptly terminates, but immediately adjoining it is seen running in a parallel direction at a distance of about twenty feet from its original course, which is exactly the width of this elvan. This is an excellent illustration of what the miners call a heave, and is interesting, as this pheno- menon is not of such frequent occurrence as in the case of veins : in this instance the elvan has the appearance of having been intersected by a thin course of clay, or jflukan, which, however, gradually tapers or dies away on the land side. At Mousehole, in Mount's Bay, a very large vein, or rather a course * of felspar porphyry, exhibits heaves by veins of quartz : the course is visible, at low water, for more than a hundred and fifty feet in length, and is sufficiently important to de- mand a particular description, which may be rendered more intelligible by the sketch, (fig. 2.) The bearing of this course is a little S. of W. and N. of E., on an irregularly undulating line, dipping towards the N. at * A large vein of a similar nature, at Cape Cornwall, ought to be considered as an elvan-course, according to Mr. Carne ; but it matters not in which light these elongated granitic masses are viewed, because no line of distinction can be drawn between them. It is worthy of remark, however, that elvan. courses generally run parallel with the strike of the laminae of the slates j whereas at both these places they cross them diagonally. Ch. IV.] SCHISTOSE GROUP OF CORNWALL. Fig. 2. Felspar Porphyry, at Mousekole, Cornwall. (Ground Plan.) a very gentle angle. The porphyritic crystals of felspar are sometimes large and distinctly defined, but more commonly they have the usual imperfect forms of such crystals : they are not confined to the central part of the course, but often abound close to the junction ; and in many cases it is difficult to say whether the slate or the compact felspar is the basis of this rock : this is particularly the case when the seams or open joints do not correspond with the line of junction, but cut off a portion of the slate, as happens at every step, so that both rocks compose the same individual mass or concretion, and cannot be separated ' from each other. At A the course divides into strings, so as to envelop portions of the slate which, together with the elvan, are mixed in irregular elon- gated portions, variously disposed : in several places portions of rock are imbedded in the elvan, which perfectly resemble the hard compact slate immediately adjacent to this course. Some of the quartz-veins abut against the elvan, others tra- verse it without any alteration ; but the vein B produces a heave of twelve feet towards the north. On the western side, the elvan is ten feet wide, gradually enlarging from eight ; E 4? 56 DESCRIPTION OF THE PRIMARY [Ch. IV. on the eastern side, it is only six, diminishing in its course to four and even three feet, when it is again heaved, by a small, quartz-vein C, towards the south, to a distance equal to its width. The vein B, in contact with the western elvan, is two feet wide ; in some parts purely quartzose, in others porphy- ritic, and even blended with irregular portions of slate, the whole mass at many points passing gradually from one rock into the other It continues of the same nature and of the same thickness towards the eastern portion of the course ; but, before reaching it, dwindles into strings, and forms a con- siderable curvature : passing the elvan a second time, it also involves porphyry in its composition for a few feet, then be- comes quartzose and schistose, and runs a curved course for seventy feet, having a variable thickness, and sending off branches, and then dies away in strings. In its southern course, however, it assumes a great contortion, and at the same time becomes a hard compact variety of slate, in which state it disappears under the sea. The courses or beds of granitic rocks, in the slates, some- times exceed three hundred feet in breadth, and at other times they dwindle to such a small size, as to be called granite veins. These courses are subject to curvatures, in common with the containing slate, as has been already noticed ; and they often send out branches or veins during their progress, as figured by Professor Buckland in the el vans of St. Agnes.* They sometimes terminate abruptly, as at Mousehole and Cape Cornwall, or, as in the northern end of the elvan at the latter place, gradually disappear in several small veins.t Lastly, near the granite or elvan-courses, these granitic rocks often occur in bunches or masses of various dimensions. The direction of these courses is generally parallel with the strata, but the dip being very commonly towards the granite at a very considerable angle, varying from 45 to 75, these granitic rocks cross the laminae of the slates, as in the diagrams, (fig. 3.) * Geol, Trans., yol. ir. f See figs. 13. 16. Ch. IV.] SCHISTOSE GROUP OF CORNWALL. Fig. 8. 57 Ground plan. Section. Pengersic Cove, Mounts Say, Cornwall. From the examples which have been adduced, we derive the important fact that the insulated beds or courses of granitic rocks always partake of the nature of the containing slate, which is not surprising, since it has been shown in this and the second chapter that both kinds of rocks have re- spectively the same bases. Let us now turn our attention to the schistose groups of other countries; referring those, who are desirous of ob- taining more minute details concerning the varieties and localities of the Cornish crystalline slates, to the fourth volume of the Geological Transactions of Cornwall. 58 A DESCRIPTION OF THE [Ch. V. CHAPTER V. DESCRIPTION OF THE SCHISTOSE ROCKS ASSOCIATED WITH GRANITE IN VARIOUS COUNTRIES. Mica-slate and clay-slate of the eastern part of Ireland. Their alternations with quartz-rock and hornblende-rock, their associations with granite. The composition of this granite, and its modes of arrangement. The gneiss > formation of the Western Isles of Scotland. Its beds of mica-slate, green- stone, compact felspar, clay-slate, talc-schist, serpentine, limestone, and quartz- rock. The nature of their connection with each other and with the gneiss. Abounds in bunches and veins of granite. Description of the primary schistose rocks of Norway, gneiss, mica-slate, clay-slate, with their sub- ordinate beds. They contain immense beds of granite, and are inter- stratified with smaller ones. The primary schistose group of Saxony, gneiss, mica-slate, clay-slate, and shorl-schist. The talcose formations. Of the Alps, including talcose granite or protogine, talc-schist, serpentine, and other magnesian rocks. Of the island of Corsica, composed of granite, eurite, protogine, hornblende-rock, euphotide, talc-schist, serpentine, and analogous rocks. IN attempting to show that the primary or crystalline schists of different countries are the equivalents of those of Cornwall, it is not intended to assert that they are all of the same nature, and referrible to precisely the same geological epoch : on the contrary, it is wished, for the present, to avoid all conjectures concerning the nature of their origin, and only to express that these slates do, as in Cornwall, bear a certain relation to the granite with which they are associated; and consequently all slates, both foreign and Cornish, which have the same relative connection and position with the granite, may be regarded as parallel or equivalent rocks. These equivalent schists are commonly of a very different nature, which might have been expected a priori ; for even in Cornwall the suites of slates vary when connected with different masses of granite ; still, however, although their composition is not identical, they undergo analogous mutations, resulting from the proportions of the constituent parts, and the manner Ch. V.] PRIMARY SCHISTOSE ROCKS. 59 in which these are united. Thus it has been shown that one insulated patch of Cornish granite is very felspathic and abounding in porphyry ; another, shorlaceous, and containing numerous beds of protogine (china-stone) ; and a third, more micaceous than ordinary, and intersected by layers of eurite ; and in each of these cases we have seen that the series of ad- jacent schists vary accordingly, and yet each series may be considered as the equivalent of the other. Now let us proceed to examine the nature of the crystalline slates of other countries, and to enquire whether they bear the same relations to the accompanying granite, as obtains in Cornwall; or whether these rocks, in the latter case, are exceptions to the general rule, and have a peculiar arrange- ment. In Ireland it has already been stated that the extensive mass of granite occurring in the eastern district presents a difference of composition in its northern and southern portions : in the former, it consists of the ordinary ingredients, being often quartzose, but exhibiting little variation of mineral character ; whilst in the latter, quartz is not so abundant, and the felspar and mica are sometimes intimately incorporated, in some varieties resembling homogeneous trap, and in others verging towards clay-slate ; and, in like manner, the adjacent schist is principally mica-slate in the north, and clay-slate in the south. " This mica-slate," says Mr. Weaver, " wherever it occurs, is in direct contact with the granite. It consists of alternate layers of quartz and mica, from one line to two and three inches thick, and some layers of quartz extend even to two feet in thickness, while the surface of the intervening mica is almost invariably studded with cruciform and stelliform aggre- gations of hollow spar."* " The mica-slate which occupies the lower part of Glenmacanass, on the western side, contains a bed of talc-slate, of uncertain thickness, both rocks dipping 25 towards the south-east. The talc-slate is of a greenish * Geol. Trans., vol. v. p. 140. 60 A DESCRIPTION OF THE [Ch. V. grey colour, and interlaced fibrous texture, yet disposed in laminae, forming a slate. It is soft, yet tenacious, works easily under the chisel, and hardens in the fire : it is quarried and wrought for various economical purposes." * " Hornblende, in crystals disseminated through this mica- slate, is by no means uncommon ; and sometimes this mineral becomes so intimately mixed with the slate as to constitute a compact tenacious compound; as, for example, in Aghole mountain. Beds of common hornblende probably occur also occasionally in the mica-slate, as detached blocks of that rock are sometimes to be seen ; for instance, in the northern part of Carrigacrow, and in the lower part of Balreagh glen, towards Glencree river ."f " Brisselstown Hill consists of mica-slate, fine granular greenstone, greenstone-slate, and greenstone porphyry. Mica-slate appears on the eastern side of Brissels- town, beneath which is seen the granite base; but the southern side and summit are composed of the trap rocks just men- tioned, principally the two first, which form a great bed in the mica-slate. This slate in the western part is porphyritic, containing numerous crystals of felspar, and it passes into greenstone porphyry by an intimate mixture of hornblende. This gradation from mica to hornblende is frequently observ- able in these rocks." J " On the southern side of Comaderry mountain are masses of trap rock : the higher part consists mostly of common hornblende rock, which varies in colour from green to black, and approaches even to the nature of massive mica. About the middle of the declivity, it is inter- mixed with felspar, mostly compact, and greyish white, the two substances constituting a coarse-grained rock ; at times the felspar greatly predominates, containing hornblende and mica disseminated in its substance. Lower down the hill, the rock consists of a compact felspar base, with prismatic crystals of hornblende interlaced, and shooting through the felspar in every direction." * Geol. Trans., vol. v, p. 146. f Idem, p. 154. J Idem, p. 161. Idem, p. 147, Ch. V.] PRIMARY SCHISTOSE ROCKS. 61 This mica-slate is in most places traversed by layers or beds of quartz rock, of various forms and dimensions ; and, in addition to the minerals already mentioned, it incidentally contains andalusite, garnet, and sphene.* On the southern part of this primary range, as already stated, the granite is not bounded by mica-slate, but by clay- slate, which is described by the same author as being yellowish, greenish, or purplish in colour; smooth, glossy, fissile, and free from admixture : but in general it is so blended with the rocks with which it is associated, that its true nature can only be understood after considering all its various relations. These rocks are clay-slate, conglomerate, greywacke*, greywacke- slate, quartz rock, greenstone, and greenstone porphyry. " The clay-slate conglomerate consists of angular fragments of clay-slate, some nearly as large as the head, with smaller fragments of quartz, imbedded in and cemented by clay-slate." " The greywacke is composed of small rounded and angular grains of quartz, numerous minute scales of white mica, small fragments of clay-slate, and sometimes portions of felspar cemented by clay-slate. The grey wacke-slate is a similar rock, with a slaty structure ; and it is the predominating rock."f " The quartz rock is generally white or yellowish-white, but is sometimes more or less stained with yellow, red, and brown. In its structure, it varies from the perfectly compact splintery to the close-grained granular ; sometimes contain- ing small, well-defined, rounded grains of quartz, which are frequently of a different colour from that of the base ; but it contains very rarely rounded and angular grains of felspar, and a few scattered minute scales of mica. Such is the cha- racter of the pure quartz-rock : but it is not only interstrati- fied with clay-slate on the large, but on the small scale ; and it is in these alternations that reciprocal incorporation takes place, presenting rocks in some instances of a homogeneous character, and in others of a distinctly compounded structure. * Geol. Trans., vol. v. p. 154 f Idem, p. 165. et seq. 62 A DESCRIPTION OF THE [Ch. V. In the former modification, quartz-rock becomes tinged throughout its mass of a yellowish, greenish, or purplish colour, by an infusion of the matter of clay-slate, and passes occasionally even into hornstone and flinty slate. As the proportion of clay-slate increases, the compound becomes entitled to the name of quartzy clay-slate ; and at last it passes into pure clay-slate, both compact and fissile. In the beds, in which quartz predominates, scales of mica sometimes occur, and are disposed without order through their mass. In the slaty beds, however, they appear frequently arranged in the direction of the laminae, silvering over the planes of separ- ation. Quartz-rock, impregnated with the matter of clay- slate, frequently envelopes grains of quartz, scales of mica, sometimes, also, grains of felspar, and rarely minute portions of clay-slate; thus constituting a firm compact variety of greywacke\ When clay-slate matter predominates, it passes into greywacke -slate. All these varieties of quartz-rock are more or less traversed by small contemporaneous veins and strings of pure white quartz, which in their range frequently follow the line of the dip." * " The greenstone, as at Dunganstown Hill, is near the sum- mit more crystalline than in other parts, composed of distinct crystals of felspar and hornblende ; but in general hornblende appears only in spots, or as colouring the felspar mass, which is partly compact and partly foliated, and in which are dis- seminated minute crystals of felspar. The grain varies from fine granular to compact ; and the common character of the rock in the latter state is that of a greyish green base with an uneven fracture, which contains disseminated minute crystals of glassy felspar. In the north-eastern part of this hill the greenstone approaches more to the nature of an ash-grey com- pact felspar, partly vesicular, the vesicles being mostly empty, or slightly coated with iron ochre, derived from the decom- position of iron pyrites, which is present in these trap rocks." f * Geol. Trans., vol. v. p. 185. f Idem, p. 170. Ch. V.] PRIMARY SCHISTOSE ROCKS. 63 Mr. Weaver also gives an excellent account of the associa- tion of granite with the slates ; from which we find that entirely detached portions of the former occur in the latter rocks, after the same manner as the granite in the slate of Cornwall. Thus, "in following the northern ravine from Glenismaule upwards, we meet with granite alone for about three fourths of the ascent, when mica-slate appears in contact with and resting upon the granite ; the strata standing nearly on their edges, ranging N. E. and S. W., and dipping 75 to the N. W. In this spot the granite exhibits some disposition towards the structure of gneiss, but to a small extent only. A little higher up the mica-slate is better exposed, and con- tains thin layers of quartz, sometimes pure, sometimes mixed in part with felspar. Considerably higher up a bed of small- grained granite occurs in the mica-slate, about six inches wide, the range and dip being the same as the slate; and within a few inches of this bed of granite is another of the same width, but of short continuance, terminating abruptly at the S. W. and edging out on the N. E., being thus completely enclosed in the mica-slate, the laminae of which follow the line of the granitic bed, which is about fifteen feet in length. We next come to a third bed of granite, but of less dimen- sions, or rather to a vein, terminating also abruptly in the mica-slate; and upon its sides we perceive portions of the latter rock impressing and indenting the former in a very singular manner. There are also layers and veins of quartz, a few inches thick, either pure, or mixed with felspar, termi- nating likewise abruptly in the mica-slate." " Higher up this ravine is a short compressed elliptical mass of granite, enclosed between the laminae of the mica-slate; and beyond this is the horizontal section of what appears to be a granite vein nearly enveloping a portion of mica- slate. The parallelism of the laminae of the mica-slate does not appear to be affected by this disposition of the granite." * * Geol. Trans., vol. v. pp. 155, 156. 64 A DESCRIPTION OF THE [Ch. V. The granite and the mica-slate not only occur in each other in the state of irregular portions, but in numerous instances were found, by Mr. Weaver, to alternate in large beds, the position of which is so distinct, that their nature cannot admit of a doubt, as in the case of Polgooth mine, in Cornwall. This phenomenon is so important in the history of primary rocks that it is desirable to quote some examples in detail. " In Glenmalur, nearly one half of Lugduff, which borders the glen on its north side, is composed of these alternations, the study of which has been greatly facilitated both by nature and art ; the northern face of Glenmalur being furrowed by several ravines, and the lead mine of Ballinafinchogue being situated in this quarter. Ascending the ravine, the water of which turns the wheel of the upper smelting-house, we find the following succession of beds : 1. Granite, at bottom, of uncertain thickness, upon which rests 2. Mica-slate, including many layers of granite, which alternate with the mica- slate, forming a body - 24 feet thick. 3. Granite - - - 12 4. Mica-slate and granite, forming an indis- tinct intermixture - - - 36 N. B. The preceding beds include also fre- quent thin layers of quartz. 5. Granite, the lower part of which is most cu- riously interwoven with contempora- neous veins of granite and quartz - 90 6. Mica-slate - 462 7. Repeated alternations of granite and mica- slate, forming a body - - - 132 8. Mica-slate - - 372 9. Granite - 9 10. Mica-slate - 120 11. Granite - - - - 30 Ch, V.] PRIMARY SCHISTOSE ROCKS. 65 N. B. The general character of the upper beds of granite is that of granular quartz, with much silvery mica, and very little felspar. 1 2. Mica-slate is traceable in the ravine up to the cattle pond, through an ascent of about - 720 feet thick. " Some of this mica-slate contains a vast quantity of gren- atite, particularly the bed marked No. 8., generally in very small crystals, but sometimes in groups of large cruciform and stelliform crystals, well displayed in alto relievo on the rock by the wasted state of its surface." " The relations of the rocks at the lead mine are equally interesting to the geologist." " The whole of these alternations may be computed at one third of a mile in thickness, measuring at right angles with the dip." * In the southern portion of this part of Ireland, granite alternates in the same manner with clay-slate. " At Croghan Kinshela," says Mr. Weaver, " granite supports clay-slate ; and some thousand fathoms of trenches having been cut down to the solid rock in quest of auriferous veins, the mineral structure of this mountain has been fully disclosed. The western arm and brow of the mountain consist wholly of granite. The northern brow exhibits the following suc- cession of rocks proceeding eastward, across the line of their direction : 1. The fundamental granite. It is composed of yellowish or greyish-white felspar, grey vitreous quartz, and blackish-green mica : the felspar predominating, and the mica being in the smallest proportion. It is traversed by veins, consisting of white quartz with a greasy lustre ; and greenish-grey clay-slate, containing cavities lined with quartz crystals. * Geol. Trans., vol. v. p. 148. et seq. F 66 A DESCRIPTION OF THE [Ch. V. 2. Greenish grey clay-slate - - 48 feet. 3. Granite, and some clay- slate - - 66 4. Greenish-grey clay-slate - - - 894 5. Granite - - 96 6. Clay-slate and granite in repeated alternations: the former sometimes containing large grains of translucent quartz, and occasionally, also, a few crystals of glassy felspar - - 1 92 7. Granite .... 960 8. Greenish-grey clay-slate - - 90 9. Granite - - - 126 10. Clay-slate, greenstone, and greenstone-slate, in alternation - - - - 120 11. Ash-grey clay-slate, with some small beds of greenstone and greenstone- slate towards the latter part - - - 666 12. Ash-grey clay-slate - - 528 1 3. Greenstone ; constituting the body of the hill at its junction with the northern arm. " This greenstone, examined by the lens, is seen to consist of minute crystals of yellowish-grey felspar, intimately mixed with spots of black hornblende ; its aspect to the naked eye, being greenish or bluish-grey, with a compact, small-grained, and uneven fracture. The compact greenstone, near the clay-slate, acquires a fissile texture."* Again, " in the line of the Avonbeg, on the side of the mail- coach road, may be seen repeated alternations. The beds in the clay-slate, at first view, seem to be granular felspar ; but a closer examination shows that they mostly consist of a fine- grained granite; the three constituents of which are, in some places, distinctly discernible. The alternation is as follows : 1. Granite - 4 feet wide, 2. Clay-slate - - 36 * Geol. Trans , vol. v. p. 172. Ch. V.] PRIMARY SCHISTOSE ROCKS. 67 3. Granite, with a few thin layers of slate 18 feet wide. 4. Clay-slate - ,-18 5. Granite - - .,,,*>* 6 " About one quarter of a mile from the meeting of the two Avons, there is a fourth and a fifth bed of granite, similarly composed ; each about nine feet thick, separated by clay-slate of thirty feet in thickness." We cannot conclude this summary of Mr. Weaver's valuable observations on the primary rocks of the East of Ireland, without recommending to the student a careful perusal of the original. Dr. M'Culloch, in his excellent work on the Western Islands of Scotland, has furnished us with copious details concerning the primary schistose rocks. We will, therefore, avail ourselves of this vast fund of information, to select such descriptions as our limits will admit of; more particularly concerning the gneiss, and the various rocks with which it is associated. The mineral composition of the gneiss of these islands is the same as that of this rock on the mainland, viz. felspar, quartz, and mica ; but the latter mineral is more frequently replaced by hornblende. The varieties of gneiss, as in the case of other primary rocks, depend not only on the propor- tion in which its respective minerals are united, but also on the mechanical manner in which these are arranged. Founded on the latter circumstance, the true and well-characterised gneiss may be divided into two kinds, the granitic and schis- tose ; both of which, however, will be found frequently to pass into each other by almost imperceptible gradations : this arrangement, therefore, is only one of convenience. These kinds are of such frequent occurrence, that a single example of each must suffice. In South Uist*, the gneiss is almost uniformly of a granitic aspect ; and its external forms * Western Islands of Scotland, vol. i. p. 95. F 2 68 A DESCRIPTION OF THE [Ch. V. are also so much like those of granite, as at first sight to mislead an observer : but a careful examination will always detect, somewhere, an indication of a foliated structure. This is particularly the case about Kilbride; but the gneiss re- sumes the more common foliated character, as it approaches the shores of Loch Boisdale. In lona and Tirey, the gneiss generally contains hornblende, the mica being more rare: and although it is found even together with the hornblende, it is most generally observed in the vicinity of the granite veins by which the gneiss is traversed. In some places, the gneiss is perfectly foliated ; but in others, it approaches so near to granite, that its nature can only be discovered by a favourable fracture. There is no regular progress from the granitic to the schistose beds, but they seem to be every where mixed without order. Indeed, the gneiss generally, in Tirey, does not exhibit any regularity of disposition, or even such an approach to it, as to show the probable course of the beds : or if, for a small space, any line of direction be traced, at the very next step the appearance of regularity vanishes.* The beds of gneiss are generally disposed in the direction of N.E. and S.W., in conformity with the other primary stratified rocks in this part of Scotland, dipping towards the east at various angles. But it often happens, that the direc- tion of these beds is very irregular, and accompanied by every possible variation of dip. Thus, in Rona there is on the eastern side a great sequence of straight beds, dipping uniformly towards the east ; while on the western, they lie in a contrary direction ; the two meeting in the middle of the island, like the roof of a house. In other places, the same beds will be found reversed ; the eastern beds dipping to the west, and vice versa : or they undergo changes still more intricate, displaying wavings and contortions. These irregularities, however, are more frequently limited to the granitic gneiss.f Since mica is an ingredient in gneiss, it is not surprising * Western Islands of Scotland, vol. j. pp. 45, 46. f Idem, vol. i. p. 213. Ch. V.] PRIMARY SCHISTOSE ROCKS. 69 that it is sometimes predominant. In the island of Coll, it gives rise to distinct beds of mica-slate, alternating with the gneiss. This mineral is also, in the same place, so developed, as to form large concretions. The mica, in this state, is always black ; in some cases crystallized, and in others, dis- posed in large plates without any definite form: and it is often so intimately mixed with hornblende, that it is difficult to distinguish one ingredient from the other.* It has been already stated, that this gneiss is characterised by the presence of hornblende; which, with felspar and quartz, often constitutes the gneiss, to the entire exclusion of mica. The quartz, also, occasionally disappears; in which case, the rock puts on an appearance intermediate between gneiss and granite : again, by becoming of a finer grain, or by the exclusion of some of the felspar, it gradually passes into a simple hornblende-schist, or into that mixture of hornblende and felspar called primitive greenstone ; and in some cases, into an unfoliated hornblende-rock. It is also not uncommon to find in this hornblendic gneiss larger or smaller laminae, lumps, and even beds of hornblende-rock. These appear- ances are well displayed in Tireyt ; and in Lewis, where they also occur, the gneiss exhibits an intermixture of a dark and pale colour, in pretty equal and parallel laminae ; the darker parts abounding in hornblende, the lighter in felspar. :f But the most curious disposition of the hornblende is to be found in the gneiss of Coll, where nodules of hornblende- rock, of considerable size, are completely enveloped by con- centric layers of gneiss. The felspar in the gneiss of the Western Islands is gener- ally uniformly intermixed with the other ingredients ; but sometimes it is distinctly developed, imparting to the rock an imperfect porphyritic character. This felspar is splendent, or nearly glassy ; and the concretions, though highly crystal- line within, have no external configuration : they are seldom * Western Islands of Scotland, vol. i. p. 63. * Idem, p. 46. $ Idem, p, 193. Idem, p. 64. F 3 70 A DESCRIPTION OF THE [Ch. V. less than a quarter of an inch in diameter, and attain to that of three inches. These concretions are often thickly dis- seminated, and disposed in a laminar direction, corresponding with the general structure of the rock ; the laminae of which, however, are sometimes bent and accommodated to the shape of the felspar.* These imbedded concretions of felspar are not always crystalline, but are found gradually passing into the common foliated and compact varieties of this mineral. In the latter state, the felspar occurs often in the form of veins, laminae, and even beds parallel with the strata of the gneiss : it exhibits various tints of yellow and green, and is often combined with other minerals ; thus becoming, in some instances, porphyritic : of this nature is the porphyry vein traversing the gneiss, in the island of lona.f The gneiss of these islands frequently passes, by the most imperceptible gradations, into a fine fissile slate : and this is always the case when the gneiss contains compact felspar. The gneiss sometimes becomes talcose, and it is then associated with magnesian rocks. This occurrence is well displayed in the island of Scalpa ; where, in consequence of extensive excavations for the lighthouse, a bed of serpentine has been exposed, traversing the promontory, and disposed in an irregular manner, like the gneiss in which it lies. The gneiss, in the vicinity of the serpentine, is in some parts almost an entire mass of compact felspar, mottled with red and white, and its laminar disposition being sometimes marked by alternations of these colours : it is occasionally interlaminated with clay-slate, and more rarely with talc; forming a talcaceous gneiss, which here marks the boundary of the serpentine. In other places, the transition between these rocks is effected by a serpentine, so full of hornblende, as to be scarcely distinguishable from a true hornblende-rock. Some parts of this rock are schistose ; their colours being likewise disposed in a laminar manner, so as to present a dark greenish basis, striped, and in some directions speckled, * Western Islands of Scotland, vol. i. p. 193. f Idem, p. 21. Ch. V.] PRIMARY SCHISTOSE ROCKS. 71 with a paler tone of the same colour. These generally lie near the gneiss; the interior gradually assuming a more massive form, and displaying, at length, a fracture from which all appearance of a foliated tendency has vanished. Talc-slate is also occasionally found at the limit of the gneiss, mixed more or less with quartz of a very greasy aspect. In the interior of the latter rock, the bed of which, at its thickest part, may be about a hundred yards, a body of potstone is found ; the harder serpentine passing into it by gentle de- grees. This serpentine is of a dark green, and somewhat translucent on the edges, like wax : it contains veins of dark green, and of pure white steatite, sometimes fibrous, together with splendent veins of greenish asbestos.* The gneiss of the Western Islands, like that of the main- land of Scotland, contains limestone. In Tirey, this cal- careous rock occurs in several places. Not far from Balphe~ trish, it is of a reddish hue ; varying from a high flesh-colour to nearly white, and from a muddy crimson to a dull pur- ple. It is of a fine splintery fracture and smooth grain. It is an irregular mass or nodule, of about a hundred feet in diameter, and is surrounded on all sides with gneiss. It is not, therefore, a bed, as it is sometimes improperly so called, but a nodule ; a form common to the greater number of lime- stones found in gneiss and in mica-slate. It is not capable of being raised in parallel- sided masses ; yet, after exposure to weather, it splits into laminae with great facility. It con- tains concretions of hornblende, but more commonly of augite ; the green colour of which contrasts well with the red colour of the ground. The contact of this rock with the gneiss is well defined ; and of all the component parts of the latter, quartz is most generally in union with the limestone; and is, probably, intimately combined throughout its mass, as indicated by the great hardness of this marble. Lumps of granite or gneiss are occasionally imbedded in this lime- * Western Islands of Scotland, vol. i, p, 168. F 4- 72 A DESCRIPTION OF THE [Ch. V. stone.* At Gott Bay, in the same island, a bluish limestone alternates with the gneiss in a nearly vertical set of irregular beds, of no great extent ; and both of these rocks are traversed by granite-veins. In the island of lona, limestone also occurs ; but, in this case, it is not associated with a true gneiss, but with a slate, intermediate, according to M'Culloch, between gneiss and clay-slate. It appears to be a schistose variety of compact felspar, which, in a massive state, occurs in this slate ; and both of which are connected with and pass into gneiss ; and it may, therefore, be considered as an in- tegrant part of this series of primary slates. This limestone also exists as a large irregular nodule, of about thirty feet in breadth, and more than a hundred yards in length. The texture of this marble is compact, its fracture splintery, and its colour white, often with a slight greenish tinge. In many parts, it is fissile ; more particularly where it approaches the schist, in which it lies : here also it becomes magnesian, and gradually passes into a steatitical calcareous schist, which contains leaves of translucent steatite; also dark-green foliated steatite, accompanied by yellowish-green and dark green noble serpentine. This limestone not only gradually passes into the adjacent schist, but both are contorted, and entangled together in a confused manner.f The masses of primary limestone are, however, much more extensively de- veloped on the mainland ; an interesting example of which, at Glen Tilt, will soon be considered more in detail. In many parts of the gneiss of the Western Islands, the quartz so predominates, either in alternating laminse with the other constituents, or in the state of a granular mixture, or even to the exclusion of the other minerals altogether, that it produces regular beds of quartz-rock of various extent. But the most interesting feature of this gneiss is its frequent intersection by granite- veins, which are so abundant, that they are rarely absent for any considerable space. * Western Islands of Scotland, vol. i. p. 48. | Idem, pp. 17, 18. Ch. V.] PRIMARY SCHISTOSE ROCKS. 73 In composition, the granite of these veins cannot sometimes be distinguished from ordinary granite ; more generally, however, it possesses a distinct character, consisting chiefly in the larger size and more decided crystallization of its con- stituent parts ; and occasionally, the mica and hornblende being absent, it gives rise to that peculiar variety called graphic granite. These veins are infinitely various in size, and in the number and intricacy of their ramifications. At times they intersect each other, so as to produce even a reticulation of the general surface, their intersections being sometimes attended with shiftings of the veins, while at other times, they cross each other without any disturbance. In some kinds of gneiss, they are so abundant as nearly to ex- clude the basis altogether, so that the mass presents little else than a congeries of veins. An instance of this occurs in the Flannan Isles, but the most remarkable one is to be seen on the mainland, between Loch Laxford and Cape Wrath, where the gneiss and hornblende-schist, in angular pieces, are entangled among the veins, resembling a variegated marble ; and these fragment-like portions do not form a twentieth part of the whole mass, whilst one set of veins is intersected by a second and third, of different textures. The granite-veins are most abundant, and most intricately arranged, in the granitic gneiss ; whilst, in the regular schistose variety, they are gene- rally disposed in laminae, which are parallel with the beds, and alternate therewith, as is well exhibited in the island of Coll. This parallelism sometimes continues for a considerable dis- tance ; at others, it is only for a short space, and the veins, quitting their even course, become either thicker or thinner, and cut through the laminae. It is also worthy of remark, that lumps of granite, apparently independent of veins, are often imbedded in the gneiss : these differ much in size, and when they are large, they commonly give out branches or veins, which diverge in a very capricious manner, cross- ing and uniting with each other, so as to produce a most intricate reticulation. These granitic veins not only intersect 74 A DESCRIPTION OF THE [Ch. V. the gneiss, but also the limestone, and other beds contained therein ; and, in traversing the serpentine of Scalpa, already noticed, they consist of the usual mixture of felspar and quartz, with talc superadded, thus partaking of the composition of the magnesian rock through which they pass. Such are the leading features of the primary schists of the Western Isles of Scotland; and to these might be added a description of the chlorite-schist, clay-slate, and quartz-rock, which compose the more southern islands ; but, by so doing, the details would far surpass the limits of this work, in con- sequence of the complicated and undefined nature of these rocks, owing to the present imperfect state of geological nomenclature. Dr. M'Culloch has indeed given a copious and circumstantial account of these islands, and his work ought to be very carefully studied by all who are desirous of obtaining a knowledge of the primary rocks. In the northern part of Europe the crystalline rocks appear to abound over very extensive tracts, and although the climate and the impracticable nature of the country oppose great ob- stacles to their minute investigation, yet they have been carefully examined by several distinguished geologists. For the present, however, Von Buch's descriptions of the rocks of Norway and Lapland must suffice ; and as far as the limits of this work will admit, it is proposed to give an outline of his observations, which bear on the subject now under consideration. Von Buch observes that the Swedish and some other foreign mineralogists have described granite as of frequent occurrence ; but that he has ascertained that this rock is a great rarity in Sweden, as well as in the whole of the north. This is in accordance with the experience of Haussmann, who has affirmed that granite, such as occurs in Saxony, Silesia, the Hartz, and in Lower Austria, was never seen by him in Sweden. The rock thus mistaken for granite is, says Von Buch, a clear and decided gneiss ; it is distinctly slaty, and its mica lies in scales one above another, and never in Oh. V.] PRIMARY SCHISTOSE ROCKS. 75 single folia, as in granite : and it contains beds of hornblende, limestone, and ores.* This gneiss, like the primary rocks already described, is constantly changing its appearance, according to the propor- tion and the nature of its constituent parts, which are com- monly felspar, quartz, and mica. Around Kongsvold the gneiss is fine, slaty, with detached folia of mica, which lie parallel to each other. This is near the mica slate ; but, farther down, in the Straits of Drivstuen, rocks of a most beautiful gneiss are to be seen, containing large, and generally twin, crystals of white felspar, which shine forth in the midst of thick scaly mica. The felspar crystals are very much heaped together, sometimes the size of the hand : the inica is shining and easily separated : all the ingredients are distinct and determinate, f In the straits of the Figa Valley, the gneiss appears in great rocks by the road-side , the mica of which is black and scaly, and the felspar distinct. It contains beds of a coal- black colour, which seem, in fact, to have been taken for coal, for they have evidently been worked ; but they are entirely composed of pure, thick, scaly, and shining mica : this mineral, in isolated masses, is not unfrequent in the gneiss ; but it is a singular circumstance to find it in whole beds. This gneiss frequently contains hornblende, and even alter- nates with hornblende rocks. At Morstue, gneiss is the pre- dominating rock, containing here and there subordinate beds of hornblende rocks : but, through the Mordskov to the eastern bank of the Miosen, the latter gradually increase in quantity, till the gneiss, with white felspar and a great deal of mica, only occurs as beds ; and, at last, black hornblende and white felspar, in a fine granular mixture, entirely prevail. " This sort of alternation of beds," says Von Buch, " which gradually appear more and more frequently, is quite usual when one rock takes the place of another : thus, mica slate is * Travels through Norway and Lapland, p. 21. f Idem, p. 102. 76 A DESCRIPTION OF THE [Ch. V. very frequent in the north, as a bed, in gneiss, before it pre- ponderates over this rock ; and in the same manner gneiss forms extensive beds in mica-slate, before the former becomes predominant." * The gneiss in the hills of Cassness abounds in garnets : granular grey felspar and quartz lie between the mica, the scales of which are not continuous, but so close together that they form uninterrupted planes. Small granite-veins here frequently traverse the gneiss : the granite is composed of white coarse granular felspar, grey quartz, and a little mica. It is worthy of remark, that whenever "granite occurs, the felspar immediately increases, and the mica diminishes or disappears. This gneiss is in immediate contact with mica- slate, which contains white crystalline marble, f In the waterfall of Muonioniska the gneiss is in situ, and the stratification distinctly marked : it is not rich in mica, and the felspar is small, granular, and white. Following the course of the river, however, it becomes more micaceous, and contains granular limestone of a dark bluish grey colour. Near Kangis the white gneiss disappears, and is changed into red granite, the Rapakivi of the Finlanders : it is a coarse granular rock ; its felspar is red, and in great abundance ; its quartz bluish grey, in small quantity, and not always to be easily distinguished ; its mica is black, in small single folia, and is exceeded in quantity by black hornblende. J On the declivity of Pullingi, this granite gives place to white gneiss ; but near its summit, the red granite again appears. This alternation continues over a large tract of country to Torneo, as is indeed also the case from this place to Christiana ; but the granite is generally white, and the gneiss abounds in crys- talline limestone. " It is singular," remarks Von Buch, " to observe the frequent changes of gneiss into granite. For several miles the granite appears to be predominant ; then the gneiss as a bed occurs in the granite, and this happens more * Travels through Norway and Lapland, p. 76. f Idem, p. 2OO. J Idem, p. 360. Ch. V.] PRIMARY SCHISTOSE ROCKS. 77 frequently, until these rocks alternate in equal abundance and extent; but slaty undulating gneiss at last prevails every where." * Thus we learn that, in these northern regions, the granite and gneiss are constantly passing into each other ; so that it is often difficult to determine to which of these the rocks are to be referred ; for the slaty structure, which is the grand distinction, is not always to be detected. For example : " The mountain of Luroe is composed of gneiss, and on the whole of the ascent the shining felspar is not concealed between the laminae, as is often the case in these islands. The detached masses would lead one to suppose that the rock of this mountain is granite, but the mixture is too indefinite, and, even in these loose blocks, the slaty structure of the rock is betrayed by the flat position of the scales of mica." t "In the island of Grydoe, the gneiss is distinct and beautiful : the large cubical blocks, at the foot of the cliffs, render it evident that the slaty ingredients have not here their usual ascend- ancy ; the mica is rare, and in small parallel stripes ; the felspar is red and small granular, and the quartz in small quantity and not distinct." :f Such are the leading facts concerning the gneiss in the north of Europe. This rock is generally succeeded by mica- slate, with which it alternates, and into which it repeatedly passes, by a gradual change of its mineral characters, just as it does with granite. In the rock of the island of Stegen, the felspar is so con- cealed, that traces of it are only here and there visible be- tween the scales of mica; which, however, are not con- tinuous, as is usually the case in the mica-slate, but appear as large scales lying close above one another. It contains gar- nets in great abundance, often as large as hazel nuts ; and beds of granular quartz and hornblende, both with and with- out felspar. This rock constitutes the intermediate link * Travels through Norway and Lapland, p. 396. f Idem, p. 149. \ Idem, p. 165. 78 A DESCRIPTION OF THE [Ch. V. between gneiss and mica-slate ; passing, on the one hand, into the former by an increase of felspar ; and, on the other hand, into the latter by the disappearance of this mineral in a distinct form. On the height of Stordalen, the undulations of the mica- slate may fairly be compared with the waves of a stormy sea ; they are so large and long, and rise and fall in such a man- ner, that it becomes extremely difficult to discover, from these slates, the inclination of the strata. Besides, the surface of these curves is so wonderfully indented, that the slates appear to be fixed into each other like swallow-tails ; and this struc- ture prevails throughout the whole length of the mountain. * At Bergen, the mica-slate first follows the great gneiss formation ; then comes the gneiss of the islands, in which beds of mica-slate are very frequent ; and this is again suc- ceeded by mica- slate, of a nature which approximates to clay- slate, f Around Forvig the mica- slate does not contain felspar, but envelops beds of white limestone. This continues to the foot of the mountain, Bevelstadsfieldt, where felspar makes its appearance in large shining crystals, which nearly displace all other ingredients ; and the slaty texture disappears. By far the greater number of these crystals lie parallel to each other, dipping towards the east at 60, exactly similar to the adjacent mica-slate. The mica between the felspar is black, and never continuous ; and the quartz is not abundant. This granite is of great extent, not in a single bed, like the gneiss with the large crystals of felspar at Kongvold and Drivstuen ; for, from the foot to the top of the mountain, a height of more than 1600 feet, its continuation is scarcely interrupted. In this granite, large crystals of tourmaline are not rare, surrounded by mica : there is no hornblende, but a few black beds, especially near the summit, consist of fine mica, interlami- nated with a little felspar, and a few thick beds of quartz. :f * Travels through Norway and Lapland, p. 117. f Idem, p. 132. t Idem P- 137 - Ch. V.] PRIMARY SCHISTOSE ROCKS. 79 Porsangerna's is a very conspicuous object, and, when seen from Kielvig, appears to be covered with snow. It is, how- ever, only white quartz in thick beds between layers of mica- slate. This quartz is so thinly stratified, that it separates into very large plates, several feet in length, and only a few inches in thickness, like marble tables. The mica-slate abounds in garnets, and in nodules which are harder than the basis ; and are composed of a fine granular mixture of white talc, red garnet, and white felspar. These nodules not unfrequently impart a very singular appearance to the whole rock.* The mica-slate of Leuvig contains neither felspar nor gar- nets, but several beds of granular dolomite; and above these beds, and even in the dolomite itself, are great layers of pure tremolite with green mica, like talc.f Von Buch gives several excellent details of the intimate connection and transition of mica-slate into magnesian rocks, which appears to be effected by the change of the mica into talc, and the pure limestones into dolomite. These excellent descriptions, however, are too long to be quoted in this work, and will scarcely bear abridgment. The mica-slate also very frequently passes into clay- slate ; that is, into a rock which is very fissile, and of a fine uniform texture : and this change is effected so gradually, that it is difficult to determine where the one begins and the other ends. We shall quote one example from Von Buch, which is also interesting in other points of view. " To what formation does the rock belong which predo- minates between the Gundal and Drontheim ? Are we to consider it as mica-slate or as clay-slate ? On the Steinberge, towards Drontheim, downwards, it appears at first sight com- pletely to resemble clay- slate : the rocks are very fissile, and the laminae are not shining, but have dark-coloured scales of mica scattered over their surface ; and these betray the true nature of the rock ; for such micaceous folia are not frequent * Travels through Norway and Lapland, p, 287. t Idem, p. 205. 80 A DESCRIPTION OF THE [Ch. V. in primitive clay-slate. Small crystals of hornblende not un- frequently also appear, but no quartz, except in isolated and rare beds. Nearer the valley of the Nid, however, the mica appears continuous and shining, in the manner of mica-slate, but always fine and straight, slaty, and without quartz, which we should rather expect to be the case in clay-slate. At Kiistad, about two miles from Drontheim, the rocks occur in other forms. The mica can no longer be mistaken : its folia surround a kernel, and form large balls of two and three feet in diameter. The kernel is extremely compact and hard, fine splintery or fine granular in the fracture, and of a bluish grey colour. It is probably a mixture of much compact fel- spar, a little quartz, and fine mica folia. The surrounding mica is also bluish-grey, glistening, and continuous, and every where covered with a multitude of beautiful pinchbeck- brown scales of mica. These balls lie close together, and whole rocks consist of them. We are frequently tempted to consider them as a conglomeration of large blocks ; but the nature of the mica, and the position of this rock between other strata of mica-slate, distinctly prove that it cannot be separated from the mica-slate formation ; this indistinctness of character and constant change in its ingredients show, however, that the mica-slate approaches very nearly the tran- sition into clay-slate.* The clay-slate, also, like mica-slate, often passes into mag- nesian rocks : first, it becomes very fissile, soft, and unctuous to the touch ; and then it is found to contain beds of a talcy nature, resembling potstone, which is composed of fine granular talc mixed with quartz, and its properties vary according to the proportion of these minerals : sometimes it is very fine grained and semi- hard, with splintery fracture, and when talc prevails it is soft and sectile.f The composition and the geological relations of the rocks of the island of Kielvig are very remarkable, and deserving * Travels through Norway and Lapland, p. 106. f Idem, p. 91. Ch. V.] PRIMARY SCHISTOSE ROCKS. 81 of accurate consideration, as they not only determine the con- stitution of the most northern part of Europe, but furnish us with information which can hardly be found either so full or so distinct in the interior of mountains. The cliffs, and small islets on the shore, are not composed of gneiss ; for the whole exterior of these rocks, their fine slaty structure, and the earthiness of their cross fracture, pro- nounce with distinctness that we are entering on clay-slate ; but the shining surface of this rock resembles mica-slate : the scales of mica, however, although abundant, are only scattered on the continuous base of this slate. These strata contain beds of massive brown quartz, in which delicate fissures are frequently coated with chlorite, with occasional imbedded crystals of felspar. Large folia of talc, and small greenish- grey splintery cones, resembling serpentine, are not of uncom- mon occurrence. Ascending Kielvigs-Eid we find small-grained granite, con- taining insulated folia of black mica and a great deal of horn- blende : the line of separation of these rocks may be followed for a considerable length, and it may be clearly seen that the clay-slate is continued beneath the granite. This granite frequently changes itself into straight slaty gneiss, in which large and beautiful garnets often occur. In some places, however, this granite contains diallage and but little quartz ; and by the former mineral gradually increasing, whilst the quartz and mica continue to decrease till they disappear, the granite becomes a fine granular greenstone without any visible separation. This change in the nature of the rock is be- trayed by weathering ; for the clove-brown diallage becomes tile-red and frequently of a garnet colour. Towards the mountains, this greenstone passes at last into a coarse granular rock, in which the ingredients of felspar and diallage are beautifully distinct, like the gabbro of the Italians. The felspar of this variety more easily disintegrates than the dial- lage, although the latter loses its colour sooner. The loose blocks are therefore rough, the diallage projecting when the 82 A DESCRIPTION OF THE [Oh. V felspar has been washed away. The stratification is not visible in the coarse granular rock, but in the pure granular greenstone it is very distinct, stretching N. E. and S. W., and dipping 60 towards the N. W. Not far from Kielvig, a small bay lies between perpen- dicular rocks, called Little-Kielvig, where the clay-slate passes into mica-slate ; for there is no longer any basis to the slate, the whole being a collection of an infinite multitude of shining folia, lying upon one another ; not such folia as appear on greywacke-slate, but fresh and scaly, as they usually are in gneiss. In this slate there are several beds of potstone, which is greenish- white, coarse, and frequently splintery, translucent, and very similar to jade, if it were only harder ; but it is only semi-hard, and contains small folia of white talc. The clay-slate only makes its appearance in the vicinity of the sound of Mageroe, and not at the North Cape, where the diallage rock is also wanting. The island of Stappen consists of gneiss ; and the nearest rocks of the steep North Cape are of the same nature. This gneiss is more striped than slaty ; its mica is black, in very fine folia, which lie single and insu- lated; and the felspar is abundant, of a pale flesh-red and white, and almost transparent : the quartz is grey and in dis- tinct grains. This rock is certainly not situated above the diallage rock. " These facts show that the diallage rock here belongs to the remotest members of the primitive formation, and nearly touches on the transition : and this rock is found in the same position in Silesia, Prato, Genoa, and Cuba." * In this brief sketch of Von Buch's excellent observations on the primary rocks of Norway, sufficient has been extracted to show that the same mineral transitions and the same modes of association occur in this country as characterise the equiva- lent rocks of Cornwall. We would willingly enter into the minute of this comparison, but they would occupy too much * Travels through Norway and Lapland, p. 279, el seq. Ch. V.] PRIMARY SCHISTOSE ROCKS. 83 time ; and cannot, after what has been said in the former chapters, escape notice on a careful perusal. In Sweden, also, the granitic masses are said to bear the same relations to the primary slates as in Norway. Most German geologists consider that the granite of Saxony is also similarly circumstanced ; and Bonnard and others, contrary to the published opinions of Haussmann, likewise regard the granite of the Hartz, not as the most ancient and fundamental rock of the district, but of a more modern origin, since it is, in many places, regularly interstratified with mica-slate and other crystalline schists, all of which are surrounded by grey- wacke, and appear to repose on the strata of this secondary rock. In Saxony, however, the appearances of this kind are of a more doubtful nature. As already observed in a preceding chapter, the granite of the Erzgebirge occurs in insulated masses surrounded by primary slates, as in Cornwall ; and the latter rocks will be found, in the following details, to present other points of similarity. Gneiss and mica-slate are the prevailing crystalline schists in this mountainous chain ; the former being the most abun- dant at the eastern, the latter at the western extremity. The gneiss varies exceedingly in the proportion of its con- stituent parts, and in the manner in which these are united. At Freyberg it is very micaceous ana schistose, being well adapted for economical purposes ,* whilst at Himmelsfurst, on the contrary, it is of a more granular texture : so that Werner's division of this rock into two kinds may be generally received. The granular variety is often coarsely crystalline, and exhi- bits frequent passages into granite : and near the town of Schwarzenberg, it contains large pieces of whitish and reddish felspar, having a tendency to a prismatic form, which imparts to the gneiss a porphyritic aspect. In the schistose variety, the component minerals are arranged in regular alternating bands. This rock contains several subordinate beds, differing from G 2 84- A DESCRIPTION OF THE [Ch. V. it in composition : thus, quartz-rock at Himmelsfiirst ; a com- pound of felspar and tourmaline at Marienbergh; porphyry at Halsbrlick ; hornblende-rock at Beschertgluck ; and lime- stone in numerous places. It also envelops granite in the form of irregular masses and veins, some of which are of con- siderable size ; and one mass, in particular, at Geyer, is well known in consequence of its having been extensively explored for tin ore, in which it abounds. The gneiss of Geyers- berg, which encloses this granite, is of the schistose variety, and composed of the common ingredients ; the felspar, how- ever, being in a less proportion than usual : its constituents are not indistinctly mixed together, but are individually ar- ranged in small and undulating veins. The granite is fine- grained, and formed of grey quartz, brownish mica, and red- dish felspar, which is always the most abundant ingredient ; it contains, here and there, a little indurated lithomarge, and shorl ; is a very compact sonorous rock, capable of receiving a polish; occurs in beds of from two to ten feet in thickness, dipping 5 to 10 towards the south-east; and is traversed by numerous fissures, which intersect each other at various angles. Between this granite and gneiss there is always interposed a layer of some inches or feet in thickness, which is granitic, but very different from that just described. It contains fel- spar and quartz in pieces of two to sixteen inches in length : the reddish felspar is still predominant in the mass ; the mica is black in less proportion, sometimes in groups of two to six inches in size ; and the quartz is often largely developed, crys- talline, and compact, and in some parts in distinct pyramids, forming layers which alternate with others of indurated litho- marge mixed with mica. Lastly, this variety of the granitic rock appears always to adhere both to the granite and the gneiss on either side : indeed, these granitic and schistose rocks are so intimately connected that they seem to belong to the same formation. The gneiss of the Erzgebirge generally passes into mica- slate ; so that it is often very difficult to distinguish the latter Ch. V.] PRIMARY SCHISTOSE ROCKS. 85 from the former, at the point of transition. The mica-slate varies much in colour, and in composition ; and it is only in a few places, that it exhibits its distinguishing character of interlaminated quartz and mica : when this happens, it abounds in disseminated garnets. This slate contains some subordinate beds of quartz-rock, of hornblende-rock, and of compounds in which garnets and actynolite are very abundant. It is generally succeeded by argillaceous schist, into which it may be seen to pass at Schneeberg. The clay-slate also sometimes reposes immediately on the gneiss, as at Joachimsthal ; where, likewise, it may be seen passing into hornblende-schist. At Johann-georgenstadt, this rock even comes in contact with the granite ; but in this position, it is always of an intermediate nature, between mica- slate and clay-slate, and contains layers of quartz-rock. But the most rare and interesting rock, in the primary schists of the Erzgebirge, occurs at Auersberg, and was first described by Freiesleben. It is composed of quartz and shorl, as mica-slate is of quartz and mica ; and it has, therefore, been called shorl-schist : it also occasionally contains mica, chlorite, garnet, and other minerals ; but these are not essen- tial. Its structure is striped or ribboned, formed by alter- nate layers of quartz, and fibrous or granular shorl : the stripes are seldom straight, but for the most part variously curved and undulating. The shorl-schist reposes on the granite : and, although this rock has been only seen in situ at Auersberg, there is every reason to believe that it exists elsewhere ; since pebbles of it are often found in the alluvial deposits which contain tin ore : and its presence may be regarded as a favourable indication of this metal, as it has hitherto always occurred associated therewith.* Before concluding the description of the primary schistose rocks, it will be desirable to consider a little more in detail the series in which talc is the characteristic mineral over * Annales des Mines, tomes 8 et 9. G 3 86 A DESCRIPTION OF THE [Ch. V. extensive districts, and not in insulated and subordinate beds, as those already noticed. This series is found well developed in the Alps and in Corsica; and to these we shall for the present confine our remarks. The rock of Mont Blanc was for a long time considered as a talcose variety of granite. M. Jurine, regarding it as dis- tinct from true granite, gave it the name of protogine ; which term has been adopted by Brongniart, and other French geologists. M. Brochant*, after an extensive examination of this part of the Alps, has also concluded that the granitic rock of Mont Blanc is not true granite; but only an extreme variety (more crystalline, and containing a greater quantity of felspar,) of a talcose rock, with which it is asso- ciated ; and which is very abundant in the adjacent moun- tains. This talcose formation extends from Mont Rosa to St. Bernard, and to Mont Blanc, and even to Mont Cenis, and, very probably, far beyond these limits ; and its most crystalline or granitic variety composes all the highest peaks along this tract. We will not, in this place, stop to enquire whether this rock is the equivalent of granite, and, therefore, referrible to the protogine of the granitic group ; or whether it is a member of the schistose group, and best distinguished by the name of felspathic talc-rock, conferred on it by Bro- chant : but, availing ourselves of the observations of this dis- tinguished geologist, we will endeavour to describe this interesting formation. The principal and prevailing rock of this series is a talc- schist. Mica-slate is also found in this part of the Alps, and in similar associations, exhibiting frequent passages from mica into talc, from mica- slate into talc-schist, and even into chlorite-schist. The talcose rocks, however, are much more abundant than the micaceous rocks; and thus it is, that micaceous limestones are more rarely met with than the talcose limestones : and this predominance of talc is more * Annales des Mines, tome 4. p. 283. Ch. V.] PRIMARY SCHISTOSE ROCKS. 87 remarkable on the Italian side of the Alps. Almost all these varieties of micaceous and talcose slates exhibit different tints of green, sometimes resembling that of chlorite : they very seldom contain distinct scales of mica or talc ; but their sur- face is always more or less glossy, and often presents a striated and somewhat fibrous appearance. This talc-schist forms the basis of the formation; and, by its combination with other minerals, gives rise to distinct species of rocks. Crystals of felspar are sometimes uniformly disseminated throughout it ; when these are of a large size, the rock resembles a variety of gneiss : generally, however, they are very small, and often scarcely discernible, except on the cross fracture of the rock ; and even then this mineral would not be recognised, but by tracing its gradual increase in size till it forms distinct crystals. Quartz is rarely present ; and when it is visible, it occurs as small grains scattered about in very irregular groups. Hornblende is no where to be met with in this series; that is, in a distinct crystalline form: but it is undoubtedly sometimes mixed with these rocks, for hornblende rocks are found associated in the same mass with felspathic talc-schist, passing into each other by insensible gradations. The transitions of all these rocks into each other lead one to conjecture, that their different degrees of hardness and tenacity must be attributed to the presence of one or other of these accessary minerals. When they are tough, and at the same time hard, hornblende may be suspected to exist in the compound ; when they are very hard, and yet easily fran- gible, quartz is probably predominant; and, lastly, when they are sufficiently tenacious to be indented by the hammer without breaking, but are not so soft as to be cut by the knife, the presence of felspar is indicated. Such are the principal talcose rocks of this district : and in those places where they are best characterised, and most abundant, will be found serpentines, either pure or blended with limestone, fine-grained hornblende rocks, crystalline G 4 88 A DESCRIPTION OF THE [Ch. V. limestones, chlorite-schist mixed frequently with magnetic ironstone, and, lastly, talc- schist; in the midst of which, the felspathic talc-rocks occur, either in the form of subordinate beds, or connected with them by gradually passing into each other. This felspathic talc-rock is the granitic rock of Mont Blanc. Its principal constituent parts are felspar and talc : the talc is generally of a greenish colour, and often traverses the rock in the form of small veins. This rock has always a tendency to the slaty texture, and even becomes sometimes decidedly foliated ; and it is worthy of remark, that the speci- mens taken from the summit of Mont Blanc present the same character. The only difference between these massive and slaty rocks is, that in the granitoid rock, the felspar most abounds ; and in the schist, the talc : just as the analogous case of greenstone and syenite. Quartz is, indeed, also found in this granitic rock, but it is of rare occurrence; and it exists rather as scattered nodules, irregularly grouped, than as crystalline grains uniformly disseminated throughout the mass, after the manner of true granite. These mineralogical characters, says M. Brochant, con- joined with the geological relations of this rock (viz. its association with talc-schist, serpentine, and greenstone, and, above ah 1 , its containing and gradually passing into felspathic talc-schist,) clearly indicate that this rock is not a granite, but only a crystalline variety of the felspathic talc-schist with which it is interstratified. And it may be farther remarked, that the metallic minerals generally occur in true granite, in gneiss, and in mica-slate, in the form of veins ; whilst, in the talcose rocks of the Alps, they are seldom met with : and when found, are always in the state of bunches or beds. Although true granite is not to be met with in the highest parts of the Alps, this rock is not entirely wanting in this mountain range : for the lower parts on the side of Piedmont, from Yvree, and indeed from Turin, as far as Lago Maggiore, are composed of granite. This rock is well characterised, Ch. V.] PRIMARY SCHISTOSE ROCKS. 89 and, according to M. Brochant, never contains talc : he states, however, that in some places its felspar is decomposed, producing kaolin, or china-clay; and it is, therefore, very possible, that these masses of granite are traversed by beds or layers of protogine, as in Cornwall. This circumstance was, probably, overlooked by this geologist ; for he does not appear to have been aware, that granitic masses, as we have already seen, are made up of various distinct rocks : as he states, that the granite of other countries is almost without any intermixture, and does not contain any subordinate rock. M. Brochant was not able to discover the junction of the true granite with the talcose rocks of the Alps ; but in Cor- sica, which appears to be a continuation of this talcose form- ation, the nature of this connection is better developed. M. Gueymard's * account of this island is accompanied by a section, drawn from Ajaccio, through Corte, to Ponte di Golo. The lowest part of the range next Ajaccio is an alternation of granite and eurite : proceeding towards the interior, granite next predominates ; but the highest part of the range is composed of eurite, protogine, and hornblende rocks, alternating with gneiss and mica-slate, which gradually give place to protogine, associated with talc-schist and lime- stone ; and these are lastly succeeded by serpentine, talc-schist, and similar rocks in which magnesian minerals prevail. The details furnished on this subject are not so copious or particular, as the interesting nature of this section seemed to demand. It may, however, be gathered from them, that the beds of granite and eurite are contemporaneous. The granite is also intersected by hornblende-rock and green- stone, in the form both of beds and of veins. These rocks are sometimes disposed in certain directions ; but, at other times, they traverse the rock without any regular order. The substance of these veins is often so intimately connected with the rock which encloses them, that they both appear to be of the same age. The granite of the ridge is associated * Annales des Mines, tome 9. p. 123. 90 A DESCRIPTION OF PRIMARY SCHISTOSE ROCKS. [Ch. V. with gneiss, talc-schist, and quartz-rock. It appears again farther towards the west, under similar circumstances ; it is generally talcose, and passes insensibly from the protogine of Corte into well-characterised protogine. M. Gueymard draws a line to separate the primary rocks from the transition ; but at the same time states, that such a distinction is not here to be found : for the line laid down rather indicates the limits of certain prevailing rocks, than a separation of geological epochs. The principal rock which succeeds the foregoing, is a stea-schist : it encloses only a few subordinate beds near the coast, but these become more abundant in proportion as the central chain is approached. They consist of blue limestone, more or less veined and crystalline ; of ollareous serpentine ; of euphotide ; of dark- coloured limestone, like that of the Alps; of compounds of quartz and felspar; and, lastly, of porphyry. The quartz- rock and limestone, near Ostriconi, appear to be enclosed in the granitic series ; but the author is not quite certain on this point. The granite, however, near Vivario, does contain quartz-rock, and, near the village of the same name, is so intimately connected with gneiss and schist, that they must be all referred to the same epoch. Such is a brief sketch of the older rocks of Corsica : some of which are extensively worked, and have been long cele- brated as furnishing the most beautiful stones for architec- tural and ornamental purposes. And with this sketch, we terminate our descriptions of the foreign primary schists ; to which we shall have repeated occasion to refer hereafter, in our attempts to establish that they are analogous to the Cornish slates. Ch. VI.] STRUCTURE OF THE PRIMARY ROCKS. 91 CHAPTER VI. ON THE STRUCTURE OF THE PRIMARY ROCKS. Two kinds of structure, the internal and concretionary. Both of these also admit of a binary subdivision. The concretionary being either simple or compound, and the internal structure either massive or fissile. Structure of granitic rocks, developed by the action of the elements, can also be ascertained by cleavage. Macculloch's remarks on the spheroidal structure of granite this structure not confined to blocks, but common to the whole mass. The general structure dependent on the aggregation of individual concretions. The layers of granite possessed of determinate bearings like strata. Structure of the primary schists. The nature and disposition of strata. The curvatures and convolutions in the beds of the primary rocks. Their angular and spheroidal structure, resembling breccia and conglomerates. THE consideration of the concretionary forms of the primary rocks, and of the manner in which these are aggregated toge- ther, so as to constitute considerable masses, may appear to some to be here out of place ; but it will be seen hereafter, that this subject will furnish some important arguments, in the discussion of the nature of these rocks : at the risk, there- fore, of being deemed unnecessarily prolix, we proceed to offer a statement of facts, in many cases, indeed, of a very elementary nature, but requisite to complete the proposed practical sketch of primary geology. The structure of these rocks may be regarded in two points of view : in the first, as internal ; and in the second, as con- cretionary. To these might be added a third, the minera- logical ; but it is, perhaps, preferable to place this under the denomination of the texture of rocks, comprising the com- pound, the homogeneous, the granular, the crystalline, the compact, the fibrous, the scaly, the porphyritic, amygdaloidal, and other textures, on which it will not be necessary to make any farther remarks. The concretionary structure is of two kinds : first, that 92 THE STRUCTURE OF THE [Ch. VI. which is exhibited in the integrant masses, of which these rocks are always composed, and which possess forms more or less determinate, such as cubes, rhombs, and spheroids ; and which may, therefore, be termed the simple concretionary structure : secondly, the compound, or that which results from the aggregation of simple concretions, giving rise to layers, beds, and other forms of rock-masses, which often par- take of the same figure as the parts of which they are com- posed. The internal structure is that which is brought to light when we attempt, by violence, to subject the simple concre- tions to a farther subdivision : it is also of two kinds, the massive and fissile ; and generally displays the texture of the rock when this has been obscured by the decomposition, which, almost universally, has taken place on the sides of the con- cretions near the surface of the earth. The massive and fissile structures mutually pass into each other ; and certain stages of this transition have been distinguished by geologists, under the terms tabular, lamellar or laminar, foliated, and schistose. This transition is not a mere abstract idea, for it actually occurs in the same rock, as in the case of greenstone and other hornblende rocks ; and many instances have been already mentioned of the passage of lamellar and foliated into compact gneiss, not to be distinguished mineral ogically from granite. With these preliminary remarks we shall now proceed to an examination of the structure of the granitic and schistose primary rocks. The internal structure of the granitic rocks is generally massive, but not always so. The fine-grained granite of Castle-an-dinas, and of other parts of Cornwall, may be easily broken into thick laminae, as may also several kinds of pro- togine, of eurite, and of felsparite ; and more particularly the latter, which sometimes shews a tendency to a schistose struc- ture. A remarkable occurrence of this kind, in the Isle of Arran, is recorded by Dr. Macculloch. * In many places, * Western Islands of Scotland, vol. ii. p. 346. Ch. VI.] PRIMARY ROCKS. 93 the granite of Goatfell presents solid and continuous faces of rock ; while in others, it offers an irregularly laminated struc- ture, very much resembling that which occurs in certain trap rocks, including the syenites and porphyries. The fine- grained granite, on the western side of this mountain, in the Glen of Catcol, is very often even schistose. The laminae, into which it exfoliates on decomposition, vary in thickness, but may be found so thin, as not to exceed the tenth of an inch. This granite is principally composed of an intimate and minute mixture of felspar and quartz, with small crystals of hornblende, and occasional scales of mica, sparingly dis- persed throughout. The schistose structure, therefore, de- pends in no wise on the mica ; and presents no analogy to that of gneiss. And it must not be omitted to notice, that this laborious and observant geologist has remarked, " that the blocks which seem likely to undergo this change, and even those in which it has actually commenced, show no symptoms of future and similar exfoliations ; and that all mechanical attempts to produce a new, or prolong an old, fissure, are unavailing ; the rock breaking before the chisel or wedge in the ordinary and irregular jnanner." This circumstance is quoted, because it is at variance with the author's experience : it will, however, be referred to shortly, when the cleavage of granite comes under consideration. The developement of the structure of rocks, by a partial and incipient decomposition, appears to be dependent on the same principle as that by which the crystalline texture of a saline or metallic mass is disclosed by the action of a weak chemical solvent : but the laws, by which this action is go- verned, have not been sufficiently investigated. We find, wherever a large section of granite is exposed to the atmo- sphere, that its surface becomes divided by two sets of fissures, which preserve an exact parallelism among themselves, and which, crossing each other at right angles, separate the mass into blocks of a quadrangular form. Whilst this process is proceeding, the large square surfaces of the granite will be 94 THE STRUCTURE OF THE [Ch. VI. found to be again subdivided into lesser squares, as frequently occurs in the cliffs of the Land's End district : or, if the quadrangular blocks be freely exposed to the action of the elements, they are gradually converted into spheroids of various configuration, by the destruction of the solid angles, as well exemplified in the tors and logan-stones which crown the most elevated hills in granitic districts. " This pecu- liarity of structure oftentimes produces vast piles of blocks, so regular in their form and arrangement, that they convey a striking resemblance to a gigantic edifice in a state of ruin ; and lead us to do homage to the works of Nature, by con- trasting them with the petty operations of human skill." When the structure of the granitic rocks is more par- ticularly examined, it will be found that the concretions, into which they are divided by the intersecting systems of fissures or joints, present various kinds of forms : thus, according as the horizontal joints approach nearer or recede farther from each other than in the cube, tabular or prismatic masses are produced ; and as the vertical joints decline more or less from the perpendicular, different modifications of the rhomboid are formed. The figure of the concretions appears in some measure to be connected with the composition of the rock. For example, the common granite (as defined in the pre- ceding pages) exhibits the most perfect cuboidal concretions ; the fine-grained granite and protogine disintegrates into tabular masses; shorlaceous granite and shorl-rock often assume the prismatic form ; and the rhomboidal structure is characteristic of all the kinds of felsparite, which, however, is not the simple form of this rock, for it separates diagonally into various angular concretions, which renders it a bad building material. These facts are too few to justify a gene- ralization, and are balanced by many exceptions ; but which, it must be confessed, have not been subjected to a minute and circumstantial scrutiny : they are, however, sufficient in number to invite more attention than has been hitherto bestowed on this subject. Ch. VI.] PRIMARY ROCKS. 95 All these concretionary forms of the granitic rocks, deve- loped by a partial decomposition, suffer a more rapid dis- integration at their solid angles, by which the latter are gradually rounded off, and the blocks are converted into spheroidal masses, varying, of course, according to the figure of the original concretions ; these are, for the most part, more or less oblate or flattened, though they may occasionally be seen as spheres, so regular, indeed, that they appear to have owed their form to the chisel, and remind one of the granitic shot fired by the Turks at our ships in the Dardanelles. This universal reduction of the granitic concretions to the spheroidal form, by the action of the elements, a property which is also common to greenstone, basalt, and other horn- blende and crystalline rocks, is a fact which calls for farther investigation. Dr. Macculloch is of opinion, that this spheroidal structure is original, and that the cuboidal masses of these rocks are subordinate thereto, being the result of the interference of the contiguous spheres with each other. He conceives " that in a homogeneous mass of fluid matter, crystallization had commenced from numerous centres at the same time; and that, while there was yet space for the formation of successive solid deposits round any set of these imaginary centres, a spherical or spheroidal figure would be the result. As the surfaces of these spheroids approached each other, the suc- cessive crusts would interfere, and the remaining intervals would be filled by portions of spheroidal crusts, until the cuboidal figures of all the contiguous masses were completed; thus forming that aggregated mass of cuboids which we witness in the granites of this aspect which remain uninjured in their places. We need not be surprised that this regu- larity is not more constant, nor the forms more perfect, as we are unacquainted with the numerous circumstances which may determine the several centres of crystallization, or which may interfere with the ultimate regularity of the resulting * Geol. Trans., vol. ii. p. 76. 96 THE STRUCTURE OF THE [Ch. VI. This view of the subject is strengthened by the fact, that it is not an uncommon arrangement in rocks to find not only spheroidal masses, but the constituent parts so disposed around common centres, as to form concentric layers. This circumstance has been noticed in the gneiss of the Western Isles of Scotland, and in the mica-slate of Norway ; but the most remarkable instance occurs in the orbicular granite of Corsica. The spheroidal structure is not confined to the component concretions of the granitic rocks, but is characteristic of the entire masses. Wherever the Cornish granite is exposed by quarries on the side of hills, near the junction with slate (as at Kitt Hill, and in the parishes of Mabe and Constantine), the surface of this rock has not only a rounded form, but it is also traversed superficially, by curved fissures or joints. Indeed, the external form of the country gives indications of this structure; for the broad rounded ridges constitute a peculiar feature of the granitic districts of Cornwall and some other countries, whilst the sharp spiry summits of the Alps and^ of Arran distinctly mark a different structure, which is confirmed by their being composed of tabular masses, resem- bling strata placed on their edges. This tabular structure is not only more prone to changes, on account of the com- paratively greater exposure of surface, but also because it offers greater facilities for the action of the elements. An excellent example of this nature is presented by the granitic elvans of Hanover Cove, which traverse the slate near the granite of Cligga Point, in Cornwall: the softer kinds of these rocks, and the intermediate slate, have decayed, and been removed by the action of the waves ; while the harder kinds of elvans project in thick tabular ridges, about two hundred feet in height, the shattered and serrated forms of which have a terrific aspect when viewed from the sea. But to return to the spheroidal structure : there is one fact which is opposed to its being the original or fundamental form of granite, and that is, that the granite is not only traversed by Ch. VI.] PRIMARY ROCKS. 97 systems of parallel joints, as already stated, but is also capable of being cleft in directions corresponding therewith ; so that, whether the spheroid be on the large or small scale, it can be subdivided into cuboidal forms. There can be no doubt, however, that these lesser cubes would, on exposure to the air, be also reduced to spheroids; so that it is a difficult matter to decide whether the latter is the original form, or whether it is accidental, resulting from the solid angles exposing a greater surface to the elements ; or, whether the cohesion being weaker in those parts, may oppose less re- sistance to a chemical change. If the decomposition con- tinue after the spheroid is formed, it then takes place uniformly over the whole surface, which consequently ex- foliates in concentric layers. Dr. Macculloch has made some interesting observations on this subject, suggested to him by observing in the British Museum, granitic columns from Leptis, in Africa. " I was surprised to find," he says, " that the shafts of these columns were in the act of desquamation, casting off crusts precisely similar to those which occur, in many cases, in natural blocks of granite; but in this instance the detached crust is not decomposed, and appears scarcely changed from its original state, except in tenderness and fragility." * This shows that the atmospheric influence may, under some circumstances, only produce a general and superficial change, just as a metal plunged into a corrosive acid, and immediately removed, will be uniformly corroded ; but if allowed to remain in for some time, the prolonged action of the agent will show that the oxidation has been more energetic at certain points, whereby the crystalline structure of the metallic mass is developed. From these facts it is not improbable, that the spheroidal forms, so commonly exhibited by the crystalline rocks, are varieties of original structure, the result of some peculiar mode in which their integrant particles have been aggregated * Journal of Science and Arts, vol. xiii. p. 238. H 98 THE STRUCTURE OF THE [Ch. VI. together, and which appear to subsist in the same mass, in conjunction with various cuboidal and prismatic arrange- ments. The fissures or joints which occur in the upper part of granitic rocks, are produced either by direct exposure to the atmosphere, or by the percolation of rain water ; and it has been stated, that they unfold the form of the concretions of which the aggregate mass is composed, just as the funda- mental forms of a crystalline mass are displayed by the partial action of a chemical solvent. And this analogy is confirmed by the fact, that in both cases the same result may be obtained by mechanical means ; thus, large blocks of granite, or the mass of this rock, at depths where the joints are no longer visible? may be cloven into cuboids, by means of several wedges applied on the same line ; and in this manner all the granitic rocks may be divided into blocks correspond- ing in form to those developed by the action of the elements. According to the case already related, concerning the schistose granite of Arran, by Dr. Macculloch, this does not always hold good; but; this discrepancy may, perhaps, be attributed to the imperfection of the mechanical power which he em- ployed, conjoined with the refractory nature of the rock operated on. The geologist would not readily arrive at the fact, that common granite can be separated into cubes ; for with his hammer and wedges he can only break off irregular fragments : but the quarryman, by first boring several holes on a given line, at an equal distance from each other, and then alternately striking wedges placed therein, can cleave the granite into regular quadrangular masses ; and, in like manner, slate may be raised in large tabular slabs by the simultaneous application of many broad wedges, whereas the geological tools would only shiver the slate into foliated frag- ments. That the cleavage of granite into regular forms depends on the structure of this rock, is proved by its only taking place in certain directions corresponding therewith. The workmen Ch. VI.] PRIMARY ROCKS. 99 are well acquainted with this fact, and therefore never at- tempt to break the rock" in a line diagonal thereto ; and geologists are indebted to them for some curious information on this subject, which has been recorded by Mr. Enys. The granite of Penryn is principally composed of two kinds : one is a hard and compact rock, which is extensively worked and shipped for the London market. It runs in parallel ranges, bearing N. E. and S. W., through the other kind, which is softer and of a coarser texture. The hard granite is cloven into quadrangular blocks ; and it has been ascertained by long experience, that the cleavage cannot be effected in every direction, but only with regularity in three, and that each of these require a different degree of force : thus, if the horizontal cleavage demands a power denoted by two, the perpendicular cleavages, crossing each other, will be in the proportion of three and five ; the latter commonly intersects the larger fel- spar crystals, whilst the former is parallel therewith, and may be termed the longitudinal cleavage. Near Penryn the last mentioned line of cleavage runs N. N. W. and S. S. E., varying 15 or 20 either way: it generally coincides with one of the vertical natural joints, though in many instances, it does not correspond therewith, but crosses them often at- an angle of 30 or 40.* This interesting fact cannot fail ultimately to lead to some important information, as it will direct the atten- tion of geologists to the condition of the granite of other countries in this respect. This subject has not altogether escaped notice, as will be seen in the following extracts ; but it does not appear to have suggested the idea that the layers into which granite is divided by such seams having a determinate bearing are similar to the beds of stratified rocks, nor has it given rise to more extended enquiries. " The granite of Reville, in the department of La Manche," says De la Beche, "generally resembles that of Dartmoor, containing large porphyritic crystals of felspar. It splits in * Lond. and Edin. Phil. Mag., vol. ii. p. 321. H 2 100 THE STRUCTURE OF THE [Ch. VI. two directions ; one east and west, the other north and south ; and forms large rhomboidal blocks : some few groups of gra- nite rocks, on the sea-shore, are split into similar blocks, which have not the same direction. The granite at St. Vaast and the opposite island of Tatihou is somewhat different from that of Reville, though evidently in connection with it ; it is split into similar blocks, and the fissures are in the same direction with those at Reville ; in both places it is traversed by granite veins, of which the predominant ingredient is flesh-coloured felspar."* Similar appearances have been observed in the granitic district of the east of Ireland. " In the quarries of Golden Hill," says Mr. Weaver, " the granite is divided by smooth parallel seams into strata, three, four, five, and more feet thick, ranging north and south, and dipping E. 75. These strata are sometimes, though rarely, intersected by cross joints, under an oblique angle, which are mostly parallel to each other : in such cases each stratum becomes naturally separated into tabular or columnar masses of a rhomboidal form." " In Glencullen, the granite is also divided into great massive strata, which range 20 east of north and west of south, and dip into the mountain to the westward, at an angle of 70. And on the Dalkey coast, a large insulated granite rock, in its natural position, is composed of parallel layers from one foot and a half, to three feet and a half thick, ranging north and south, and dipping 70 east."f The numerous instances on record, of the stratification of granite, are of the same nature, the mass being divided by seams into regular beds: thus, " the granite on the French coast, near the mouth of the Loire, exhibits this appearance of stratification; the beds running S. E. and N. W., dipping towards the S. W., under the sea, at a moderate angle of inclination." f From these facts we arrive at the important conclusion, that the granitic rocks are not rude and shapeless masses, as * Geol. Trans. (New Series), vol. i. p. 87. t Geol. Trans., vol. v. p. 138. J Annales des Mines, tome iv. p. 25. Ch. VI.] PRIMARY ROCKS. is commonly supposed, but that the structure and arrange- ment of their parts exhibit as great marks of order and design as any department of the mineral kingdom. Though the three systems of cross joints often afford indications of apparent complication and disorder, yet we can frequently detect a more regular arrangement of layers or beds than is to be found in some kinds of gneiss. We also learn, that in Ireland these granitic layers are inclined at various, though elevated, angles ; and the same circumstance is of very common occur- rence in Cornwall, the dip varying from 45 to the perpen- dicular ; and in these cases the other joints are so disposed, as to divide the layers into rhomboidal concretions. And since the different modifications of this form have some con- nection with the mineral composition, as already stated, it might be expected that the inclination of the layers would correspond with the nature of their component concretions ; and thus the lesser angles more generally belong to eurite and felsparite ; the higher ones, to shorl rock and fine-grained granite ; whilst the layers of true granite are sometimes per- pendicular. In thus asserting that the beds of granitic rocks exhibit regular bearings, both of strike and dip, it must be clearly understood that these can only be occasionally detected ; and are often, as in the case of the schistose rocks, complicated and obscured by the curvatures of the beds. If we examine the sea-shore and cliffs of the Land's End district, the parallel seams of the granite will be generally found more or less serpentine ; and frequently their interference with each other will produce beds tapering away to a certain point, and then enlarging, or even terminating in wedge-shaped masses. This form of the granitic beds holds good both in perpendicu- lar and horizontal sections ; and it will be soon shown that the schistose rocks are subject to the same apparent irregularities. It is particularly worthy of remark, that the beds of granite exhibited in the cliffs of the Land's End district are always more or less inclined; they often indeed present perpendi- H 3 102 '- " f ;T&E STRUCTURE OF THE [Ch. VI. cular faces which, being traversed by upright lines, represent vast piles of columns : but these sections cross the beds at a considerable angle, and their nature may be easily detected by a close inspection of the mass, when they will be seen to result from the rhomboidal concretions being so arranged that one of the three parallel systems of planes of which they are formed, has a perpendicular position. This prevalence of the rhomboidal structure, in the cliffs of this district, is an interesting fact, because the granite is here always at or near the junction with the slate, and generally exhibits such a variation in its mineral composition from the central masses as characterises this rock when it comes into contact with the primary schists, and on which account it is often difficult to decide whether the rocks at the junction be granite or slate, without putting them to the test of the hammer. Now in all the cases just quoted, of the bearings of granitic layers or beds, in France and Ireland, the same rhomboidal structure prevails, as indicated by the beds dipping at various angles ; and the granite, in all these places, occurs near the junction with the slate. It becomes, therefore, an interesting enquiry to ascertain, if possible, the limits of this structure ; and under what circumstances the granite assumes a regular cuboidal structure. It is well known that the central part of a mass of granite is not wholly of the latter kind, because it is traversed by beds of eurite, felsparite, and other granitic com- pounds, which are composed of rhomboidal concretions. It will, perhaps, be found that cuboidal blocks are not so common pr characteristic of granitic rocks as is generally assumed. Before concluding this topic it may be observed, that the granitic beds necessarily dip at several angles, according to the kind of rhomboids of which they are composed, and to the position in which they are placed : thus, in granitic cliffs, the rock will be commonly found to present a perpendicular face, and two inclined ones ; one of the latter dipping at a very elevated angle, and the other at an angle of less than 4-5 : Ch. VI.] PRIMARY ROCKS. 103 bearing this in mind, and making allowances for the spheroidal tendency of the mass, and the curvatures of the layers, a regu- larity may be generally detected, not inferior to that of the crystalline schists. This structure is more or less visible in most published sketches of granitic cliffs; more particularly if the sea has excavated some portions of the perpendicular, layers, in con* sequence of a different composition rendering them more sus- ceptible of decay and degradation. We have attempted in the subjoined figure to convey an idea of this arrangement, having for this purpose intentionally exaggerated the regu- larity of the lines. Fig. 4* The lines corresponding with the gentle declination of the nearest hill towards the point, mark the manner in which the inclined face of the rhomboid. slopes in one direction; while the greater slope of the hill towards the cliff is denoted by the lines on the sides of the indentations of the cliff, which run parallel therewith. Concerning the structure of the primary schistose rocks a few remarks will suffice. It may be divided, as in tlje case of granite, into the concretionary and, the internal*; and the H 4 104 THE STRUCTURE OF THE [Ch. VI. latter is only to be distinguished from that of granite by its foliated modifications predominating over the massive, whereas it is the reverse in granite. The various kinds of structure so frequently pass into each other in the same mass, that it is oftentimes very difficult to observe where the one begins and the other ends. When the rock has a compact texture, but more particularly if it be crystalline, it most commonly exhibits a massive structure; and as it gradually becomes more and more fine-grained and homogeneous, it generally acquires a proportional degree of fissility, acquiring at last a perfect slaty structure : on this account it is often impossible to draw a line of demarcation between the true slates and the massive, crystalline, and gra- nitic rocks, with which they are associated ; since they gra- dually pass into each other in their mechanical arrangement, as well as in their mineral composition. This is not only the case in Cornwall, but in all primary districts. It would lead us into too many details, and into many repetitions, if we were to enter fully on this subject ; let it therefore suffice to quote one example from Dr. Macculloch, on Scotland. " The primitive greenstone and greenstone- slate consist of mixtures of felspar and hornblende ; in the latter case disposed in a laminar form, and in the former, without that regular struc- ture." " These greenstones constitute a class of rocks entirely different from those that are intruded amongst the regular system of secondary strata; and have evidently been formed together with the gneiss and mica-slate which they accompany ; and are, like them, referable to the same epoch, never intrud- ing to their disturbance, but occupying situations and main- taining characters in every respect conformable to them."* Thus we find that the internal structure of the primary rocks, both granitic and schistose, is of the same nature ; the only difference that can be observed is, that the massive Structure prevails amongst the former, and the latter rocks * Gaol. Trans., vol. ii. p. 393. Ch. VI.] PRIMARY ROCKS. 105 are more frequently fissile. Let us then entirely overlook the nature of the internal, whilst we examine the concre- tionary, structure of the schistose rocks. In Cornwall, all the varieties of rocks belonging to the schistose group are composed of rhomboidal concretions ; these exhibit various modifications, and are more distinctly formed in some rocks than in others, especially in those which are principally composed of compact felspar. This structure is not only detected by the fracture, but it is also, on a large scale, developed by a partial disintegration, by which parallel systems of fissures or joints are produced, as in the case of granite. These are sometimes not so evident, and are com- plicated by the degradation of the slate, in the direction of its internal laminae. One set of these fissures may be often seen traversing these rocks, in a perpendicular manner ; as shewn by the smooth upright surface of some cliffs, and by the re- gular parallel surfaces, when an intermediate layer has been removed by the action of the elements. And, indeed, the same fact may be very frequently observed in quarries. In these cases, if the adjacent parallel joints be disclosed, they will be found running in opposite directions, the one at a greater angle than the other. This is owing to the form of the layers, resulting from the aggregation of rhomboidal con- cretions ; and the nature of it will be plainly illustrated by so placing a rhomboid that two of its sides may be per- pendicular. In this manner are arranged layers of rocks, that are sometimes massive. and sometimes fissile; or which, indeed, individually partake of all the forms of internal structure. There can be little doubt but that the primary slates of other countries possess the same structure ; that we have not many facts in corroboration of its universal occurrence may perhaps be, in some measure, attributed to these rocks not being found under circumstances so favourable for observation as those of Cornwall. That this structure, however, is not peculiar to the latter country may be gathered from the 106 THE STRUCTURE OF THE [Ch. VI. following statement: "In the course of the operations at- tending the driving of levels and sinking of shafts in Crone- bane and Tigrony, I have observed," says Mr. Weaver, " that the slaty rocks (whose general range is nearly N. E. and S. W., and dip S. E. at an angle of 65) are divided into great beds, commonly about five fathoms thick, by parallel seams or joints, which intersect the inclined plane of the clay-slate at right angles, dipping 25 towards the N. W. These seams are open fissures, which will sometimes admit one or two fingers, and at other times scarcely the blade of a knife. In their progress they pass uninterruptedly through all the beds and contemporaneous veins included in the slaty rocks, dividing them, and sometimes producing, as it were, a sensible alteration in their disposition, though to no great extent. This tendency towards a division into horizontal beds (in- dependent of the slaty structure) may also be observed in several parts of this district." * From the preceding observations on the arrangement of the Cornish schistose rocks, it is evident that the dip of the layers or strata may be referred to more than one direction : which of these is to be regarded as the true one ? This is a question not easily answered in a satisfactory manner ; and yet we find that geologists have not often expressed any hesitation in determining this point, though one of great importance, as many of the arguments advanced in favour of the prevailing theory depend on the direction and the inclination of the primary strata. In Cornwall, the choice of the true dip lies between the two largest faces or surfaces of the rhomboidal layers, which run in the same direction, most commonly N. E. and S. W., but dip at opposite angles. In the description of this county, published in the Cornish Transactions, that surface of the layer which is parallel with the laminae of the rock has always been selected : this, indeed, generally appears to be the most * Geol. Trans., vol. v, p. 217. Ch. VI.] PRIMARY ROCKS. 107 extended surface ; but it may be in some measure deceptive, arising from the formation of seams or fissures between the laminae, as well as between the layers or strata. Adopting, however, this method, the dip of these schists is commonly found to be from the granite in the vicinity of this rock, and at a distance therefrom, very frequently in an opposite direc- tion : it is, however, very various ; and if the inclination does follow any general rule, it is that of the great curvatures of these rocks, with which it almost universally corresponds. The angle of the dip scarcely, in any instance, exceeds 40, generally fluctuating between 25 and 35 : and the amount of this angle bears no relation to the vicinity of the granite ; for next this rock it is sometimes great, at others, exceed- ingly small. If the inclined surface of the layer, which dips in the opposite direction, were adopted as the surface of the stratum, then the slate of Cornwall would have an inclination varying from 45 to 75, and even approaching still nearer to the perpendicular. There is one consideration, that might induce some to prefer this view of the case ; viz., that the layers of massive crystalline and granitic rocks very commonly lie in directions parallel thereto. Thus, the elvan courses, near the granite, almost uniformly dip towards this rock, whilst the laminae of the slate are disposed in an opposite direction, so that they often appear to be intersected by the elvans. The following diagram will, perhaps, render this arrangement more intelligible : Fig. 5. a, porphyry in granite; b, the same rock traversing both slate and granite; c, the same in slate ; d, d, d, compact rock in slate. 108 THE STRUCTURE OF THE [Ch. VI. Now on what principle can it be decided which of these two directions, exhibited by the fissures or joints traversing the schistose rocks, is the true dip of the strata. In the secondary rocks, the laminae of the slate are frequently found at considerable angles to the strata, as determined by the alternation of the slate with other rocks ; and, therefore, it may be given in favour of the greater angle inclining towards the granite, as in the above diagram : but it may be stated that, in adopting the other, we are not singular ; for Professor Sedgwick, in his excellent communications, pub- lished in the Transactions of the Cambridge Philosophical Society, has assigned the same dip to the Cornish strata. This subject will be reverted to again hereafter ; at present, we must continue the description of the primary slates. Complicated as the structure of these rocks may have appeared, in the foregoing details they have hitherto beeen only considered under their most simple forms ; for, in addition to what has been already said, it must be stated that all the schistose rocks are more or less curved, not only in the direction of their beds, but also at right angles thereto. These curvatures are sometimes contorted, but never to that degree which is so characteristic of gneiss and mica-slate. In looking down on the slate district from a granitic eminence, the surface will be found to consist of two systems of curves, which cross each other at right angles; and wherever the subjacent rocks are laid bare, they exhibit a corresponding curvature. These curves are not, indeed, always parallel with the surface ; for the latter has been more or less modified by the operation of atmospheric and other agents : thus, the sides of some hills are serrated or irregular, by the protrusion of some strata more indestructible than those with which they alternate ; and some valleys are not concave, but flat, from the accumulation of detrital deposits. But such variations may be easily allowed for, and, indeed, are not perceptible when viewed at a distance : the general external appearance, though modified, accords well with the internal Ch. VI.] PRIMARY ROCKS. 109 structure of these rocks. That this circumstance is not so apparent in other countries, may be attributed to the presence of secondary and tertiary rocks, as well as of recent debris, by which the surface has assumed a new and different feature. For farther particulars on this subject the Cornish Trans- actions may be consulted ; but we must not omit to observe, that the laminae of the schist very frequently correspond with the ascending and descending curves, which are parallel with the granite : or, in other words, in traversing the slate at right angles with the granite, the laminae dip in the same direction as the slopes of the hills : and that the strata forming the limb of the curve nearest to the granite are commonly thinner than those on the other side of the curve, and are more abrupt, dipping at a greater angle ; in consequence of which structure, it often happens that the thin and highly inclined strata, at the summit of the hills, have been entirely degraded; whilst the more extended strata are persistent, exhibiting an underlie conformable with the granite. In like manner, the small islands on the coast, which frequently con- sist of the upper parts of such individual curves, have some- times lost the highly inclined layers by the action of the waves, only retaining those strata which dip from the shore, and which correspond, or not, with the inclination of the nearest mainland, according as this is composed of the ascending or descending portion of the curves. The north coast of Corn- wall, particularly at Portreath, and the adjacent coves, well illustrates this disposition of the strata. It is time, however, to turn our attention to the structure of the primary schistose rocks of other countries. The disposition of the beds of gneiss in the Western Isles of Scotland is very irregular, more particularly those of the granitic species of this rock. Towards the southern and western parts of Coll, the gneiss is often arranged in regular beds, of which the alignement can be traced for a consider- able space, straight and free from all that disturbance which marks the beds of Tirey. Towards the middle and northern 110 THE STRUCTURE OF THE [Ch. VI. end of this island, however, all order disappears, and the masses are disposed in a dislocated manner. The direction where it can be traced is north-easterly, and the dip towards the east ; as is pretty generally the case in these islands. The angle of the dip is various, often reaching to 70 or even to 80, and seldom declining so low as 1 5. In Barra, also, the beds of gneiss can rarely be traced for more than a few yards together in a straight line : they are commonly bent, and even much contorted, while their inclinations to the horizon are very irregular. In East Rona, as already noticed, the gneiss often dips in opposite directions. In short, in these islands, as on the mainland, the gneiss sometimes cannot be detected in the form of beds ; and when the beds are apparent, they exhibit all the complication of direction and dip which might be expected, from their being curved on the great scale in two lines, which nearly cross each other at right angles, besides the lesser and subordinate curves, to which the larger ones are continually subjected. To this structure of the primary schistose rocks, assisted by the action of the ele- ments, may be attributed the parallel arrangement of these ranges of islands, and the subdivision of these ranges into the individual isles. The strata in- the southern clusters are, indeed, more regularly disposed ; but still, on the great scale, they present the same characters : their composition is also different, consisting in a great measure of clay-slate which, at a distance from granite, is commonly arranged with a greater degree of regularity than gneiss. Thus Macculloch* informs us, that the prevailing directions in the shores of these islands will be found to correspond in a great degree with the direction of the strata of which they are composed, and to vary in a great measure according to the changes in the lines of bearings ; and a similar general correspondence will be discovered on the mainland between the direction of the strata and the general forms of the * Western Islands of Scotland, vol. ii. p. 298. Ch. VI.] PRIMARY ROCKS. Ill shores. If the lines of direction are connected throughout, it will be seen that they are curved ; and the reality of the curvature is proved by its taking place, not only in the inter- vals between these islands, but even in the isle of Luing itself. This curvature, however, is not so great as to implicate all the parallel strata ; as it is not found in the Garveloch Isles, which lie immediately to the westward of Luing. Although the direction of these schistose strata is thus constant, with certain trifling exceptions, which must be rather viewed as temporary undulations than as serious deviations from their general course, the dip is in different places not only various in quantity, but also reverse in position. In Gigha, it is to the westward ; while it is to the eastward, not only in Isla and Jura, but in all the northern isles. To whatever quarter, however, the strata dip, the angle of inclination is seldom less than 20, and rarely exceeds 60. Over a considerable tract on the mainland the mica-slate and clay-slate series accom- pany each other, alternating together, and exhibiting fre- quent transitions which terminate upwards by the predomin- ance of the clay-slate. These stratified rocks extend in one belt from the easternmost point of Scotland to Bute, run- ning N.E. and S.W., and dipping towards the east ; and they are subject to no other deviation or irregularity, than those slight undulations from which none of the Scottish strata are exempt. Now these two series of slate are also continued into Arran ; but their continuity is there inter- rupted, for a certain space, by a mass of granite. They are no longer parallel to each other, nor do their strata possess the same direction or dip. Indeed, such is the apparent con- fusion, that the dips cannot be exactly ascertained ; yet it is evident, on comparing a great number, that the predominant tendency of the whole is conformable to the surface of the granite on all sides ; or, in other words, that the strata repose on the mountain mass. And since the micaceous series pre- vails on the south-western side of the granite, and the argil- laceous on the north-eastern side, it is evident, that the strata 112 THE STRUCTURE OF THE [Ch. VT of these series respectively dip to opposite points of the compass. Such is the disposition of the primary strata on the western coast of Scotland, which is similar to that of the analogous rocks in Cornwall ; and on referring to the details on Nor- way, in the fifth chapter, the gneiss, and other schistose rocks, will be found to exhibit the same characters : we will, there- fore, conclude this topic with one more example taken from the east of Ireland. It has been remarked by Mr. Weaver, " that, in following the line of the two Avons, the Daragh and the Avoca, from the granite chain on the west to the sea on the east, the slaty rocks gradually acquire a higher angle of elevation. In the glens, the angle of apposition is from 20 to 25 ; in the meridian of Cronebane, the angle is 65 ; and nearer to the coast, it varies from 70 to 80. On the western side of the mountains, this order is in general inverted, the highest angle being in the vicinity of granite, and the lowest most remote from it, with this additional distinction, that the dip of the slate is commonly opposed to that of the granite declivity on the western side; while, on the eastern, their position is more conformable to the declivity."* In order to show the nature of these dips, it will be desirable to enu- merate several examples from the same author.f Thus, we find, that on the eastern side of the range, the mica-slate at Killiney Bay leans against the granite, and dips from it at an angle of 80 ; at the eastern side of Rochetown road, near the martello tower, it dips in the same direction at 40 ; whilst, on the eastern brow of Rochetown Hill, in a natural hollow in the granite, its dip is 75 : on the southern side of the Scalp, this slate rests on the granite, inclining towards the S. E. at 35 ; but the general mass ranges through the hill, dipping N. of E. at 40 : at Maulin Hill it dips on the summit 25 towards the S.E.; but on the west, near its * Geol, Trans., vol. v. p. 178. f Idem, HO. Ch. VI.] PRIMARY ROCKS. 113 contact with the granite, only 5 : and in the adjoining glen of Balveagh, a precipitate mass of this slate reposes on the granite in a nearly horizontal position : in Glenmacanass, the mica-slate has various dips in different places, varying from 20, 25, to even 70 : on the western side of the mountain range, the mica-slate adjoins the granite at Glenismaule, and dips towards the N. W. at 75 ; and near the same spot, 52 in the same direction : at Seefiniane, it dips 45 north : in the northern part of Seechon, it inclines towards the south at an angle of 75; in this neighbourhood there appears to be a gradual reversion in the direction of the dip, as the slate is seen in a nearly horizontal position, inclined at an angle of 45, then 52, and, lastly, of 75 : at the eastern side of Bris- selstown, it dips 65 towards the east : at Kilranela Hill, the mica-slate is remarkably contorted, dipping near the summit at 30 ; while, on the eastern side of the hill, the dip is 65 : in Baltinglass, Tinoran, and Manger Hills, the inclination of this slate is generally about 60 towards the N. E. ; and near Graige, on the eastern bank of the Barrow, it dips 65 to 70 S.E." From these statements we learn, that in Ireland, as in Cornwall, the crystalline slates dip at very different angles, not only in distant places, but also within a narrow compass, even on the side of the same hill, owing to the curvature of the strata : and, moreover, that the strata are variously situated ; reclining, in some places, in positions conformable with the granite ; and, in others, they are so disposed, that if the dip were prolonged, they would abut against or pass under the granite. Although, however, this disposition in the direc- tion of the dip often corresponds with the curvatures of these rocks, it must not be considered as dependant thereon ; since these variations of dip often occur when the rocks are dis- posed in straight and regular beds. Details on this subject might be multiplied to a great length, for the convolutions of rocks has always been a favourite object of research to the geologist, but the purpose 114 . THE STRUCTURE OF THE [Ch. VI. of this work does not require it ; we shall, therefore, conclude with observing, that the curves are sometimes " simple, like the superficies of a cylinder ; at other times double, like that of a sphere;" and sometimes they are angular or contorted, exhibiting the most complicated sinuosities. And these cir- cumstances of curvature are not confined to a mass of strata, but are possessed by individual strata alternating with others, that are straight and regular ; and may be often observed even in the laminae, or smaller concretions, of which a stratum is composed. There yet remains another variety of structure to be de- scribed, not uncommon among the primary rocks, and which is generally considered as a proof of a mechanical origin. Without, however, expressing any opinion on its nature, let us in this place confine our attention to a description of this structure, which is characterised by an aggregation of angular and spheroidal concretions. It has been already stated, that, on the large scale, the granitic rocks appear to be composed of spheroidal masses ; and that, during disintegration, the open seams divide the mass into tabular and angular portions: but the structure now under consideration is distinct from this, being confined to the individual concretions of which the mass is com- posed. First, as regards the spheroidal or conglomerated struc- ture. An instance of this nature has not as yet been noticed in the Cornish granite, with the exception of those crystalline nuclei, of a dark colour and fine grain, which occur here and there insulated in the granite, and which, indeed, if suffi- ciently abundant to predominate over the containing rock, would exhibit this kind of appearance. In the globular granite of Corsica, however, we have a noted example, in which the constituent minerals are arranged around certain centres in concentric laminae. In the schistose rocks, this structure is of a more frequent occurrence. In the gneiss of Coll, nodules of hornblende-rock are completely enveloped in Ch. VI.] PRIMARY ROCKS. 115 the laminae of the former rock, which are every where bent over them, as if they had, in a soft state, been moulded on the previously indurated concretions. * So likewise near Drontheim, Norway, in a rock intermediate between mica- slate and clay -slate, folia of mica surround kernels, which are very hard and compact, and appear to be a granular mixture of compact felspar, a little quartz, and fine scales of mica. The kernels, with their envelope of mica, are of various sizes, sometimes equal to two or three feet in diameter : these concretions lie close together, forming whole rocks. In Cornwall, the compact variety of chlorite-schist near Relistian mine exhibits this conglomerated structure : its nodules are not globular, but flattened, and are composed of concentric layers, the outer ones being softer and more chlo- ride, the central more compact and crystalline. Many examples of this conglomerate have been recorded : that of Valorsine is particularly noted, and is regarded as a detrital rock ; but if it alternates with gneiss, as has been stated, it is probably of the same nature as those just enumer- ated. " The lower part of the Col de Balme, according to Saussure, as quoted by Dr. Kidd, is a moderately grained grey granite ; above which are rocks intermediate to gneiss and mica-slate : higher up are beds of a grey and sometimes greenish slate, containing a great quantity of rolled pebbles, some angular and others rounded, varying from the size of sand to masses of six or seven inches in diameter. Some of the beds are very thin, remarkably fine grained and micaceous, and entirely free from pebbles ; others are of considerable thickness, and filled with pebbles." Brongniart, also, in the Cotentin, in Brittany, observed a similar fact. A series of granitic and syenitic rocks is followed by a shining argilla- ceous slate of a greenish colour, the laminae of which gradually becoming more and more undulated, and traversed by veins of quartz, and penetrated with oval nodules of the same * Western Isles of Scotland, vol. i. p. G4. I 2 116 THE STRUCTURE OF THE [Ch. VI. substance, the rock passes insensibly, without any change in its structure, or in the degree of inclination of its strata, into a steatitic schist. And he remarks, " that this schist also contains numerous nodules of quartz, which, being penetrated by the matter of the talc, cannot be considered as rolled pebbles, but as nodules cotemporaneous in their formation with the rock itself." * Lastly, Mr. Weaver states, that, near Cronebane in Ireland, the clay-slate occurs in almost every stage of union with quartz, which varies in size from large compressed lenticular nodules to the smallest grains, terminating in a most perfect intermixture of the quartz with the substance of the slate. These nodules are arranged parallel with the direction of the strata ; and they often terminate in thin edges, diffused between the laminae of the slate, f The primary rocks which are composed of angular con- cretions, are of more frequent occurrence in Cornwall than those of a conglomerated structure. The quartzose varieties of the granitic rocks, in which quartz predominates, often exhibit an angular appearance ; and this is much more dis- tinctly developed in the same species of the primary schists. The layer of felspathic rock, which is worked for tin ore near the Indian Queens, in the central district, so abounds in small angular portions of different colours and texture, that it resembles a coarse lime-ash floor : this rock appears to be a regular elvan-course, equivalent to the beds of porphyry which are associated with the Cornish slates. At Huel Virgin mine, near Marazion, one of the beds of slate is intersected by small irregular veins of flesh-coloured felspar and quartz ; and, in some parts, the former so predominate in the mass, that the slate is divided into small angular portions. This rock assumes every variety of form, according to the proportion of its constituent parts, exhibiting a most perfect likeness of a brecciated marble. This bed, according to Mr. Kenwood, is considered as one of the lodes (as in the case of Relistian * Geological Essay, p. 83. f Geol. Trans, vol. v. p. 171. Ch. VI.] PRIMARY ROCKS. 1 ] 7 mine), on account of its being metalliferous. And it is, in- deed, worthy of observation, that the contents of the Cornish veins are commonly arranged in a brecciated manner, con- sisting of quartz and of angular pieces of the adjacent rock ; whilst the conglomerated structure is of rare occurrence, but has been observed in veins traversing the granite of St. Just, as well as in those which are situated in the slate. A remarkable instance of this kind was seen by Macculloch in the island of Coll, near Ben Feoul. Distinct beds of mica slate alternate, there, with gneiss : one of these abounds in frag- ments of quartz, so as to form a breccia. * Whether a similar kind of rock in Rasay is really a variety of gneiss, or a true fragmentary rock, is not very apparent. " It entirely consists of fragments of gneiss, broken and reunited with very little change of character : it can scarcely be separated from the principal rock." In the same situation a breccia " is formed of dark red felspar and hornblende-schist equally mixed in distinct fragments, and producing, from the contrast of colour, a very peculiar appearance." f At Ben-na-chie, in Aberdeenshire, says Dr. Macculloch, the quartz-rock in contact with the granite exhibits an in- teresting appearance. " The regularity of the stratification is disturbed, and, in many places, the strata are fractured and displaced. Where the fractures are considerable, the parts are sometimes reunited by minuter fragments and by crys- talline quartz ; and, in many places, these fragments are so numerous, that the whole mass forms a breccia, or an angular local conglomerate of a peculiar character." J The gneiss, near Formo, in Norway, says Von Buch, rises to a great thickness ; and in the Rostenberg, and in the ravine towards Lessbe, it becomes very remarkable in its composition. The gneiss, here, generally abounds in mica, which is not scaly foliated, but in large continuous folia ; and it also contains * Western Isles, vol. i. p. 63. t Idem, p. 247. f Geol. Trans. New Series, vol. i. p. 58. I 3 118 THE STRUCTURE OF THE PRIMARY ROCKS. [Ch. VI. numerous beds of quartz : throughout this rock appear, in every direction, considerable pieces of gneiss in which the felspar predominates, the mica being only in isolated folia, and the quartz in very small quantity. The mica in these pieces only gives rise to straight and parallel streaks, while in the surrounding gneiss it renders the slaty texture more strongly marked and distinct. These pieces are all angular, and most of them even quadrangular; and are a foot and upwards in size : they are very thickly heaped together, but still in such a manner that the intervening mass of gneiss may always be distinguished. The streaks of different pieces lying near each other are often parallel ; but they also frequently take different directions, and do not, therefore, follow the direction of the cleavage of the basis. " This wonderful rock is not a conglomerate ; the basis is too distinctly charac- terised as gneiss : but it must be owned that this appearance bears some resemblance to the manner in which the pudding- stone is found in gneiss at Valorsine, and in the lower Valais, according to Saussure. It appears to be an older gneiss which was destroyed during the formation of the newer gneiss."* Mr. Weaver has remarked, that the apparently brecciated rocks are connected by insensible gradations with mica-slate, and even with the fundamental granite itself: and is of opinion that neither the mere occurrence of matter mechanic- ally divided in the composition of rocks, nor the curvatures or inflections which their strata sometimes display, entitle them to a place among the transition formations ; for these are characters common to all classes of rocks, f Having now examined the structure of the primary rocks, we are the better prepared to enquire into the nature of the relative position of the granitic and schistose rocks. * Leopold Von Buch's Travels in Norway and Lapland, Jameson's edition P. 94. f Geol. Trans, vol. v. p. 196. Ch. VII.] GRANITIC AND SCHISTOSE ROCKS. 119 CHAPTER VII. ON THE RELATIVE POSITION, AND ON THE NATURE OF THE GRANITIC AND SCHISTOSE ROCKS AT THEIR JUNCTION. Position of the primary schists next granite. They extend far over this rock through the transverse valleys in Cornwall. The mica-slate of Ireland similarly situated, capping the granitic mountain of Lugnaquilla. The Cornish strata skirt the granite in an irregularly undulating curve. The same arrangement obtains in the isle of Arran, in the north of Scotland, and in the Alps. Dip of the strata at the junction, in Arran, in Corn- wall, in Galloway, apparently irregular not so dependent on the structure of the primary rocks illustrated by sections. The anticlinal axis of primary districts said to be simple in each geographical range not so in the Ocrynian range, nor in the Pyrenees. Composition of granite and primary schists at their junction, in Cornwall, near Cherbourg, in Ireland in lona and Mull, in Glen Tilt, and near St. Paul Fenouilletin the Pyrenees. IT has been already stated that the primary rocks have an un- dulated structure ; and, that the curvatures of these rocks cross each other at right angles, thus producing, at the surface, two parallel systems of valleys. If we examine those valleys, in Cornwall, which are at right angles with the central range, it will be frequently found that the schistose rocks run up there to a considerable distance, forming, as it were, bays and inlets on the sides of the large insulated masses of granite: sometimes, indeed, these intruding beds of slate meet from opposite sides, and thus cut off patches of granite from the main masses. In this manner the Tregonning and Godolphin Hills are separated from the granitic hills of Crowan and Wendron, by the inter- vention of a narrow valley of slate : and even the hollow, which divides the former hills from each other, appears to be superficially covered with a schistose rock. It has been lately ascertained that the granitic mass of Kingston Down is not continuous at the surface, as laid down in the map, but is also 120 THE RELATIVE POSITION OF THE [Ch. VII. divided into two parts by the slate which ascends very near to the summit even of Kit Hill, the highest part of this range : and one of the projecting granitic hills near Redruth is said to be separated from the main mass by a narrow and shallow band of slate, which passes over the concave curve south of that town. Thus we learn how difficult it is, even in a country compara- tively so well known as Cornwall, to trace the precise bound- aries of the rocks, since even the distinctly characterized granite and slate cannot be accurately laid down, unless mi- nutely examined step by step. It is, therefore, very probable that every succeeding enquirer will find the granitic masses of Cornwall more penetrated and intersected by slate than his predecessor. This circumstance, combined with the facts, that the thin and superficial portions of slate are often com- pletely disintegrated, and therefore subject to be obliterated by the next current of water which may pass over them ; and that in many primary districts the granite is almost entirely concealed, only appearing here and there in small conical masses, which, in consequence of their greater durability, must increase in extent by the progressive removal of the adjacent decomposing slates resulting from the action of the elements ; all these things tend to indicate that the granitic masses were, at one time, more extensively enveloped in slate than at pre- sent; and to render it probable that they were originally entirely surrounded thereby, and that therefore their present protrusion must, in some measure, be attributed to the subse- quent decay and removal of the stratified rocks by the operation of causes now in action. This explanation of the relative position of these rocks is, however, only considered as applicable to the more recent and subordinate changes which have taken place : but this subject will be discussed hereafter ; we must, for the present, confine ourselves to a description of the situation of these rocks near their junction. " The mountain of Lugnaquilla, in the east of Ireland," says Ch. VII.] GRANITIC AND SCHISTOSE ROCKS. 121 Mr. Weaver*, " rises in bold granitic precipices above Augh- avanagh, and is capped with mica-slate, which occupies its summit and a considerable part of its western declivity; while the steep brows around this mountain, its precipitous sides towards Glenmalur on the east, and towards the glen of Imale on the north, and its long southern arm, consist of granite. This cap of mica-slate is also partly interstratified with beds of granite, and these alternations are sometimes traversed by veins of granite. The summit of this mountain forms a kind of table land, presenting a smooth green sod, and is strongly contrasted with many of its neighbours." This cap of mica-slate appears to be a continuation of the main mass which forms inlets in the adjacent glens : but the picturesque summit of Cadeen consists of a shield- shaped mass of mica-slate, for granite occupies the foot in every direction .f In tracing the slate near its junction with the granite, in Cornwall, it will be found, in consequence of its undulations, sometimes to rise high on the side of the granite ; and again, at a short distance therefrom, to sink to a much lower level, where it lies concealed under several layers of recent deposits, such as alternating beds of gravel, debris, and vegetable remains, which, when tin ore is present, are provincially known by the name of stream- ivorks. This undulating line is not only higher and lower, at any one part, according to the con- vexity or concavity of the curve, but the whole line is found to decline gradually from N. E. to S. W., so that the slate which, in the eastern part of the county, attains to 1000 feet in height, is, at the Scilly Islands, below the present sea level. This statement is, however, only correct as a general view ; for, at many intermediate points, between the begin- ning and termination of this curved line, the slate will be found more elevated than in places which lie more to the north-east ; as is also the case as regards the relative altitudes of the highest points of the granitic range which traverses * Geol. Trans, vol. v. p. 153. f Idem, p. 160. 122 THE RELATIVE POSITION OF THE [Ch. VII. Cornwall in the same direction. These facts may appear to be trivial to those persons who have not paid much atten- tion to this subject ; but they will be found hereafter to be important, as furnishing some aid in the examination into the nature of these rocks. In the Alps, and other granitic countries, the primary slates are disposed near the granite, in a similar undulating form; though the curves do not always run in the same direction. In the isle of Arran, for instance, the primary slates encircle the central mass of granite in the form of undulating hills, with the exception of a part on the eastern side, between North Sannox and the hills above Brodick Wood; where the chain of schistose rocks is wanting, and enormous masses of secondary conglomerate may be traced nearly to the base of the precipices of granite.* It has been supposed, that the primary slates are not only absent from the surface, but that the secondary rock rests immediately upon the granite : it may, in part, repose on granite ; but analogy would lead us to conclude that it is also deposited on the slate which, probably, is here continued beneath in the form of a hollow curve. And the primary rocks of the central axis of the Eastern Alps appear to be disposed in the same manner ; descending in a curved line from the eastern borders of the Tyrol, where they attain the height of more than 12,000 feet, until they are lost beneath the Gratz and Vienna basins : from these, however, they again emerge in the low ridge of Leitha Gebirge, which separates Austria from Hungary ; and after again disappearing under the recent deposits, they rise once more in elevated curves near Presburg, and are prolonged into the mountains which range in a north-easterly direction towards the Carpathian chain. The old rocks of the Leitha Gebirge ridge form a true anticlinal axis, from which the tertiary deposits dip in opposite directions ; and there can no longer be any doubt of * Geol. Trans. New Series, vol. iii. p. 22. Ch. VII.] GRANITIC AND SCHISTOSE ROCKS. 123 the prolongation of the Alpine chain in the direction above indicated.* Thus we learn that the absence of either the granitic or schistose rocks, or of both, at the surface, does not indicate that they are altogether wanting ; for they are frequently found to exist at a lower level, being only concealed by a subsequent and more recent formation. In the Brora dis- trict, for example, rocks of the oolitic series are in contact with the granite ; and the primary slates might be supposed to be absent, if they had not been detected in small patches at Clyne Kirk and near Loch Brora. It is, therefore, very probable, that the granite is always surrounded by primary slates, as is commonly the case : that the latter are not always visible, may be attributed to the form of their sur- face, or to an extraordinary accumulation of the newer deposits. By these remarks, it is by no means intended to advocate a universal order of succession among the rocks ; but only to contend that the primary slates do succeed the granitic rocks, when they are not visible at the surface ; and that this circumstance can be explained by the analogy of those places where the slate sometimes rises high on the granite, and at other times falls to a low level in an undulating line ; thus permitting newer formations to occupy its hollows, and to come into contact with the granite. The next point for consideration is the angle at which the primary strata are placed when in the vicinity of granite. In Arran, this position of the beds of schist can be deter- mined with certainty in many parts of this junction ; and they may in general be said to recline against it : yet if, in some cases, a given bed of schist be prolonged towards the granite, it will be found to indicate the contact of the edges, instead of the faces ; while, in tracing the linear direction of the strata in a third set, they will tend to abut endwise * Geol. Trans. New Series, vol. iii. p. 305. Professor Sedgwick, and Mr, Murchison on the Eastern Alps. 124- THE RELATIVE POSITION OF THE [Ch. VII. against the mass of granite. No general conclusion can, therefore, be drawn with respect to this connection; since even the slight kind of conformity visible at one point of junction, may not be continued perhaps for ten yards.* This accords with the position of the slate in Cornwall, near its junction with the granite ; as will be seen by the following remarks of Professor Sedgwickf on this subject : " At the junctions of the granite with the slate, the latter rock may be generally observed to dip from the mass of the former ; yet the inclination arid line of bearing of the slate are not found universally to correspond with the surface of the rock on which they rest. Thus, in crossing from Mara- zion to St. Michael's Mount, the slates suddenly change their dip, and rise towards the granite of the Mount ; but though they undoubtedly rest thereon, yet their beds do not reach an inclination of more than 10 or 12, and conse- quently appear rather to abut against, than repose upon, the almost perpendicular mass of granite. Again, the slate which commences to the west of Lamorna Cove, has nearly a uni- form dip towards the E. and S. E. : and the line of bearing of its strata does not afford any indication of the very uneven surface of the granite on which they rest. The whole mass of slate appears to repose obliquely on the granite, and at its western junction rises into high cliffs, the upper beds of which spread over the fundamental rock, while the lower beds seem to lean against, rather than repose upon, the pre- cipitous face with which they are in contact. At the eastern junction of these rocks, the slate rests on the granite ; but in such a position, that their great line of cleavage, if produced, would abut against the headlands of the latter rock." This disposition is well exemplified at Mousehole, on the same range of coast, where the junction of the granite and slate is exposed for more than a hundred feet in length, run- ning in an undulating line across the direction of the strata ; * Western Islands of Scotland, vol. ii. p. 353. f Trans, of the Philos. Soc. of Cambridge. On Devon and Cornwall. Ch. VII.] GRANITIC AND SCHISTOSE ROCKS. 125 the ends of which, however, do not abut against the granite in an abrupt manner, but repose on it in such a manner as to fill all the inequalities of the irregular curvatures of the fundamental rock : so that, at the surface of these rocks, the seams of structure developed by partial disintegration, not only run in the same direction, but traverse both rocks in continuous lines parallel with the bearings or strike of the strata. The granite of Galloway, according to Sir James Hall, presents similar indications, at its junction with the slate. " In the bed of the river, at the High Bridge of Dee, the granite dips from the centre of the mass at an angle of 45, and the strata lie upon it in a conformable position : but, at the southern extremity of the Hill of Lauren, at Loch Ken, the character of the junction is very different; the strata here are nearly vertical, stretching from north to south. The line of junction which occurs on the face of the hill towards its summit, cuts the strata at various angles, sometimes nearly at right angles ; and the strata thus abutting endwise against the granite, the two substances are, as it were, spliced into each other."* Other examples might be quoted on this subject, but the fact that the granite and slate meet under such different aspects, is well established : and, indeed, it follows, from the curved outline and projecting masses of the former rock, which run into the slate after the same manner, as the land into the sea, that the strata having a regular strike or bearing must sometimes abut against the granite. But, notwithstand- ing this apparent irregularity of position in these rocks at their junction, there is in reality no want of order ; a careful and minute inspection will remove the appearance of con- fusion which seems, on a cursory view, to prevail ; and the clue by which this labyrinth may be unravelled, is the struc- ture of these rocks, or that arrangement by which their con- * Royal Transactions of Edinburgh, vol. vii. p. 99. 126 THE RELATIVE POSITION OF THE [Ch. VII cretions are aggregated together : a subject which lias not as yet sufficiently occupied the attention of the geologist. In the last chapter, we gave a sketch of the structure of the granite and of the slate ; we now propose to offer a few examples of the structure of these rocks at their junction, which occur in the Land's End district, where the union of these rocks is so instructively displayed. Thus, in the first place, at Polmear Cove, on the north coast, the slate reposes on the granite at an angle of about 45; and these rocks, at their junction, not only form parts of the same concretions, but also of the same beds ; the under part consisting of granite, the upper of slate, owing to both rocks possessing the same rhomboidal structure ; as may be seen in the annexed diagram. Fig. 6. Junction of Granite and Slate at Polmear Cove, Cornwall. (Cliff Section.) In this place, the beds of granite are often separated from each other by thin parallel layers, or veins, of shorl-rock ; so that the beds are more distinctly developed, in the direc- tion of the greater angle, whilst the laminae of the slate follow the lesser : these beds also decline in the direction of their strike towards the sea, and along this slope the waves are gradually removing the slate. At St. Michael's Mount, and at Mousehole, the slate rests on the granite at a less angle ; otherwise the appearance is the same as that of Polmear, Ch. VII.] GRANITIC AND SCHISTOSE ROCKS. 127 though not so distinctly displayed. At Mousehole, the slate thus situated only occurs in thin portions, apparently the residue of the under surface of a mass of slate which once existed : the slate, here, for the most part meets the granite endwise, and is exposed in a horizontal section; so that, following the beds of granite along their course, we enter on the beds of slate, which appear to be a continuation of each other, thus : Fig. 7. Junction of Granite and Slate at Mousehole. V (Ground Plan.) Along the line of junction, which is much undulated, slate sometimes constitutes a portion of a granitic block or concre- tion ; and at others, the reverse holds good : that is, the line of structure is often coincident with the line of junction, but is sometimes indifferent thereto, separating a portion of slate or granite as the case may be ; and, when this happens, the one rock cannot be detached from the other ; as they form a compact and individual concretion. At Mousehole, the nature of the union in perpendicular sections may be detected on a small scale, at many points, owing to the rocks having been irregularly abraded by the waves. On the eastern side of Rosemodris Point, the slate is not only seen reposing on the granite, at a considerable angle in the upper part of the cliff', but the beds of both rocks meet 128 THE RELATIVE POSITION OF THE [Ch. VII. each other endwise as at Mousehole ; but with this difference, that the line of union is nearly perpendicular, corresponding with the parallel fissures in the granite, which are clearly seen to owe their position to one system of planes of the rhomboidal concretions being placed upright. The other two planes dip at different angles, and the strike of these beds de- clines towards the nearest granitic headland; so that in tracing the lines of structure through both rocks, they seem to form the continuation of the same beds ; the eastern ends being of granite, the western of slate. Fig. 8. Junction of Granite and Slate at Rosemodris. (Cliff Section.) The junction at Forth Just, near Cape Cornwall, is simi- larly circumstanced, though the appearance is more com- plicated by the occurrence of a large metalliferous vein which is situated precisely between the granite and the slate. This vein, in the perpendicular section of the cliff, appears to form, as it were, the end of a regular bed of rock corresponding with the beds of the granite and slate on either side ; but the lines of concretion are not regularly continued through the vein : some slight indications of such a disposition are visible at several points ; and that they are not more distinct, may be attributed to the very compact and comparatively inde- composable nature of this quartzose vein. The structure and composition of this vein and the adjacent rocks will be con- sidered more in detail in another chapter: at present the Ch. VII.] THE GRANITIC AND SCHISTOSE ROCKS. 129 relation of the granite and slate may be seen in the following figure : Fig. 9. Junction of Granite and Slate at Cape Comtvall. (Cli ft' Section.) In several spots, at the places now referred to, the lines of structure are distinctly and regularly continued through both the granite and the slate ; but it must be also understood, that in adjacent spots this continuation cannot be seen, the corresponding lines not being in the same direction, or alto- gether wanting in one of the rocks : it is, however, to be borne in mind, that the individual beds of both the granite and the slate exhibit precisely the same appearances. This disposition of the primary slates at their junction with the granite appears to indicate that the latter was, at one time, completely enveloped at the surface by the former. Be this, however, as it may, we now find the granite, at various points, protruding through the slates, and generally occupying the most elevated positions. These masses sometimes are arranged in nearly circular forms of various extent, from several leagues to a few hundred feet in diameter ; at other times they are elongated into considerable ridges, forming what has been termed anticlinal axes, on account of the strata dipping from them on either side, like the roof of a house. The Ocrynian ridge, extending from Dartmoor to the Land's End, has been regarded as such an anticlinal axis : in a general point of view this is correct, but when each of the insulated masses of granite are examined, they will be found more or less concentrically enveloped by the adjacent slate ; and not only so, but, if lines be drawn through 130 POSITION AND NATURE OF [Ch. VII, their centres, parallel with the general strike of strata, N. E. and S. W., it will be perceived that there are several lines or axes many miles remote from each other. Whether this con- dition of the Ocrynian ridge accords with all mountain-chains, the granite of which is subject to like arrangements, we are not prepared to answer : but this we know, that the Cornish range is not a solitary example : " La recherche d'un axe granitique," says M. Reboul, " me parait, jusqu'a ce jour, avoir ete infructueuse. Les masses de cette roche forment au sein des Pyrenees comme de grandes iles qui ne s'eloignent, ni entre elles, ni avec 1'axe geographique," " Ainsi, la chaine des Pyrenees, quoiqu'elle soit Tune des plus simples, est neanmoins composee de plusieurs aretes qui affectent des directions differentes, soit dans Talignement de leurs masses, soit dans celui de leurs strates : et cette disposition Passim ile aux autres chaines plus compliquees." * Having considered the relative position of the granitic and schistose rocks, it is now proposed to examine the composition and nature of these rocks at their junction ; and, as might be expected, Cornwall also affords excellent opportunities for this investigation. It has been already stated, in the description of the schistose rocks, that those which immediately succeed the granite, differ in their composition from those that are more remote ; and so it likewise happens, that the beds of slate adjacent to the granite generally assume a different aspect as they come into actual contact with this rock. Thus, the slate of the eastern district (proteolite) becomes more granular and micaceous as it approaches the granite ; and the slate of the western district (cornubianite) loses the fine-grained and compact texture, so characteristic of this felspathic rock, acquires a granular texture, and at the same time the acces- sory mineral, whether mica, shorl, or hornblende, is developed into distinct scales or crystalline granules, imparting to the * Bulletin de la Soc. Gdol. de France, tome ii. p. 79. Ch. VII.] THE GRANITIC AND SCHISTOSE ROCKS. 131 slate appearances which have been likened to some varieties of gneiss : and it is no uncommon occurrence, in all these cases, for the compact felspar base, by the gradual increase of its silica, to become more and more quartzose ; the beds of these slates next the granite being hard and brittle, exhibiting a sharp and splintery fracture, and very little tendency to separate into laminae. Particular references for each of these appearances need not be given, for the same bed, even within very narrow limits, assumes various aspects : indeed, even in the same concretions, the slate may in one part be quartzose, in another micaceous, resembling gneiss, and in a third (but often in adjoining concretions) the texture may be fine and uniform, and as fissile as the slate at a mile distant from the granite. These facts are well illustrated in the extensive horizontal section at Mousehole, and may, in a lesser or greater degree, be observed at all the junctions on the coast of the Land's End district: so that, although there is no determinate order in the arrangement of the varieties which each kind of slate presents when in immediate contact with the granite, yet it is established that the slate, in such a position, always partakes of different characters, arising from alterations in its composition and texture. The granitic rocks, also, as they approach the junction, are likewise subject to similar transitions from the most perfect form to a state which, if viewed apart from the main mass, would sometimes 'not be easily recognised as granite. The most common mode of this metamorphosis is effected by the basis of a porphyritic granite becoming gradually very fine- grained, and alternating with layers of the same, in which the crystals of felspar are wanting. When the basis is very fine- grained, it perfectly resembles the compact felspar of elvan- courses; and when, in this state, it comes in contact with granular slate, they can only be distinguished from each othei by the dark colour of the latter, and cannot be perfectly separated by the hammer: sometimes, however, they are distinctly divided by a seam or joint, each rock being con- K 2 132 POSITION AND NATURE OF [Ch. VII. fined to distinct concretions ; and this more generally obtains when the granite abounds in crystals of felspar at the junction: but even this does not constantly hold good; for sometimes these prophyritic crystals, as at Polmear Cove, are also contained in the dark-coloured slate, for many inches from the point of contact of the two rocks, and this intimate union is farther displayed by this phenomenon occurring in the same individual block or concretion. Oftentimes the granite is very quartzose at its junction, and gradually passes into quartzose varieties of slate ; or, as at Forth Just, Cape Cornwall, this transition takes place through the medium of a large quartz vein (Little Bound's Lode), into which the granite and the slate, on either hand, distinctly graduate. In short, the granite and slate of Cornwall, at their junction, are frequently so similar, both in composition and concre- tionary structure, that the detached blocks, as they lie side by side, cannot at a little distance be distinguished from each other ; the darker colour of the slate, and its tendency, often slight, to break into laminae, are sometimes the only differences to be detected on a closer inspection. We will now examine whether the junctions of other coun- tries afford similar facts. M. de Caumont, in his account of the department of Manche, in France, has detailed the ap- pearances of the passage of slate into granite. Near Cher- bourg, a greenish schist contains fine particles of felspar, which gradually become more and more abundant; at the same time grains of quartz become visible, and the rock assumes a granular texture : and, in these places, it resembles a kind of gneiss or talc-schist, or, rather, the nodular steaschist of Brongniart. At last, the grains of felspar and quartz, elongated and imbedded between the laminaB of the schist, gradually lose this regular arrangement, acquire a more crystalline form, till the mass becomes a granite or syenite.* The junction of the granite and mica-slate is thus described * Bulletin de la Soc. Gdol. de France, tomeiii. p. 11. Ch. VII.] THE GRANITIC AND SCHISTOSE ROCKS. 133 by Mr. Weaver, in his account of the east of Ireland : " In a ravine, which marks the northern face of Crogan Moira, we may observe that the granite acquires more and more of a quartzy nature as it approaches the mica-slate, becoming almost slaty-quartz at the point of contact ; while the mica slate itself abounds in beds of quartz of variable thickness, and the granite is traversed in all directions by contem- poraneous veins of quartz of greater or less magnitude, and also by veins of granite." * In the Island of lona, according to Dr. Macculloch, a few rocks of red large-grained granite occur on the shore below the Bay of Martyrs, and are in contact with the schist. The latter rock, at a distance from the granite, is described as a black compact clay-slate, occasionally containing hornblende, and sometimes mica, and seems to hold an intermediate place between clay-slate and gneiss : but when in contact with the granite, it puts on a remarkable appearance, displaying exter- nally a singular mixture of black and red. It is very hard, and its fragments are as sharp-edged as siliceous flint. In many places this schist loses its black colour and becomes grey ; in others it is mottled with red felspar, and interspersed with quartz, which minerals increasing in quantity, the slate appears to be on the very verge of passing into the granite with which it is nearly in contact, the sea, in this place, pre- venting the actual contact from being examined; else it is probable that a still more perfect series of this transition might be observed. The junction of argillaceous schist with granite is not rare in Scotland ; but this is the only instance seen by Dr. Macculloch, in which the interference of the latter is of such a nature as to produce the appearance of a real transition from the one to the other rock.f In the Island of Mull, says the same author J, a long and interesting line of the junction of granite and the primary * Geol. Trans., vol. v. p. 152. f Western Islands of Scotland, vol. v. p. 15. | Idem, vol. i. p. 554. et seq. K 3 134 POSITION AND NATURE OF [Ch. VII. strata can be traced at Loch Laigh, the shore fortunately lying in a direction oblique to the line of junction. It pre- sents a very distinct view of appearances of considerable value towards the history of granite, which, however frequent in the mainland of Scotland, are nowhere to be seen under more favourable circumstances. There are two kinds of granite in Mull, the one of a pale flesh colour, and the other of a high red: these rocks are large-grained, yet compact, and their mica is black. The strata consist of an alternation of mi- caceous schist and quartz-rock ; and in some places the former puts on the characters of a schistose gneiss. When these rocks come in contact with the granite, they assume the fol- lowing appearances: The quartz-rock becomes red, and is found to contain felspar in large proportion, often resembling fine-grained gneiss ; and it thus frequently passes into granite by a transition nearly imperceptible. The mica-slate, in the same situation, becomes a true gneiss. These changes, how- ever, are very partial, commonly confined within a few feet of the junction, and gradually vanishing as they recede. At a distance from the granite the strata are thin and regular, but next this rock they are variously disposed : in some places they continue in the same direction, and in others their ends, or even their sides, are united to the granite, and so firmly adherent, that they cannot be separated ; and this irregularity is greatest where veins pass from the granite into the strata, It is worthy of remark, that the junction sometimes takes place between the granite and the mica-slate ; at other times, between the former and the quartz-rock. This is an im- portant circumstance, since these beds being parallel, the granite is not parallel to them ; being found in contact some- times with one, and sometimes with the other, stratum. Here, also, as frequently happens at the junction of these rocks, fragments of the strata are detached from the main body and imbedded in the granite : and in some cases these are so distinct, that the parts whence they were separated may be traced, and the fragments, in imagination, re-united. Ch. VII.] THE GRANITIC AND SCHISTOSE HOCKS. 135 Such is the nature of the junction of granite with the primary slates. Many of the appearances which it presents might be expected, when it is remembered that these slates do not differ much in composition from the granite; and it therefore becomes an interesting enquiry what would be the result if granite were thus to be united with a stratified rock of a different nature. This desideratum is, indeed, already supplied by the exhibition, in several countries, of the inter- ference of granite with calcareous rocks. The junction of granite and limestone is beautifully ex- hibited at Glen Tilt, in Scotland, and has been carefully examined and minutely described by Dr. Macculloch : indeed, his whole account of Glen Tilt * is so very interesting, and so replete with excellent details, showing the nature of the strati- fied rocks, both at a distance from, and in immediate contact with, the granite, that we propose to make some lengthened extracts. The granite which forms the right ridge of Glen Tilt ap- pears to be a continuation of the great central granite of the Grampians, constituting its termination in a southern direc- tion. The varieties of this granite are mixtures of quartz, felspar, and mica ; differing, however, as much in colour and texture as they do in structure. To these minerals horn- blende is often added, sometimes to the exclusion of the mica. These syenitic granites are almost invariably grey, or even black, from the predominance of the hornblende : their texture is as various as their colour, the crystals of hornblende being sometimes very large, and imbedded, as it were, in a paste of quartz and felspar ; while in other extreme cases the mixture of the several ingredients is so fine, that the constituents can scarcely be discerned ; and it is only to be distinguished from greenstones of the trap family by tracing it for some space until it passes into well-characterised granite. The chief varieties of syenitic granite are found on the slopes which * Geol. Trans., vol. iii. p. 259. et seq. K 4 136 POSITION AND NATURE OF [Ch. VII. descend to the junction, with the schistose rocks, in the vicinity of the Tilt. In this place, as in many others, the different kinds of granite are found together, not forming veins nor distinct masses, but graduating into each other by an indistinct transition. The granite, at its junction with the schistose rocks, consists chiefly of red felspar and white quartz, of which the latter is generally in the smallest propor- tion, the compound mass containing obscure crystals of dark green hornblende. These are seldom in large quantity; more commonly they are thinly dispersed through the rock, and not seldom, even in the largest masses of rock, they are altogether absent. Mica has not been observed in the granite at its junction with the slate. Amongst the stratified rocks, limestone is the most extensive and the most characteristic of this district. The great mass of the limestone, which forms nearly the whole of the left boundary of Glen Tilt, is of a lead blue colour of various intensity, and its texture is almost universally large-grained and highly crystalline : here and there, how- ever, it presents some beds of the most beautiful ornamental marbles which Scotland has yet produced. These marbles are seldom pure, at least to such an extent as to be adapted for the purpose of statuary, but are combined in various pro- portions with a great variety of minerals. Mica is more fre- quently present than any other substance, and, near the junction of the Tilt with the Garry, this mineral is so abundant, that the rock may be easily mistaken for micaceous schist : it has a foliated structure, owing to the mica and calcareous matter being disposed in alternating laminae, the latter rarely exceeding the twentieth of an inch in thickness. Besides mica, this limestone is mixed with steatite, serpentine, talc, asbestus, tremolite, and various other minerals, which are dis- posed in the form of veins or patches ; or, by their intimate union with the limestone, produce mottled, clouded, and otherwise variegated marbles. Such is the nature of the limestone at a distance from the Ch. VII.] THE GRANITIC AND SCHISTOSE ROCKS. 137 granite, where it forms a continuous bed over a very large tract of country, with an elevation somewhat varying, and, consequently, in an undulating plane : its dip is invariably to the south, at an angle of from 5 to 50, and even to 60. Tn one place, a considerable contortion of the bed occurs, and in many others, fractures and dislocations of it may be seen. " Yet, with such partial irregularity," observes Dr. Macculloch, " we may still safely consider the general pa- rallelism and stratification as regular, and the dip as a medium constant quantity of, perhaps, twenty degrees." But when the limestone is contiguous to the granite, it assumes, with some few exceptions, a very different aspect. Its large- grained and crystalline texture disappears, and it more nearly resembles hornstone or compact felspar, having a smooth texture, with a thin-edged fracture intermediate between the splintery and flat conchoidal : it is very hard, effervesces slowly with acids, and gives on analysis a large portion of siliceous matter ; and its external aspect, where it has been worn by the action of the river, is not much unlike that of granite or porphyry. In every instance, all particular or minute regularity disappears, wherever the limestone beds are found in the immediate vicinity of the granite : they are generally much contorted, and so intermingled and blended with the accompanying strata, and with the granite, that the whole mass appears to be in a state of utter confusion. The other strata, which accompany and alternate with the limestone, are schist and quartz-rock, terms which have also such an extensive signification, that it is absolutely necessary to describe them at length. The schist of Glen Tilt rarely maintains the same cha- racter for any considerable space : the most abundant is a clay-slate, rarely fissile, and generally of a very compact texture, and dark blackish-blue colour. It sometimes pos- sesses a glossy and unctuous surface, and passes into a kind of talcaceous schist. It is often much penetrated with quartz, and sometimes finely interlaminated with the same substance : 138 POSITION AND NATURE OF [Ch. VII. at other times it is intermixed with hornblende, and passes into hornblende schist: this occurrence is frequent at the junction with granite, where also the schist, when merely argillaceous, generally displays unusual hardness. Micaceous schist also occurs, and, in general, it is characterised by the predominance of the quartzose ingredient; and in this way it passes into quartz-rock by insensible gradations. The dis- position of the strata of schist is the same as that of the limestone; and when these two rocks alternate together, the limestone is often interfoliated with thin laminae of the schist, in consequence of which they appear to graduate into each other. These two rocks are not always regularly parallel with each other. " Near Gow's Bridge, a white marble is surmounted by a bed of hornblende schist ; this is followed by marble about two feet in thickness, of which the greater part is suddenly cut off at right angles to the bed by a second mass of hornblende schist ; while the lower part continues in the same direction, but bent, and of the breadth of only two or three inches. The lower part of the mass of schist, which thus intersects the marble, is also protracted in a thin plane, continuous with the thin part of the marble, and lying above it ; while the upper side of the same becomes continuous with a regular and thick bed of the schist. A thick bed of marble is therefore included on three sides within the hornblende schist, its outer extremity being cut through at right angles by the schist. As the beds dip into the hill, the further pro- gress of this arrangement cannot be traced." The quartz-rock of the Tilt is similar to that generally associated in Scotland with gneiss and mica-slate in the vicinity of the granite. It is frequently of a bluish colour, very uniform and compact, and approaching nearly to common quartz in character, but still showing evident marks of foliation. When it approaches the limestone and schist, with which it alternates, all these rocks are found to pass gradually into each other. This quartz-rock, however, very frequently contains felspar; Ch. VII.] THE GRANITIC AND SCHISTOSE ROCKS. 139 and, when this is the case, its beds sometimes exhibit all the constituents of granite, so disposed as to resemble the foliated texture of gneiss, and, in some instances, a perfect granite ; and, when this is the case, this quartz rock is fol- lowed by a schist, which seems intermediate between gneiss and mica-slate, having the aspect of the latter with its shining surface, and even foliated structure, but showing in its cross fracture the grains of felspar which belong to the former. This granitic variety of quartz-rock occurs at a considerable distance from the mass of granite, and the schistose beds are straight and undisturbed. Such is the nature of the granite and the adjacent schistose rocks at Glen Tilt ; and, before proceeding to point out their analogy to the primary formation of Cornwall, there are some phenomena exhibited by these rocks which require to be noticed. It has been already remarked, that, at a distance from the granite, the alternations between the limestone, schist, and quartz-rock are regular and well-defined: but when the granite is approached, the lines of stratification become irre- gular, and the dip of the strata is in almost every direction ; and immediately adjoining the granite there is a general mixture of all the stratified rocks, and a total discomposure of their regularity, which is increased by the presence of elongated and detached portions of the granite. It is worthy of remark, that at the junction the strata are not continuously in contact with the granite : on the contrary, it is sometimes the schist, sometimes the quartz-rock, sometimes the limestone, or there is a want of conformity between the granite and the rocks which lie above it. The granite and the strata in the bed of the Tilt have frequently the appearance of alternating together ; but all the circumstances are nowhere so clearly exposed to view as to warrant a positive conclusion. Dr. Macculloch is of opinion, that "those parts of the granite which seem to alternate with these strata can only be considered as portions of veins, the 140 POSITION AND NATURE OF [Ch. VII. disposition of which, like that of trap veins in similar cases, has accidentally coincided with the direction of the beds." Hornstone and felspar porphyries frequently occur in the course of the Tilt on both sides of the valley. These some- times appear to be veins, and at other times assume the form of beds: but it is difficult to ascertain their true nature. When the porphyry is parallel with the strata, resembling a bed, it has oftentimes the appearance of alternating with the schist, quartz-rock, and limestone. The excellent and circumstantial description of the junction of granite and the limestone series in Glen Tilt has led us to make longer extracts than was anticipated ; but the im- portance of the subject required that the nature of this connection should be fully and clearly detailed. Examples of a similar description are of frequent occurrence in the Alps and Pyrenees]; with this difference, that the fossiliferous strata with which the limestone is connected are of a more recent formation. In these countries, however, we cannot obtain such distinct and minute displays of these rocks at their actual contact ; but, on the other hand, the limestone of these mountainous regions, which are crystalline, and imme- diately associated with the granite, are often of less extent, and, consequently, the secondary strata, into which they appear to pass, are not far removed from the granite. These phe- nomena furnish some of the most important evidences in favour of the plutonic theory, and, therefore, it is necessary to subjoin to these lengthened details at least one example from the eastern side of the Pyrenees, as described by Dufrenoy. " Near Saint Paul de Fenouillet the limestone is associated with a black marl enclosing fossils which belong to the inferior beds of chalk, and are disseminated in the form of black casts imbedded in a bluish-grey crystalline limestone : these remains are not easily detected, unless a great number of them have been examined ; for they appear to have been so compressed, and adhere so strongly to the limestone, that it is difficult to separate even characteristic fragments. From Ch. VII.] THE GRANITIC AND SCHISTOSE ROCKS. 141 the bridge over the Fou, to a place where the Gly enters into a narrow and deep gorge opened in the nearly vertical beds of crystalline limestone, the characters of the latter rock are such as have been described ; but here, at the distance of about a thousand feet from the granite, the limestone is per- fectly saccharoid, and does not contain a trace of organic remains. Here the following disposition of the rocks may be observed. The strata dip towards the E. 25 S., at an angle of 75, appearing to repose on the granite which forms the hills of St. Martin; This crystalline limestone continues to within about a hundred yards of the main mass of granite, and, indeed, within a hundred feet of a branch of this rock, which will be presently described. Then succeeds, 1. A reddish ferruginous limestone, about fifty feet in thickness, and disposed in beds regularly stratified. It is worthy of remark, that no passage can be observed between this rock and the grey limestone by which it is covered. The line which separates them is well defined ; which is not the case with the rock immediately subjacent. 2. Beneath the red limestone, is a dolomite, which is very solid, although composed of small distinct rhomboids aggre- gated together. This rock is more than fifty feet in thick- ness, and is not stratified. It decomposes in a very irregular manner : its surface is deeply coloured, although in a fresh fracture it is of a light yellowish tint. It contains some very irregular veins of spathose iron ore. 3. The dolomite rests on a very quartzose felspar rock, which forms a kind of venous bed, or floor, about sixty-five feet in thickness. It is difficult to convey an exact idea of this rock : it seems to have been produced by the penetration of the granite into these strata, and is, consequently, a mixture of very different elements. This mass is in nowise stratified. It contains spathose iron ore, pyrites, and a little iron glance. 4. The compound of dolomite and spathose iron ore, already described as superimposed on the quartzose felspar rock, again occurs beneath the latter, in a layer about two yards thick ; and this, again, rests on, 142 POSITION AND NATURE, ETC. [Ch. VII. 5. A granitic rock, unstratified ; forming, nevertheless, a mass disposed in parallel beds, the thickness of which exceeds a hundred feet. This rock consists of large-grained felspar, green mica, and a very small proportion of quartz : it is in- termixed with the above mentioned ores of iron, in small groups, or nests ; and in the vicinity of these minerals, the rock appears to be altered. 6. This granitic rock is again succeeded by dolomite; which is about twelve yards in thickness, and is not so regular as the two preceding beds above described : its surface is not even ; for the dolomite penetrates a little into the granitic rock, and also into the granite on which it reposes. It still contains spathose iron ore, but is particularly rich in various kinds of iron glance. 7. Lastly, we arrive at the granite which constitutes the hills of St. Martin. It differs essentially from the rock 5, being fine-grained with black mica : but, notwithstanding this difference, we may rest assured that the granite rock contained in the dolomite is a ramification from the granite.* The description of the junction of granite and limestone, in this interesting locality, is somewhat blended with con- jecture, and does not furnish such minute details as Glen Tilt, owing, probably, to the sections being less favourable for observation. We shall revert to this example when we examine the nature of the evidence on which it has been concluded that this granite must be of more recent origin than limestone. Such is the condition of various primary strata at their junctions with the granitic rocks ; and though they do not all exhibit precisely the same appearances as those of Cornwall, yet the points of resemblance are sufficiently numerous to render it probable that the phenomena are of the same nature, being only somewhat modified by local circumstances. * Bulletin de la Soc. Grfol. de France, tomeii. p. 71. 14-3 CHAPTER VIII. ON THE VARIOUS MODES IN WHICH THE GRANITIC AND SCHISTOSE ROCKS ARE ASSOCIATED TOGETHER. The occurrence of granitic rocks !n the primary schists in the form of beds or courses in irregular bunches or masses in Cornwall, Scotland, Ire- land, and Norway. Portions of these schists also contained in granite in Cornwall, in various parts of Scotland, and in France. The primary strata traversed by granite-veins in Cornwall and in Scotland. Sum- mary of the various appearances presented by these veins. HAVING detailed in the preceding chapter the nature and position of the schistose rocks at their point of contact with granite, it is now proposed to consider how these stratified and unstratified rocks comport themselves, when more in- timately intermixed in the form of beds, dykes, veins, and detached portions of various irregular shapes and dimen- sions. Some of these circumstances have been necessarily an- ticipated, more or less, in the foregoing descriptions ; more especially the occurrence of large and regular masses of granitic rocks in the primary slates, as those of Cornwall, and analogous examples in other countries, of which those of Ireland, recorded by Mr. Weaver, are most minutely de- scribed. These granitic beds, commonly known in Cornwall by the name of elvan-courses, are sometimes so numerous, and so regularly placed in the slate, that they appear to alternate therewith; and, indeed, this mode of association is more regular and persistent than is often to be met with among the different kinds of primary slates ; for these pass into each other, at various points, through such frequent and almost impercept- ible gradations, that the same order cannot be detected. These elvan-courses, however, also exhibit great irregularities, if we descend from the general outline to a minute examination 144 ASSOCIATION OF [Ch. VIII. of their junction with the containing schists: they, in fact, display all the phenomena which occur at the junction of the main masses of granite and slate. Thus, the elvans often pro- trude or bulge out in the form of angular or rounded pro- tuberances, which sometimes appear to be separated from the course by open seams, which run parallel with the regular part of the elvan, and continue uninterruptedly in the same line ; so that if the projecting portion be small, it will often be found to form a part of the adjacent schistose concretion ; they, likewise, are sometimes elongated into lateral branches, or terminate in minute strings, after the manner of veins ; and it is no uncommon occurrence for insulated pieces of the elvan to be imbedded in the slate, and, vice versa, for those of the latter rock to be contained in the former. For farther particulars concerning the situation of these granitic beds in the slates, we must refer to the fourth and fifth chapters : those details ought to have been given in the present chapter, but by so doing we should have separated from them one of the most characteristic features of the schis- tose rocks, and thereby rendered the description very im- perfect. Besides these somewhat regular beds or courses of granitic rocks, irregular masses or bunches of various dimensions also occur in the primary slates. These are particularly abundant near the junction, as is well illustrated on the sea-shore near the village of Mousehole, in Cornwall ; where they are seen to be connected with the slate on all sides, sending out veins, passing into the slate by mineral gradations, forming, with portions of the slate, the same concretions, and also containing insulated pieces of slate of different sizes and shapes. It might be contended that these bunches of granite are merely pro- tuberances of the subjacent granite : in some cases, even at Mousehole, they probably are ; but that they are not always so, is demonstrated by the workings of Dolcoath mine, where several of these outlying masses of granite have been found to be perfectly insulated. Ch. VIII.] THE GRANITIC AND SCHISTOSE ROCKS. 145 That these appearances have not been more commonly observed, may be attributed to the want of favourable sections, and not to the primary rocks of other countries being dif- ferently constituted : an example of this nature has, indeed, been recorded by Mr. Weaver, as occurring in the eastern part of Ireland. " The brow of Tonelagee," he says, " ex- hibits bold precipices from four to five hundred feet in height. The northern and southern portions of this are composed of granite ; but, in the interval, there is a body of mica-slate about two hundred yards wide, including a bed of granite which varies from six to ten yards in width, besides irre- gularly formed masses of the same rock imbedded in and in- corporated with the mica-slate. It appears to be a prolongation of the body of mica- slate at the head of Glenmacanas, gradually narrowed in its western progress, and probably tapering to an edge, so as to constitute a kind of wedge- shaped mass inserted in the body of the granite. " * If any doubt should still remain as to the occurrence of granitic rocks as insulated masses in slate, it may be stated, that this does actually take place in the Herland mines, which are more than a mile distant from the nearest body of granite. Indeed, the irregular veins and bunches of granite in the gneiss of Scotland and Norway are of the same nature : they have been sometimes called granitic gneiss, and have been attributed by others to an injection of granite ; but let their origin be what it may, they are evidently analogous to the insulated portions of the granitic rocks in the slate of Corn- wall. Refer to the examples quoted in the fifth chapter, and more particularly to Macculloch's description of the Western Islands of Scotland, where " lumps of granite, apparently independent of veins, are often imbedded in the gneiss ; and when they are large, commonly give out branches or veins, which diverge in a very capricious manner." The granitic rocks are thus disseminated throughout the * Geol. Trans., vol. v. p. 145. L 146 ASSOCIATIONS OF [Ch. VIII. crystalline schist, under various circumstances, and in different forms ; so likewise the latter rocks occur in detached portions in the granite, not only at the junction, but in the main mass some distance therefrom. Examples of this kind in Cornwall are on a very small scale, and of rather an obscure nature : at the junction, in- deed, portions of slate are entangled with, or even enveloped in, the granite ; but even in this position they are not always very distinct, owing to the slate being of a compact and crystalline character, not readily distinguishable from the granite, when this rock has not its ordinary appearance. In every portion, however, of the granitic mass, small concretions may be observed, which have a dark colour and compact texture, owing, apparently, to the constituents of the granite being, at such points, very fine-grained, and intimately united with black mica, shorl, pinite, or other minerals, which are more abundant than in the general mass, attracted as it were round certain centres. But these appearances are better displayed in some other countries. In various parts of Galloway, Sir James Hall observed portions of slate in the granite at the junction of these rocks : at the Hill of Lauren, " the granite actually contains a mass of the stratified body included in its substance;" and "in the island upon which the Castle of Doon stands, angular fragments of killas are surrounded on all sides with granite." * And on the shores of Loch Doon, Dr. Grierson observed fragments of compact gneiss in the granite, which is here a common and prominent feature, but of more rare occurrence in the Dee district : " one of these fragments, of a tabular shape, was two feet long, about ten inches broad, and four in thickness. Some of them have disintegrated more rapidly than the granite, and so have left hollows in it, and have, indeed, fallen out : others have weathered more slowly than the surrounding granite, and are seen projecting from the t * Royal Trans, of Edinburgh, vol. vii. p. 102. Ch. VIII.] THE GRANITIC AND SCHISTOSE ROCKS. 147 surface. These fragments and the granite do not pass into each other, unless the manner in which they unite be called a transition ; they are perfectly distinct, but as intimately united as the alburnum of a tree with its wood : the termin- ation of the one, and the beginning of the other, are seen distinctly, but the mass is equally strong at the junction as any where else."* In the mountains called Ben na vear, near Balahulish, the schist which is traversed by the granite is often much indu- rated, and approaches by various undefmable gradations to a sort of hornblende-slate. Masses of a similar substance may be found imbedded in the granite. Occasionally these masses appear, on close examination, to be only irregular spots of hornblende, such as occur not unfrequently in those granites of which this mineral forms an ingredient. More often, how- ever, their shape is perfectly defined, being laminae with trun- cated edges : and in some places, this appearance of fracture is so precise, that when two fragments occur together in the granite, the imagination as easily replaces the separated parts, as it does in the brecciated marbles or agates : nay, farther, the fragments will be sometimes found to consist of an argillaceous or slightly micaceous schist, maintaining this character with scarcely a perceptible alteration, and sometimes only ap- proaching to hornblende-schist at its exterior parts. It is also worthy of remark, that these fragments sometimes exhibit, at their edges, stripes of different colours arid degrees of hard- ness, arising from the varying texture of the laminae which compose them. These masses vary in size from an inch to a foot and upwards ; but, whatever their size may be, they have almost invariably parallel sides. At Loch Rannoch, the schist, which is imbedded in the granite, is often composed of black shining mica. Towards the junction of the fragment with the surrounding rock, it generally contains crystals of hornblende. The fragments * Wernerian Trans., vol. ii. p. 384. L 2 14-8 ASSOCIATIONS OF [Ch. VIIL vary much in size ; and differ completely in aspect from those accumulated plates of mica, which are found in the granite of Aberdeen, as well as in many other granites. In other cases, the imbedded fragments consist of the same quartz-rock and gneiss which form the general body of these schistose rocks. The head of Loch Spey, a tract far removed, yet possibly not unconnected with this, is also composed of granite; and among this are found perfect granitic conglomerates, in which fragments of mica-schist, equalling in quantity the substance which connects them, are seen imbedded in a paste of granite. Occasionally these fragments are confounded with the granite at their edges; but at times are so defined, and even so separable, that I procured a specimen with a distinct vacant impression of a rectangular fragment, which had probably been detached; as cavities left in this way, by the wearing out of the schist, occur frequently in the rocks of the moor of Rannoch. * Although the schistose rocks are seen only at the two ends of this prolonged tract of granite, the imbedded fragments can be traced throughout the whole. Dr. Macculloch adds, " that if any one be unwilling to consider these imbedded portions of schist as fragments, it can only be said, that if they were really detached fragments, they could possess 110 other aspect than that which they now have." In short, the occurrence of angular and rounded portions of schistose rocks in granite has been observed in almost every primary country, exhibiting various appearances, according to their size and the proportion which they bear to that of the containing mass. " The granite of Braemar, of Aberdeen, and of other districts n Scotland," says Professor Jameson, contains portions of gneiss, mica-slate, clay- slate, and also of porphyry, syenite, and trap, varying in size from a few inches to many fathoms. These masses are to be seen passing by imperceptible shades into the bounding granite; thus showing * Geol. Trans., vol. iv. p. 126. et se but to confine our remarks to the question whether horizontal strata, re- posing on others which are inclined, furnish sufficient evidence that the latter have been upheaved before the deposition of the former. " To determine the general unconformable position of two rocks, sometimes requires very great care," says De la Beche*; * Geological Manual, 8vo, p. 481, 288 AN ENQUIRY INTO THE NATURE OF [Ch. XIII. " though, at first sight, it may appear extremely easy to observe, whether one rock rests on the upturned edges of another, or not ; and so it undoubtedly is, in many cases ; but when they meet at small angles, or the one rests on the contortions of the other, the enquiry becomes more difficult, and it requires numerous observations to be certain of the general fact. When the contortions are small, the evidence is distinct; but when on the large scale, the great bends being measured by miles, instead of fathoms, the subject is not so easy. It may be stated, as an example, that the mass of the calcareous Alps is considered to rest unconformably on the mass of those composed of protogine, gneiss, &c. ; but the situations where the contrary opinion may be formed are very numerous, the sections there exposing perfect conforma- bility. It also requires great care in tracing strata up to a mountain range, for the purpose of ascertaining its relative antiquity, to distinguish between those beds which have been decidedly upturned subsequently to deposition, and those which may have originally taken a small angle during their formation, on the flanks of a chain previously elevated to a certain extent." Again, the same author has observed, that " a general unconformability does not always prove a move- ment in the inferior rocks, prior to the deposition of the superior : for supposing a given series to be so produced, that the newer rocks may be formed within successively diminish- ing areas, and another deposit to cover the whole ; it is evident that the upper mass will so far rest unconformably on the in- ferior rocks, that it will cover them all in succession. Now, this is what has happened with the chalk and oolite groups in England. The angles at which the cretaceous and other rocks meet, in Dorset and Devon, are so small, that their unconformability could scarcely be determined at any par- ticular point, though in the mass it is evident." Thus we learn that, without entering on the more intricate problem of the nature of these inclined and horizontal strata, Ch. XIII.] THE INCLINED POSITIONS OF STRATA. 289 it is not even an easy matter, at all times, to determine the fact, whether strata are or are not conformable. Sometimes groups of strata are found in mountain ranges, so differently inclined, that it has been deemed necessary to call in the aid of successive elevations, instead of one single convulsion, in order to account for these occurrences : indeed, they exhibit on the large scale the same intermixture of in- clined and horizontal, straight and curved strata, as the laminae of individual strata do, on a small scale, as detailed in the preceding chapter ; and which it was attempted to show may depend on the angle at which the beds have been deposited, or on the structure which they have assumed during consolidation. Now the known irregularity of the present surface of the earth on which sedimentary rocks are forming, and the variously inclined seams which occur in shapeless masses of gravel and sand, would, a priori, lead us to expect, that the strata in any given district would exhibit different bearings and different degrees of inclination ; and if these various dips be evidence of the elevation of the strata, how numerous and complicated these actions must have been. And thus it is, that most of the advocates of this doctrine have found it necessary, whenever they extend their observa- tions, to call in the aid of many convulsions, where the author of the theory only laid down one anticlinal axis : Dr. Hibbert, for instance, in his researches on the Basin of Neuwied, ascertained that one derangement, corresponding with the valley of the Rhine, would not explain all the phenomena, as these require at least six or seven catastrophes for their satisfactory elucidation. But we need not dwell on a topic which has already been so ably handled by Boue, who has satisfactorily shown that De Beaumont has founded his specu- lations on too narrow a basis in this respect, since all sub- sequent details concerning the bearings of mountain ranges, have proved that the inclinations are much more complicated than was originally supposed. It remains, therefore, for 290 AN ENQUIRY INTO THE NATURE OF [Ch. XIIL farther investigations to ascertain, if possible, whether many of the appearances, which are now attributed to forcible elevations by the protrusion of igneous rocks, may not be explained b} the operation of the less energetic, but not inefficient causes which have been indicated. If, then, in the examination of the secondary rocks, so many difficulties present themselves, in determining the nature and Origin of their inclined strata, it is not surprising that the more complicated structure of the primary slates renders this enquiry still more perplexing. Some may be of opinion that, in the latter case, the evidence of elevation is much more decisive, since the crystalline stratified rocks almost universally dip at great angles, and abound in curves, contortions, and other supposed marks of violent catastrophes. On reference to the former chapter, it will be seen to what extent the primary slates are admitted to be analogous to the secondary rocks, it having been attempted to show that both are traversed by seams or joints, which arise from their concretional structure; and it is now proposed to offer arguments in support of the opinion, that the layers interposed betwen these parallel seams are in their original position. It is, however, generally asserted, that the primary strata have been tilted up to their present highly inclined positions. But let us enquire, what are primary strata ? This may be deemed superfluous, since they are such well-known objects, described by most geological writers, who have found no difficulty in recording their directions, dip, and other bear- ings. We must, however, confess that, after much personal examination, and the attentive perusal of the labours of others on this subject, we cannot answer this question. Professor Sedgwick and some other geologists have re- garded the primary strata of Cornwall to be parallel with the laminae of these rocks. In conformity with the usual practice of giving the dip of the schistose rocks, we also followed the Ch. XIII.] THE INCLINED POSITIONS OF STRATA. 291 same course, and selected the bearings of the laminae, because it is a point easily determined, whenever a small portion of rock is exposed ; and because there is an advantage in having a certain criterion to guide us in forming our judgment. But it does not follow that this selection is correct. It is true that the position of the laminae frequently corresponds with that which horizontal beds might assume, if tilted up ; but this is not always the case, for the laminae sometimes dip towards the granite, as well as in the opposite direction. Again, it has been shown that these rocks are intersected by other parallel planes, which cross the laminae at various angles, but have the same bearing or strike ; so that, as the planes of the laminae most frequently dip from the granite, those of the other series incline most commonly towards this rock. Now it has been already stated, that the fissile struc- ture of the slates is perfectly independent of the so called strata, since they are arranged at various angles thereto, in rocks not only of the primary, but also of every other geologi- cal epoch. Since, therefore, this structure does not infallibly denote the supposed order of deposition, why determine on the planes parallel thereto, in preference to those which have an opposite dip ? There is one reason why the latter might be preferred: viz., that these planes are for the most part parallel with those of the massive rocks (greenstones, porphyries, and the like) with which these slates are as- sociated. In Cornwall, the lamellar system of planes seldom attains great angles, but fluctuates between 20 and 4-0; whereas the others vary from 4?5 to 70, and even higher inclinations. What has been the rule for determining the selection in other countries ? It does not appear that any fixed plan has been followed : sometimes, however, it has been cursorily remarked, that the laminae cross the planes of stratification. On referring to Weaver's description of Ireland, in the fifth chapter, it will be seen that the mica- schist and clay-slate, u 2 292 AN ENQUIRY INTO THE NATURE OF [Ch. XIIL next the granite, are stated, in some places, to dip at moder- ate, and in others, at considerable angles, sometimes from the granite, and at others towards this rock ; so that, if con- tinued, they would either abut against, or underlie the granite. The same statements have been made by Maccul- loch, and other writers. Now, considering the hitherto un- defined nature of the term stratum, we would enquire whether it is probable, that geologists have always selected the same denomination of planes ? and whether they may not have regarded as the strata, sometimes one series of planes, and sometimes another ; of which there are three series ; one per- pendicular, and two others variously inclined, as described in the chapter on the structure of the primary slates. May not this have happened, in some cases, from one set of planes being occasionally more distinctly developed than the other ? or from the degradation, in cliffs and similar places, not having been always confined to the same direction, having been more extensive in one than another, and vice versa ? Lastly, we would ask, if these strata have been upheaved by the granite, how comes it to pass, that whichever series of planes be adopted as true strata, that, in Cornwall, the lines or seams which mark these planes are continued into the granite itself, dividing this rock into corresponding layers? This fact is not obscurely indicated on a single spot, but is displayed, in many places, in clear and bold characters, which cannot be mistaken ; and which certainly cannot be recon- ciled with the idea of the forcible elevation of the strata, and of the protrusion of the granite. On the grounds detailed in this chapter, we therefore con- clude that the inclined position of strata is not an infallible criterion of mechanical elevation ; that this appearance in the primary slates may be more justly attributed to their original structure, and in many of the secondary strata to the same cause conjointly with their deposition on inclined surfaces; and, that the notion of granite having been protruded through and tilting up the strata, either in a fluid or solid state, does not Ch. XIII.] THE INCLINED POSITIONS OF STRATA. 293 appear to be countenanced by reference to the known effects of igneous agency ; and, lastly, that the situation of strata adjacent to trap-rocks, the supposed connecting link between the granitic rocks and existing volcanic products, is of too intricate and conflicting a nature to be received as positive evidence of such an occurrence. u 3 294 ON THE IGNEOUS METAMORPHOSIS [Ch. XIV. CHAPTER XIV. ARE THE PRIMARY SCHISTOSE ROCKS SEDIMENTARY DEPOSITS ALTERED BY THE CONTACT OF IGNEOUS ROCKS? Passage of primary slates into secondary rocks. Transitions of two kinds - mineral and mechanical. The latter no criterion of identity. The relative position of strata not easily determined. The lias group of the Botzberg. The capability of caloric to alter rocks. Lyell's remarks on this subject. Sir James Hall's experiments on the fusion of rocks. The state of rocks next actual lavas also next trap in Skye in Anglesea in Ireland. Dolomization. Strata next igneous rocks only partially altered. These changes similar to the effects of caloric condition of primary strata next granite indicates a transmutation of one earth into another. The contact of ignited granite and sedimentary deposits disputed. The arrangement of primary slates incompatible with their being altered deposits. These slates contain elements not found in sedimentary rocks. The transfusion of alkalis analogous to that of magnesia in dolomite. Objections to such transfusions. The primary slates of Dartmoor and the Hartz said to be altered greywacke". Those of the Alps altered oolitic strata. The association of gneiss and limestone in Jungfrau of granite and limestone in Glen Tilt and the Pyrenees. Conclusion. THE metamorphosis of stratified deposits by the contact of igneous rocks in a state of fusion or incandescence, appears to be established by such a legitimate train of deductions, that it will probably be esteemed an act of great presumption to dispute its accuracy. And any objections advanced against this hypothesis, may naturally meet with a less candid consid- eration than the topics already discussed; because it is the offspring of the Plutonic theory, which has grown with its growth, and which certainly is so plausible, and so well adapted to give strength and support to its parent, and is withal so fascinating, that even its adversaries must attack it with no little reluctance. In a former chapter it was attempted to prove, that the evidence by which the analogy between the primary and Ch. XIV.] OF THE STRATIFIED ROCKS. 295 secondary strata is thought to be substantiated, is not free from objections. The same difficulties may be equally advanced on the present occasion ; for, if the prevailing theory fails to show that the primary slates were once in the condition of sedimentary deposits if it fails to demonstrate that the former, at the period of their formation, did perfectly resem- ble the latter then it does not follow that the present nature of the crystalline slates has resulted from any superinduced action or metamorphosis ; on the contrary, their existing state may be original. In the previous examination of this question, one important consideration which has been ad- vanced in support of the crystalline slates being altered rocks, was omitted : before proceeding, therefore, to discuss the capability of caloric to effect such changes, the subject of this omission must engage our attention. In tracing the nature and position of the stratified rocks which surround granite, from distant points up to the central unstratified mass, it has been remarked, that the fossiliferous strata appear to pass gradually into the primary slates which are in immediate contact with the granite. And, since these strata belong to various geological epochs, in different coun- tries, it has been concluded, that the primary slates are altered rocks belonging to the same periods as the adjoining sedimentary deposits. This is clearly and distinctly propounded ; and is an argu- ment not to be easily opposed, for it is based on the observa- tions of the most experienced geologists ; and it can, therefore, be only disputed by questioning the accuracy of their con- clusions. This is a painful alternative, and more especially as we cannot appeal to Nature, and positively demonstrate the fallacy of the data from which they have formed their deductions ; but can only advance our views of the subject, which may, very probably, prove equally fallacious. To differ from the best authorities on such grounds, may be deemed an unpardonable proceeding ; but, as these distinguished phi- losophers are lovers of truth, we feel assured that they will u 4f 296 ON THE IGNEOUS METAMORPHOSIS [Ch. XIV. candidly receive any suggestions which may lead either to the subversion, or to the firmer establishment of the theory which they advocate. It is proposed, in the first place, to examine the nature of the transition or passage which occurs among the secondary rocks, and between these and the primary slates ; and ascer- tain whether the evidence thus furnished is conclusive, that all rocks so connected have had the same origin, and belong to formations of the same epoch. In the primary rocks, transitions appear to be effected in two ways : in the one, the crystalline constituents, when the rocks are compound, or the component particles, when sim- ple, gradually pass from large and coarse concretions to par- ticles so minute as not to be detected by the eye ; and during this change, rocks of very various aspects are produced, which are farther varied by a difference in the proportion of their component parts : this transition is very similar to that which commonly obtains amongst the secondary rocks, owing to the intermixture of two deposits ; either in consequence of the sediment following the laws of gravity during its accumula- tion, or to the gradual change of circumstances under which they have been produced. In the other case, however, the transition between different primary rocks is accomplished by a gradual change, not only in the proportions, but also in the mineralogical characters of the constituent parts. Thus, the felspar and quartz of granite pass into granular and compact felspars, that is, into eurite and felsparite; mica, by degrees, assumes the properties of talc, shorl of horn- blende, hornblende of diallage, or of chlorite ; in short, many other transformations well known to geologists, and which, of course, are accompanied by a corresponding difference in the nature of the rocks. The latter may be called a mineral ; the former, a mechanical transition. When crystalline rocks are found to pass into each other by mineral transitions, it may be presumed that they do belong to the same formation : but when the passage is Ch. XIV.] OF THE STRATIFIED ROCKS. 297 effected by a mechanical transition, numerous facts clearly show that the same conclusion must not be drawn ; thus, it is difficult to lay down the precise line between the tertiary and the secondary formations, or, indeed, between some of the individual beds of their respective groups ; all of which have evidently been formed at distinct periods. This being the case, is it at all surprising, that it is difficult to separate the primary slates from the fossiliferous strata by which they are immediately succeeded ? In Cornwall, a re- mote portion of the calcareous series contains organic remains characteristic of the oldest beds of the greywacke, or transi- tion group ; and there is no point between the slates of the porphyritic series in contact with the granite, and these fos- siliferous beds, which can be asserted to mark the boundary between these series : though, as we have stated in another place, the same facilities for examination cannot be obtained as are offered at the junction of the slate with granite. Now, what happens in Cornwall has been frequently observed in several parts of the Alps and other districts, with the excep- tion of the secondary strata belonging to different groups : but surely this apparent transition is no positive evidence that all the rocks so connected are of the same epoch ; we have said apparent transition, as we are not aware that this has been actually determined by one continuous section in- tersecting these rocks, and perfectly exposed to view through- out its whole length. It has already been shown, that the conglomerates and greywacke of Nare Point, Helford Harbour, in Cornwall, pass by insensible degrees into the adjacent slates, from the detritus of which they have been formed : and it is therefore evident, that the parent slates and the derivative rocks can- not have had the same origin. And the same conclusion is applicable to some appearances of this kind in Caithness, referred to in the last chapter. The strata of the old red conglomerate have been, according to Sedgwick and Mur- chison, tilted up by the protrusion of solid granite ; during 298 ON THE IGNEOUS METAMORPHOSIS [Ch. XIV. which catastrophe the latter rock was so shattered as to form a considerable bed of granitic conglomerate, interposed between the red conglomerate and the intrusive mass : here, then, we have three rocks, not only produced in different ways, but at three distinct periods, and yet they all gradually pass into each other. Thus, also, according to De la Beche, " the undoubtedly mechanical greywacke around Dartmoor gradually passes into rocks having the characters of mica-slate next the granite, as shown on a line drawn from the southern part of Dartmoor to the sea, at the Bolt Head or the Prawle, a line which can be examined for the greater part along the sea- cliffs." It is very probable that the fragmentary rock has, in this case, been formed of debris not far removed from the cry- stalline slates ; and it may, in many parts, appear to pass into the latter, because, when the parent rock is partially changed by an incipient decomposition, it cannot be easily distin- guished from the finer varieties of greywacke. In the cliffs in Mount's Bay, in Cornwall, the felspathic rocks, for instance the greenstones, become of a different colour during decomposition, which of course dies away by imperceptible shades, as we descend to the perfect rock : now suppose the disintegrated part to be again consolidated, the transition would then be perfect, and yet the upper and lower strata would not be precisely the same. But this case may be put in a much stronger point of view : for this bed of argil- laceous debris, containing fragments of quartz and the harder portions of the parent rock, is sometimes thirty feet or more in thickness, and, at the bottom of the cliff, reposes on a horizontal stratum of sea-sand; and this deposit tapers upwards towards the hill, till it terminates in a wedge-shaped mass, such as so commonly belongs to secondary strata : at its upper part it is connected with the loose part of the decom- posing rock just described, which has clearly not been moved, for the quartz-veins continue their course therein without Ch. XIV.] OF THE STRATIFIED ROCKS. 299 alteration, whilst they are cut off and disappear in the wedge- shaped mass, which circumstance, conjoined with the position on the sand, indicates that the upper bed of debris has been transported : the time may come when these deposits will be converted into stone, and then would future geologists find derivative rocks, associated with sandstone, containing organic remains of the modern epoch, and yet passing by the most perfect gradation into crystalline slates, which are in contact with granite, and abundantly intersected by granitic veins. It will be again necessary to revert to this topic, when the well-known observations of Hugi, Studer, Beaumont, and others, on the alteration of the strata by granite in the Alps, come under consideration ; and, after making allowance for the distorted and disproportionate appearance of their sections, the dissimilarity of these phenomena and those just detailed is not so great but that it may be possible hereafter, when we are more minutely acquainted with the facts, to offer a plausible explanation, without having recourse to the supposition that the crystalline slates were once sedimentary deposits. By way of illustration, take the Botzberg in the Swiss Alps, described by Hugi and Studer. It is stated that the strata dip towards the granite, and that the latter rock, in descending the mountain, is succeeded by gneiss, which, in its turn, gives place to mica-schist, followed by limestones and slates belonging to the lias group. De la Beche, from whom we quote, remarks that, " assuming the section to be correct, the superposition of the crystalline rocks, in this case, is evi- dent * ; " and he offers two explanations of this unusual position ; one referring it to the strata being thrown over, the other to the granite overflowing the fossiliferous beds. Now it has been already shown, that it is no uncommon occurrence to find parallel seams or joints traversing the secondary rocks as well as the primary, and dipping toward the granite, but * Geological Manual, Svo. p. 456. 300 ON THE IGNEOUS METAMORPHOSIS [Ch. XIV. it does not follow that these correspond to the planes of deposition ; so the liassic slates and limestones of the Botz- berg may successively present themselves at the surface, exhibiting layers which decline towards the granite, and yet the entire mass of each of these rocks may be wedge-shaped, basin-shaped, or of any other form, ranging even in the op- posite direction. The term stratum, and all its properties of bearings and dip, both longitudinal and lateral, have been so variously and indefinitely used, that we cannot form any correct idea of the relative position of rocks, unless all the minute circumstances of the case have been carefully de- tailed. If, in addition to this difficulty, it be remembered that the earthy materials of the liassic slates have very pro- bably been derived from the debris of the crystalline rocks, immediately or through the medium of a previous migration ; and, since this debris uniformly contains scales of mica, more or less abundant, it is not surprising that a mechanical transition should here occur ; but, at the same time, we cannot admit this as infallible evidence that the mica-slate and gneiss are identical with the strata of lias, only differing therefrom in consequence of having assumed a new aspect, superinduced by their contact with granite in a state of incandescence. It is time, however, to discuss the capability of caloric to effect such metamorphoses, a topic which can be more satis- factorily approached, since the arguments advanced in its support depend more on facts than opinions. " The geo- logist," says Lyell, " has been conducted, step by step, to this theory, by direct experiments on the fusion of rocks in the laboratory, and by observation of the changes in the composition and texture of stratified masses, as they approach or come in contact with igneous veins or dikes. In studying the latter class of phenomena, we have the advantage of examining the condition of the rock at some distance from the dike, where it has escaped the influence of heat, and its state where it has been neai' to or in contact with the fused Ch. XIV.] OF THE STRATIFIED ROCKS. 301 mass. The changes thus exhibited may be regarded as the results of a series of experiments, made on a great scale, by nature, under every variety of condition, both as relates to the mineral ingredients of the rocks, the intensity of heat or pressure, the celerity or slowness of the cooling process, and other circumstances." * We need not enlarge our details by relating the well- known experiments of Sir James Hall on the fusion of rocks, by which we learn that a melted rock assumes various ap- pearances on consolidation, according to the rate of cooling. We would, however, gladly enumerate the conditions of sedimentary rocks at their point of contact with currents and dikes of recent lava, that is, of undoubted volcanic rocks ; but we do not remember that such have been recorded, for although the interference of lava of the modern epoch with strata has been described, yet such lava is ancient, and cannot furnish evidence so important as that which is now actually produced by existing volcanos. The altered appearance of sedimentary deposits next trap is of very common occurrence, and we shall therefore quote a few examples, in order to show the precise nature of this phenomenon. At Duntulm, in the north-western part of Skye, an ob- scurely columnar trap covers beds of shelly limestone and of sandstone, containing shells and carbonised wood, alternating with shale : the junction of the trap and strata is, in many places, attended with great confusion. The upper part of these strata, about twelve or fifteen feet in thickness, is divided into thin laminae, which have the appearance of shale alter- nating with limestone : but when fragments are broken off and closely examined, the schist is found to be of a black colour, very hard and brittle, sharp in the fragments, and with an obscurely rhomboidal fracture ; in short, it is a kind of sili- ceous schist, a Lydian-stone : the sandstone is found to possess * Principles of Geology, vol. iii. p. 367. 302 ON THE IGNEOUS METAMORPHOSIS [Ch. XIV. great hardness and a jaspideous aspect, similar to that sandstone which is in contact with the greenstone of Stirling Castle. " Considering, therefore, the analogy of these two sand- stones, we may fairly conclude," says Dr. Macculloch, " that they have, in these instances, been altered from their original texture, in consequence of the proximity of the trap-rock : and we have an equal right to conclude that the same influ- ence has also converted the shale into Lydian-stone. This, in fact, is the position of every specimen of this stone which I have seen in Scotland. In Conachan, in Raasa, in Shiant, at Talisker, it forms beds in contact with and involved in trap, which, from their connection and position, appear to have been common clay-slate."* But one of the most important examples which has hitherto been adduced on this subject, is the modification of strata next a basalt dike, near Plas Newydd, in Anglesea, described by Professor Henslowe in the Cambridge Philosophical Transactions. " The dike (we quote from Lyell) is 134 feet wide, and cuts perpendicularly through strata of shale and argillaceous limestone, which are altered to the extent of thirty feet from the dike. The shale, as it approaches the basalt, becomes gradually more compact, and is most in- durated nearest the junction, where it loses part of its schis- tose structure, but the separation into parallel layers is still discernible. In several places the shale is converted into hard porcelainous jasper : in the hardest parts of which the fossil shells, principally Productse, are nearly obliterated ; yet even here their impressions may be frequently traced. The argillaceous limestone undergoes analogous mutations, losing its earthy texture as it approaches the dike, and becoming granular and crystalline. But the most extraordinary phe- nomenon is the appearance in the shale of numerous crystals of analcime and garnet, which are distinctly confined to those portions of the rock affected by the dike."f * Geol. Trans, vol. iii. p. 99. t Principles of Geology, vol. iii. p. 368. Ch. XIV.] OF THE STRATIFIED ROCKS. 503 " The induration," says Dr. Berger, " which the secondary rocks undergo, when traversed by dykes of trap, has often been noticed ; it is not my intention now to discuss this sub- ject: I shall only mention that the induration does not extend far from the dyke, and that the phenomena, though very fre- quent, are not universal." * " In general, there is no foreign matter between the substance of the dyke and the rock it divides, except a slight rusty appearance on the surface of the latter. The contact between the two is pretty close, but they may always be disjointed by the blow of a hammer." f Before proceeding to make any remarks on these observa- tions, it will be well to relate the nature of the phenomena alluded to, as detailed in the same paper, on the north-east of Ireland, by the Rev. W. Conybeare. " At Redbay Castle, several basaltic dikes traverse the conglomerate : one of these, remarkable for its great thickness, having resisted the action of the waves, which have encroached considerably on the adjacent cliffs, presents some bold detached crags projecting from the beach. The conglomerate forming the wall of the dike has undergone a great degree of induration, its cement assuming the appearance of a compact hornstone ; thus it has been enabled to oppose to the sea a resistance almost equal to the basalt itself, and is still seen adhering on the sides of the advanced crags." J " The whin dike of Carrick Mawr forms a broad causeway, traversing the beach, and terminating in a nearly insulated mass of rocks rising about thirty feet ; of this mass only the central line consists of the dike itself, the sides being evidently composed of portions of the strata traversed by it, but much altered in their character and degree of induration by its contact. These beds appear to have been chiefly derived from the slate-clay of the coal measures, which has become so compact as to assume the character of flinty slate. In one point, this rock may be seen on one side, and on the other, * Geol. Trans., vol. iii. p. 230. f Idem, p. 231. I Idem, p. 201. 304? ON THE IGNEOUS METAMORPHOSIS [Ch. XIV. the sandstone grit which usually accompanies the coal beds, also in a highly indurated state ; its colour changed from red to white, and its mass penetrated by minute grains of iron pyrites. At fifteen yards distance from the dike, the alter- ation ceases, and the sandstone resumes its usual character, becoming reddish and destitute of pyrites. Where the dike traverses the great insulated mass of slate, it is very irregular both in thickness and direction. The works of the Gob col- liery have reached this dike 500 yards inland from the face of the cliff : the coal is altered by it to a considerable distance from its point of contact, being reduced to the state of a cinder, which can be employed only for burning lime. This dike throws out the measures of Gob colliery, which are not recovered on its eastern side : its breadth is about twelve feet where it comes to the surface of the cliff, but varies consider- ably in different parts of its course." * " The peninsula of Portrush, which may be about a mile in circumference, is fenced with low cliffs on the west, north, and east ; those on the west present a rude prismatic green- stone ; those on the north and east, tabular masses of green- stone, overlying, and in some places appearing to alternate with a very remarkable rock. It is a flinty slate, exactly similar to the indurated slate-clay which forms the wall of the Carrick Mawr dike, in the Ballycastle collieries ; and the analogy is rendered the more striking from the further resem- blance of the greenstone of that dike to the greenstone of these cliffs. In this flinty slate are contained numerous im- pressions of Cornua ammonis invested with pyrites, the shells being similar to those found in the slate-clay underlying the chalk near Ballintoy : and we felt convinced, while examining the spot, that the rock was no other than the slate- clay of the lias formation, in an indurated state." f " The chalk is frequently traversed by basaltic dykes, and often undergoes a remarkable alteration near the point of * Gcol. Trans., vol. iii. p. 205. f Idem, p. 212. Ch. XIV ] OF THE STRATIFIED ROCKS. 305 I contact ; where this is the case, the change sometimes extends eight or ten feet from the wall of the dike, being at that point greatest, and thence gradually decreasing, till it becomes eva- nescent. The extreme effect presents a dark-brown crystalline limestone, the crystals running in flakes as large as those of coarse primitive limestone ; the next state is saccharine, then fine-grained and arenaceous ; a compact variety, having a por- celainous aspect, and a bluish grey colour, succeeds : this, to- wards the outer edge, becomes yellowish white, and insensibly graduates into the unaltered chalk. The flints in the altered chalk usually assume a grey yellowish colour; the altered chalk is highly phosphorescent when subjected to heat." * We must not quit the subject of the metamorphosis of limestone by the contact of trap, without noticing the crys- talline magnesian limestones which are supposed to have been originally common limestone converted into dolomite by porphyry and similar igneous rocks. Many objections might be urged against the possibility of the introduction of mag- nesia into this rock ; but the difficulties of the case have been so fairly stated and discussed by De la Beche f, that we need not insist on the imperfect state of this hypothesis. There is, however, one point which may be alluded to, because it is directly opposed to the nature of the supposed altered chalk of Ireland; viz., the occurrence of organic remains in dolo- mite, which, in the other case, are said to have been obliter- ated by the fusion requisite to impart a crystalline texture to the earthy carbonate. We have been thus particular in detailing these examples at some length, not only because the original works may not be within the reach of all our readers, but also because it is important to bring all the minutiae of these cases into one point of view. It is, therefore, by these, and numerous phenomena of the same nature, clearly established that the stratified rocks, * Geol. Trans., vol. lit. p. 172. f Geol. Manual, vo., p. 474. etseq. X 306 ON THE IGNEOUS METAMORPHOSIS [Ch. XIV. immediately in contact with trap, basalt, and analogous rocks, do not always present the same appearance as at a distance therefrom : and since the latter rocks, in all probability, are of igneous origin, it has been concluded that this difference has been caused by these rocks having been brought into contact with the strata in a state of ignition ; whereby the sedimentary deposits have been variously affected, according to their composition. This view of the subject is further corro- borated by the fact that these changes, with the exception of dolomite, are such as might be produced by heat. There is, however, a most important consideration arising out of this investigation, which must not be overlooked. Admitting that these changes may have arisen by the presence of intensely heated trap rocks, how comes it to pass that a like cause has not always produced a corresponding effect ? How is it, if these rocks have been intruded among the strata in a state of ignition, that they have not equally altered the same rock throughout their entire course ? For it is not easy to conceive the reason why the basalt- dikes in Ireland should change the chalk only in insulated patches ; or why dikes of the same nature sometimes reduce coal into coke or cinder, and at others traverse it without producing any alteration ; or why the sandstones and shales are not always converted into horns tone, jasper, Lydian stone, and flinty slate, when intersected by igneous rocks. That this anomaly really does exist, might be proved by numerous details ; but it is a fact so often noticed, that it may suffice to refer to the statement above quoted from Berger, that the phenomena of induration of strata by the contact of basalt, though very frequent, are not universal ; and to the works of Macculloch, which, among many other examples, record that, in Skye, large dikes of basalt, running perpendicularly through the sandstone at Stratnaird, have not affected the texture of the rock ; whilst at Duntulm, in the same island, the sandstone at the points of contact have assumed a jaspideous aspect. The same anomaly also obtains in the case of the dolomite Ch. XIV.] OF THE STRATIFIED ROCKS. 307 in contact with augite porphyry in the districts of the Lakes Orta, Maggiore, and Lugano, as described by Von Buch : dolomite is generally interposed between the limestone and porphyry, but, in several instances, the limestone joins the porphyry, without having experienced the metamorphosis attributed to the presence of this igneous rock. The supporters of the Plutonic theory must often have observed similar facts, but we do not know whether they have made an attempt to explain how a heated mass, capable of effecting such changes, could have remained in contact with strata, without producing any alteration : if this subject has not yet attracted their attention, it is deserving of their serious consideration. In conducting this enquiry, it may be suggested that the position of the igneous rock should always be marked, and the precise parts, whether upper or under, where the strata have been changed ; and the degree of alter- ation that has taken place, in short, all particulars, should be noted; for circumstances, which may often appear to the enquirer minute and trivial, may be hereafter regarded as very important, and tend to throw some light on this subject. In the mean time, it is proposed to admit, for the sake of argument, that the alteration of the strata, above considered, has been produced by heat; and, after having stated the nature of this change, to enquire whether the metamorphoses which the primary slates are supposed to have undergone are analogous. The strata next the trap rocks are sometimes indurated, assuming such an appearance as might be supposed would result from a long-continued and high temperature; and sometimes they appear to have been altered by a partial or total fusion : thus, sandstone assumes the character of hornstone, jasper, or even solid quartz: clay-slate and shales are converted into flinty slate and Lydian stone, and even garnets are then found which did not previously exist in the schistose rock ; and secondary limestones become perfectly crystalline, their organic remains at the same time disappearing, so that they x 2 308 ON THE IGNEOUS METAMORPHOSIS [Ch. XIV. cannot be distinguished from some primary marbles. But, great as these changes undoubtedly are, they may all be attributable to the action of heat alone, except in the case of dolomite, where a new element is supposed to be introduced ; a supposition, however, which must be established on more certain evidence before it be an acknowledged exception to this general rule. Much light cannot at present be thrown on this subject by a reference to existing chemical analyses ; but a table is sub- joined, in the hope of directing attention to the importance of this evidence. That experiments of this nature have not hitherto been either numerous, or very accurate, is not surprising ; for an indiscriminate analysis of these rocks offers little inducement to the chemist, because the compounds are very indefinite, on account of the constant variation in the proportions of the constituent parts. But if the geologist would carefully select important examples, minutely recording all the circumstances of position and association, then the assistance of the chemist would be a valuable acquisition. Such an union has, indeed, already commenced ; and the ex- ample of Turner and De la Beche must be hailed as an important event in the history of our science. Imperfect as the following table is, it teaches us that the vol- canic or igneous rocks generally contain an alkali, whilst the derivative slates are devoid of this substance ; indeed, we might conclude, a priori, that all rocks composed of earthy sub- stances, derived from the decomposition of crystalline form- ations, would be so circumstanced, since the alkali is always extracted from such during chemical disintegration. Ch. XIV.l OF THE STRATIFIED ROCKS. 309 A TABLE OF THE COMPOSITION OF SOME ROCKS WHICH OCCUR IN THE FOSSILIFEROUS GROUPS. Sil. Alum. Mag. Lime, Pot. Soda, Iron Oxide. Mang. Carb. Basalt (Klaproth) 44-50 16-75 2-25 9-50 2-06 20 0-12 (Kennedy) 48 16 9 4 16 Compact felspar (Mackenzie) 71-17 13-60 0-40 3-19 1-40 0-10 Porcelain jasper (Rose) 60-75 27-25 3 3-66 2-50 Clinkstone (Klaproth) 57-25 25-50 2-75 8-10 3-25 0-25 Pitch stone (Klaproth) 73 14-50 1OO 1-75 1-00 0-01 (Knox) 72-80 11-50 1-20 2-85 3-03 Pumice 77-05 17-05 3-00 1-75 Flinty slate (Wiegleb) 75 the remainder being lime, magnesia, and oxide of iron. Lydian stone Clay slate (Kirwan) 38 26 8 4 14 _ Drawing slate (Wiegleb) 64-06 11 2-75 11 Adhesive slate (Bucholz) 58 5 6-50 1-50 9 (Klaproth) 82-50 0-75 8 0-25 4 , 0-75 Kaolin (Rose) 52 37 ~~ 6-33 It will also be remarked, in this and a succeeding table, that porcelain jasper, and analcine, both of which are said to have been formed by the metamorphic action of basalt on sedimentary strata in Anglesea, contain an alkali. These substances, therefore, differ from all similar altered rocks, and must be added to dolomite in the list of exceptions. We might attempt to solve this difficulty, by suggesting that, as part of a bed of compact felspar often possesses a schistose structure, as does also even a mass of crystalline lava, so might this basalt, and the analcine might be confined to such slate at the junction of the igneous and aqueous rocks ; or the jaspideous rock might have resulted from the combination of the fixed basalt with the shale at their point of contact. x 3 310 ON THE IGNEOUS METAMORPHOSIS [Ch. XIV. It is time, however, to turn our attention to the primary rocks ; in order to enquire whether the slates of this class owe their crystalline condition, and their exemption from organic remains, to the influence of granite, and other un- stratified rocks, in a state of ignition. " According to these (Plutonic) views," says Lyell, "gneiss and mica-schist may be nothing more than micaceous and argillaceous sandstones, altered by heat ; and, certainly, in the mode of their stra- tification and lamination, they correspond most exactly. Granular quartz may have been derived from siliceous sand- stone, compact quartz from the same. Clay-slate may be altered shale ; and shale appears to be clay that has been subject to great pressure."* Without, however, following this exposition farther, we will proceed to show that the changes which the sedimentary strata exhibit, next trap, are not perfectly similar to the condition of the primary slates ; thus furnishing an additional argument against the analogy which is supposed to exist between the primary and fossili- ferous strata. It has been attempted to show, in a former chapter, that the lamination or structure of these rocks, though often similar, does not depend on stratification, or rather on the mode in which they have been deposited ; and in the begin- ning of this chapter, it has been advanced, that the transition between the primary slates and the adjacent secondary strata does not infallibly prove that they belong to one and the same epoch. Still, however, even considering these topics to be disposed of, the question whether the primary strata are altered rocks, involves some other important considerations i and none of greater moment, than whether granite has actually been in a state of igneous fusion since the formation of the primary slates; for the whole question necessarily depends on this point, which we think has not been satis- factorily established, but which for the present may be conceded. * Principles of Geology, vol. iii. p. 373. Ch. XIV.] OF THE STRATIFIED ROCKS. 311 If, then, granite has been brought into contact with the .stratified rocks in a state of incandescence, what effect has it produced thereon ? If its action has been analogous to that of trap, " it must have altered their texture ; and this alter- ation must exhibit every intermediate gradation between that resulting from perfect fusion, and the slightest modifica- tion which heat can produce." Now, what is the fact ? The primary slates, at their junc- tion with granite, have generally a different appearance from those parts of the rock that are more distant j and they, in such situations, very commonly possess a finer texture, and a greater degree of hardness : but here the analogy with the altered condition of the secondary strata next trap terminates ; for a closer scrutiny will show, that the difference, in the case of the primary slates, is not such as heat alone could effect, it has not arisen from any modification of induration or fusion, but by a chemical change in the proportions, or the nature of their constituent parts : and this circumstance not only applies to the schistose rocks, but in very many cases is participated in by the granite. Refer to the seventh chapter, describing the nature of these rocks at their junction, where several examples are minutely detailed. Can it be supposed, however potent caloric may be to change clay-slate into flinty slate, and sandstone into jasper, that it can convert one earth into another, alumina into silica, as indicated by the quartzose nature of clay-slate and other schists, when in con- tact with granite ; or lime into silica, as exemplified in the limestone of Glen Tilt, where, as it approaches the granite, " its crystalline texture disappears, and it gradually assumes the characters of hornstone, effervesces slowly with acids, and gives, on analysis, a large portion of siliceous matter." This transmutation of one earth into another, may not present any serious difficulty to those who can give credence to the hypothesis concerning the dolomization of limestone : but it is to be hoped that geologists will not sanction an opinion so utterly at variance with the principles of chemical x 4 312 ON THE IGNEOUS METAMORPHOSIS [Ch. XIV. science ; for, if we attempt to pursue a path of our own, re- jecting the aid of the kindred sciences, vague and groundless speculations will speedily usurp the place of cautious and rigid inductions. The metamorphosis, however, which is supposed to have been produced on the strata by granite, has not, according to this theory, been confined to the point of contact, but has extended to very considerable distances therefrom ; converting sandstones into gneiss and mica-slate, shale into clay-slate, and the latter into hornblende, schist, and various other crystalline slates. In order to conceive the possibility of such an occurrence, we are called upon to imagine voluminous masses of melted granite to have been for ages in an incandescent state, in contact with sedimentary deposits : by which the latter, to the whole extent of the primary slates, were intensely heated for a long period, but nowhere to the point of fusion, though this is said to have been nearly accomplished in the case of gneiss. That a volume of melted lava does remain for a long time in contact with the stratified rocks in volcanic vents, can- not be denied ; but the amount of change thereby produced does not seem to have been ascertained : indeed, in this case, it cannot be easily examined ; but, in the dykes which com- municate with immense superimposed beds of trap, we might expect to find some data on which to form an estimate of this power, more especially as such trap is supposed to have been ejected from beneath under considerable pressure ; therefore intensely heated, and under circumstances favourable to the metamorphosis of the adjacent strata. But these are not always changed; on the contrary, these igneous and aqueous rocks often lie side by side, undisturbed and unaltered. It appears to us to be a conclusion too hastily drawn, that the granite has been in a state of fusion subsequent to the supposed deposition of the primary strata : for it follows, that such aqueous sediments must have been formed on a basis of Ch. XIV.] OF THE STRATIFIED ROCKS. 313 previously existing rocks ; and, therefore, the melted granite must have been injected between the sediments and funda- mental rock, or the latter must have been converted by fusion into granite. The latter idea appears to be enter- tained by Lyell, for he imagines that the repetition of another series of movements, similar to those that have taken place during the secondary periods, may upraise the hypogene rocks (granite and crystalline slates) formed, and now form- ing, during the tertiary and recent epochs ; " by which time, we imagine, that nearly all the sedimentary strata now in sight would either have been destroyed by the action of water, or would have assumed the metamorphic structure, or would have been melted down into plutonic and volcanic rocks." * This is certainly a very ingenious and beautiful theory : it maintains a most perfect balance of power between the con- flicting elements of fire and water, since the earth, the sub- ject of these contentions, is always preserved nearly in statu quo; for whilst the water overruns and subjugates the upper regions, the fire, pari passu, takes possession of the nether- most aqueous conquests, and annexes them to its dominions. Such a successive series of action and reaction is certainly in accordance with the usual routine of natural operations ; but it is not sufficient that a theory be ingeniously devised to meet the exigencies of certain difficulties, but its principles must be deduced from facts. Before, therefore, this theory be accepted, it ought to be clearly demonstrated that the internal fire is gradually extending its influence farther from the centre of the earth, contrary to the common opinion that the crust of the earth is increasing by secular refrigeration ; and, in the next place, it ought also to be proved, that the internal fire has converted sedimentary deposits into granite and crystalline slates : and if these points cannot be esta- blished, of what value is the conjecture, however ingenious ? Some may think it a great waste of time to enquire into * Principles of Geology, vol. iii. p. 382. 314- ON THE IGNEOUS METAMORPHOSIS [Ch. XIV. the worth of every hypothesis which has been from time to time engrafted on the original Huttonian theory : but it seems to us, that such examinations, if conducted with can- dour and caution, may be of utility, by leading to more ex- tended researches. In this case, such good results may be expected ; for not only farther geological scrutiny will be sug- gested, but the astronomer, meteorologist, and chemist, must also be applied to for assistance. If our planet were originally in a total state of igneous fusion, its present condition shows that it has been cooled to such a considerable extent, that the general temperature of its surface is not now influenced by this internal fire, but depends on another and remoter cause ; and what has taken place during the countless ages required for the production of the aqueous deposits, may be presumed to be still in operation. We can form some idea of the successive changes which might arise during the secular refrigeration of a melted mass, and we may offer plausible explanations of these changes on the known principles by .which matter is governed under similar circumstances : but on what analogies can we picture to our imaginations a central fire, not only maintaining its existence, by some unknown supplies, but even extending its dominion, so as to fuse the rocks on which the oldest strata were deposited, and to bring the latter into a state of intense ignition over a considerable portion of the globe, and within a comparatively insignificant distance from the surface; a distance far less than the total thickness of the secondary and tertiary rocks, many of which must have been formed since the supposed conversion of these strata into the existing primary rocks. We have said that many of the fossiliferous deposits are supposed to have been formed anterior to some of the primary slates, the latter proceeding from the meta- morphic state of the former ; but we might, perhaps, have asserted, on evidence not inferior to that on which this hypo- thesis is founded, that this metamorphosis, if it ever hap- pened, must have preceded the formation of all the secondary Ch. XIV.] OF THE STRATIFIED ROCKS. 315 and tertiary deposits. " The principal effect of these volcanic operations in the nether regions," says Lyell *, " during the tertiary periods, or since the existing species began to flourish, has been to heave up to the surface hypogine (primary) form- ations of an age anterior to the carboniferous." Now, if he admits that these primary rocks are older than the carboni- ferous series, our assertion holds good, and it follows that the central fire did, according to the views under consideration, extend to the surface, once more reducing the whole globe to a state of incandescence. But we presume that this is not his meaning : he only intends to assert that their original deposition as aqueous sediments, and not their altered or metamorphic state, preceded the carboniferous formation. It matters however little which view be adopted, for the latter must be ultimately reduced to the same terms as the former ; for since it is a well-attested fact, that the oldest fossiliferous strata contain, and indeed sometimes are made up of portions of the primary strata, or the so-called metamorphic rocks, which are immediately adjacent ; it seems to be a necessary consequence, that the primary slates could not have been in the nether regions of the earth when the oldest secondary strata were forming, and not upraised until the tertiary period ; for under such circumstances, how could their frag- ments and debris enter into the composition of the latter deposits ? On the contrary, if we estimate former changes by reference to causes now in action, this fact clearly indicates, that the primary slates must have existed before the oldest secondary rocks ; that is, the parent must have been in being before the birth of its offspring : and not only so, but the former could not have been situated deep in the bowels of the earth when the latter were formed, but must have been previously in an elevated position at the surface, in order to have been subject to atmospheric and aqueous operations, which alone are capable of producing the detritus found in the composition of these secondary formations. * Principles of Geology, vol. iii. p. 382. 316 ON THE IGNEOUS METAMORPHOSIS [Ch. XIV. This supposed metamorphosis of sedimentary rocks is, however, said to belong not only to strata older than the carboniferous, but actually to occur in strata belonging to some of the more recent secondary groups. The nature of the transition between these groups .and primary slates having been already considered, it remains to enquire whether caloric alone is capable of accounting for the difference between these two kinds of rocks. It has been shown, that the schistose rocks, at their junction with granite, not only differ from fossiliferous slates in their texture, as in the case of such slates next trap, but also in their com- position ; a chemical difference, which heat alone cannot be supposed to have effected. This is rather a serious difficulty in limine, for it cannot be attributed to the union of the granite and slate by fusion, since the lines of stratification, though obscured in gneiss, are allowed to be very distinct in limestone, hornblende, and argillaceous schists, and similar rocks ; and it appears to us that this difficulty is not dimi- nished when the scrutiny is extended to the whole mass of primary slates. Presuming that the analogy of our furnaces, through the sides of which the effects of the fire do not extend beyond a certain point, though continued for several years ; and of the glacier on Etna *, preserved from the action of a burning flood of lava by the sole intervention of a layer of volcanic sand, to be objectionable; and, admitting that the central fire can exert its influence through such an immense mass of non-conductors of caloric as the primary strata, we proceed to examine the changes which have been attributed to this igneous agent. In the first place, the contact of melted granite (how called into existence is at present immaterial) has, according to the Plutonists, converted argillaceous sandstone, or some other sedimentary rock, into gneiss ; which, adjacent to the granite, * Lyell's Principles of Geology, vol. i. p. 371. Ch. XIV.] OF THE STRATIFIED ROCKS. 3 1 7 is very crystalline, not to be distinguished mineralogically from granite, and which, at a distance therefrom, though re- taining the same component parts, is more perfectly stratified, and not so abounding in granite-veins. How comes it to pass, it may be asked, since heat alone can produce gneiss from the sedimentary strata, that this primary schist is not always found in contact with granite ? It may be answered, that the degree of intensity of the melted mass varied, and that other schists were produced when the temperature was not sufficient to form gneiss. This, how- ever, would be mere conjecture; but it might be more plausibly urged, that the nature of the sedimentary rock induced a corresponding difference in the metamorphic rock : indeed, it has been stated, " that, as in the secondary form- ations, we find an indefinite series of clay, marl, sand, and limestone ; so, in the primary, gneiss, mica-slate, hornblende- schist, quartz-rock, and marble, have no invariable order of superposition." This explanation, however, is more specious than correct ; for all the primary slates, associated together in every possible combination, pass into each other so frequently by the most imperceptible gradations, and appear to be ultimately composed of the same elements, that it is difficult to suppose that they have been individually produced from such dissimilar substances as clay, marl, sand, and other de- posits. To render this objection more obvious, we must refer to the primary slates generally. Clay-slate is said to be derived from shale, and when clay-slate comes into contact with granite, it passes oftentimes into hornblende- schist, which has been adduced as evidence of a more perfect igneous change, bearing the same relation to clay-slate, as granitic gneiss does to that which is distinctly stratified. Now, in Scotland, Norway, and those countries where granite and gneiss are characterised by containing hornblende, it is a common occurrence for gneiss and hornblende-schist to al- ternate together : so far the igneous theory goes hand in hand with nature ; for if clay-slate be converted into horn- 318 ON THE IGNEOUS METAMORPHOSIS [Ch. XIV. blende-schist by the contact of fused granite, it might be expected that the same change would happen when interposed between two beds of gneiss ; for the heat transmitted must have been very great, in order to form the more distant bed of gneiss. Is it, however, equally satisfactory, when the clay- slate is found alternating with gneiss, as in the islands of lona and of Isla; or when, as in the same isles, and in various parts of Scotland, this slate gradually passes into and alternates with hornblende-schist ? For if clay-slate, when heated to a certain degree, becomes hornblende-schist, by what laws of caloric could the intermediate beds of clay-slate have escaped this conversion? This argument is also ap- plicable to gneiss : thus, in Tirey, and in other Scottish islands, the highly crystalline or granitic variety is found interlaminated with that which is regularly schistose ; and not only so, but the latter sometimes completely envelopes the former, and this again encloses not only irregular veins and layers, but also entirely insulated nodules or masses of perfect granite. Turn again to the mica-slate and clay-slate of the eastern part of Ireland, as described by Weaver, and there too occur phenomena of the same import ; and in Cornwall, the misnamed clay-slates not only envelope and are inter- laminated with hornblende and other crystalline substances, but also contain insulated masses of granite, which, in one instance, at Herland Mines, have been discovered at a distance of two or three miles from the main granite. It is useless to multiply examples, for one is sufficient ; the fact cannot be disputed, that rocks supposed to require an intense heat for their production, such as would fuse granite, are found enveloped in other rocks which could not have expe- rienced such an elevated temperature. How are such capri- cious operations of caloric to be accounted for ? The easiest method would be to doubt the perfection of a theory which has involved such a dilemma, or at least to reject the con- dition of the primary slates as evidence in favour of an igneous metamorphosis. Ch. XIV.] OF THE STRATIFIED ROCKS. 319 The objections already advanced appear to us to have no little weight, but our dissent from the prevailing theory does not solely rest on these grounds ; for we are prepared to show that the slates, at their junction with granite, not only exhibit a difference of composition from that of the adjacent mass, but that the latter also contains an element which is not to be found in sedimentary clays, marls, and sands, from which they are supposed to have been derived. And, in order to establish this point, chemical analyses must be again referred to, and though these are certainly not so numerous or accurate as might be desired, yet they, perhaps, approxi- mate near enough to the truth, to enable us to estimate the value of this argument. The compositions of several substances, which bear on this question, are given in the subjoined table. From this table we learn that the proximate principles of all the compound crystalline slates, with the exception of quartz, contain no inconsiderable proportion of alkali, either of potash or of soda : but this substance does not form a com- ponent part of the secondary slates. This, indeed, is what might be expected a priori ; for felspathic rocks, during their decomposition, lose their alkali, as in the formation of porce- lain clay from granitic protogine and some kinds of eurite : indeed, the solution and abstraction of alkali from decom- posing felspar appears to facilitate the rapid disintegration of the rocks which contain this mineral. Those who support the conjecture concerning the dolomiza- tion of limestone will, of course, attribute the presence of alkali to a similar cause, to the transfusion of this substance through the primary slates ; and in this case the opinion seems more tenable, magnesia being exceedingly refractory, whilst the alkalies may be fused and even dissipated in vapour, by an intense heat. Here, however, the same difficulty occurs as in the case of dolomite ; the layer of rock immediately adjoining the granite does not always contain alkali, but is sometimes almost entirely quartzose. 320 ON THE IGNEOUS METAMORPHOSIS [Ch. XIV. A TABLE OF THE COMPOSITION OF MINERALS CONTAINED IN THE PRIMARY ROCKS. Sil. Alum. Mag. Lime. Pot. Sod. Iron oxide. Mang. Felspar (Rose) 66-75 17-50 1-25 12 0'75 (Vauquelin) 62-83 17-02 3 13 1 Glassy felspar (Klaproth) 68 15 14-50 0-50 Analcime (Vauquelin) 58 18 2 10 Axinite (Klaproth) 50-50 16 17 0-25 9-50 Garnet (Wiegleb) 26-46 23-70 17-91 16-25 Compact felspar (Klaproth) 68 19 1 5-50 2-50 4 Saussurite (Ditto) 49 24 3-75 10-50 5-50 6-50 Mica (Ditto) 48 34-25 8-75 4-50 0-50 (Vauquelin) 50 35 1-33 7 Talc (Klaproth) 61-75 30-50 2-75 2-50 Shorl ( Ditto) 36'75 34-50 0-25 6 21 trace Chlorite mur. of (Vauquelin) 26 18-50 8 2 43 Hornblende (Klaproth) Diallage 42 12 2-25 11 trace 30 0-25 (Ditto) 60 27-50 __ ___ 10-50 Serpentine (Rose) 28 23 34-50 0-50 _ 4-50 Asbestos (Chenevix) 59 3 25 9-05 . 225 Actynolite (Bergman) 64 2-70 20 9-30 4 Steatite (Klaproth) 45 9-25 24-75 0-75 1 (Vauquelin) 64 ~~ 22 r ~~ ~ -"" 3 It may also be objected that some beds of primary rocks do not contain alkali, whilst others, with which they alternate, abound in felspar : that is, the same condition of these rocks which intimates that an emanation of caloric from a central igneous mass could not have produced the supposed meta- Ch. XIV.] OF THE STRATIFIED ROCKS. 321 morphosis, also renders it probable that the additional element, alkali, could not have proceeded from the same source. For, if clay-slate has been converted into hornblende and other crystalline schists, and the latter contain an alkali which is wanting in the clay-slate, and if the last-mentioned rock also alternates with mica-slate and even with gneiss, it is somewhat difficult to conceive how the alkali could have been transmitted through these interposed beds of clay-slate. The value of this argument rests on the composition of clay- slate, and we have made use of it on the generally received opinion of the nature of clay-slate : but we do not attach any importance to this example, because we do not subscribe to the general notion concerning this slate; indeed, the term clay-slate has been so commonly and vaguely applied to most schistose rocks of an uniform and fine texture, that it is, in fact, perfectly useless, as it conveys no definite meaning. On the contrary, we think that all true primary clay-slates, that is, such as are associated and alternate with gneiss and analogous rocks, do really contain an alkali ; that their basis is a com- pact felspar, varying in composition, sometimes silica and sometimes felspar prevailing : it is probable that alkali may not be found in every specimen of this rock ; for, sometimes, the clay-slate has assumed the character of a sedimentary rock by a partial decomposition which has altered its appearance, and may have removed the alkali without destroying the tenacity and structure of the slate : this state of things often happens in Cornwall, and is probably of universal occurrence. But in offering to withdraw the objection urged against the transfusion of alkali through the primary slates, in the case of clay-slate, it is not intended to drop it altogether, but to claim its validity in the instance of quartz -rock, which, even in a state of considerable purity, and often in beds of no incon- siderable thickness, alternates not only with gneiss, but also with every member of the primary rocks. Let the appeal be made to the chemist, and by his answer let the question stand or fall. Can potassa or soda be transmitted through a mass of 322 ON THE IGNEOUS METAMORPHOSIS [Ch. XIV silica, whilst in a state of intense ignition, without entering into combination therewith ? And if the temperature of the quartzose mass be so elevated as to permit the formation of a crystalline gneiss on its side most remote from the granite, and also to reduce its supposed primeval state of sandstone into a compact and crystalline quartz ; would not the union of alkali therewith cause the whole to enter into a perfect state of fusion, and could it be expected to find in this mass of glass, when cooled, any trace of lines derived from stratification ? It is unnecessary to add any further remarks to enforce the value of this argument ; it speaks a plain and intelligible lan- guage, which cannot be misunderstood. What shall we say, then ? Are the objections which have been advanced frivolous, or are they sufficiently important to induce geologists to enter on a careful scrutiny whether the primary slates be really altered sedimentary deposits? If, indeed, there be any weight in the foregoing arguments, the primary schistose rocks are certainly not analogous to those altered portions of secondary rocks adjacent to trap and other igneous rocks ; nor is the transition of these crystalline slates into fossiliferous strata indicative of a coeval formation : on what grounds, then, it may be asked, can it be demonstrated that the secondary rocks have been altered by granite, and converted into gneiss, mica-slate, and other primary schists ? The importance of this subject claims a more particular con- sideration. "The slates surrounding Dartmoor have been changed," says De la Beche, " by the intrusion of granite ; some being simply micaceous, others more indurated and with the cha- racters of mica-slate and gneiss, while others, again, appear converted into a hard zoned rock strongly impregnated with felspar. The alteration of the rocks in this case is of very easy explanation. The greywacke, which is for the most part the altered rock, is, when taken in the mass, only the conso- lidated detritus of more ancient crystalline rocks, composed of a few simple substances. If long-continued heat, insufficient Ch. XIV.] OF THE STRATIFIED ROCKS. 323 to produce fusion, be applied to the ends of the greywacke beds, the various substances of which they consist would have a tendency to resume their original state, at least that state in which they existed in the crystalline rocks whence they have been derived ; and, consequently, we should have com- pounds resembling various crystalline stratified rocks." * Now, before this explanation can be admitted, it must be proved that the granite has been thus protruded through the strata over an extent of about twenty miles in diameter, and that crystalline slates, which once existed, have disappeared, after having provided materials for the structure of these metamorphic and unaltered masses of greywacke. It would be more consonant with reason to conclude that the fragments in the greywacke have been derived from the existing crys- talline strata which they resemble, than to have recourse to rocks of which we have no knowledge, except through the medium of the Plutonic hypothesis : for such a proceeding is neither according to the general rules of induction, nor ac- cording to the acknowledged principles of geology, by which the relative ages of rocks are determined. Thus it has been observed by Lyell, that, " in investigating a district composed of two distinct formations, it is sometimes difficult to ascer- tain their respective ages, from want of sections exhibiting the order of their superposition. In such cases, another kind of evidence, of a character no less conclusive s can sometimes be obtained. One group of strata has frequently been de- rived from the degradation of another in the immediate neighbourhood, and may be observed to include within it fragments of such rocks; from which we may confidently infer that the group from which the fragments have been derived, is the oldest of the two formations."f Thus, " there can be no doubt that some granites are more ancient than any of our regular series which we identify by organic re- mains, because there are rounded pebbles of granite, as well * Geological Manual, p. 479. f Principles of Geology, vol. iii. p. 36. Y 2 324 ON THE IGNEOUS METAMORPHOSIS [Ch. XIV. as gneiss, in the conglomerates of the oldest fossiliferous groups.* But even conceding these points, it may be presumed that the debris, of which this fragmentary rock consists, was pro- duced from the crystalline slates by causes similar to those now in action : turn, then, to decomposing primary slates (the supposed metamorphic schists, which are now interposed between the granite and unchanged greywacke), and it will be found that the oxide of iron contained in the perfect rock has become a peroxide, and the greater part of the alkali has disappeared from the argillaceous mass ; and then explain how a degree of heat short of fusion could have reduced the metal to a lower degree of oxidation, and have restored the alkali to the felspar, in which mineral these crystalline rocks abound ? We are likewise called upon to admit that the Hartz furnishes similar evidence in favour of the metamorphic in- fluence of the granite. Thus, according to M. C. Zincken, the strata of hornfeh, which surround the granitic masses of the Ramberg and Rosstrap mountains, are, for the most part, igneous modifications of greywacke ; the granite intersects these strata at various angles, and penetrates them in the form of veins, near which the laminae of the slate are some- times broken and bent, and fragments of the hornfels are imbedded in the granite : in short, the phenomena are such as commonly occur in Cornwall, and other countries, at the junction of the granite and slate, f According to this state- ment, it is supposed that granite has been violently injected among the strata, in a state of igneous fusion : but from the observations of Bonnard on the Hartz, of Brochant on the Alps, and of Von Buch on Norway, it might be concluded that the granitic masses of these countries are not distinct formations, being often regularly instratified with the ad- jacent schistose rocks, into which they on all sides gradually * Principles of Geology, vol. iii. p. 359. f Bulletin de la Soc. G^ol. de France, tome iii. p. 32. Ch. XIV.] OF THE STRATIFIED ROCKS. 325 pass, and with which they are so intimately associated ; that the slates sometimes completely envelope smaller layers or beds of the granitic compounds ; and, vice versa, the latter occasionally contain the former. Granting, however, the in- jection of the granite, it could not be the cause of the eleva- tion of strata, which, as in the Hartz, underlie the granite on one side, and repose upon it on the other ; nor could the granite of Mont Blanc have tilted up the adjacent strata, since it is only a crystalline variety of the talc-schist, similarly circumstanced as to position, being, indeed, analogous to the granite of Sweden and Norway, which is generally considered to be granitic gneiss. It may be argued that these elevations have been effected by concealed masses of granite, from which the injected portions have proceeded, and by which the metamorphosis of the strata has been effected : but this is only conjecture, and by such reasoning all diffi- culties may be as easily overcome ; for it may be imagined that vast mountainous tracts once existed, which, after having supplied the materials for the immense masses of primary strata in the north of Europe, and other regions, were swal- lowed up, or in some wise obliterated by the elevation of the formation to which they had given birth. Ought such specu- lations to be preferred to the simple and self-evident in- duction, that secondary deposits, containing fragments of contiguous primary rocks, are more recent than the form- ations whence the detritus was derived. The fel spathic rocks of Cornwall are also adduced by Lyell, as the first example of the alteration of strata, in contact with granite : these various and beautiful rocks are called a coarse argillaceous schist, which is converted into hornblende-schist at its junction with the main mass, or with the veins of granite. If the dark-coloured slate sometimes re- sembles hornblende-schist, it, in numerous other instances, does not assume this appearance : and, even in the former case, we think that this name has only been applied for want of a better. So much, however, has already been said on this Y 3 326 ON THE IGNEOUS METAMORPHOSIS [Ch. XIV. subject, that it is not necessary to make any further remarks : and, therefore, we will turn to those examples which more strongly illustrate the metamorphic hypothesis. The instance quoted by the same author, which occurs at Champoleon in the Alps, is very unsatisfactory in every respect : in the first place, the section does not appear to be natural, and the condition of the strata is not such as might be expected ; we do not detect any of those changes of composition which in- dicate an analogy with the primary rocks ; the argillaceous bed is only hardened, instead of being converted into horn- blende-schist, or even gneiss ; the grits are quartzose, and contain a thin layer of imperfect granite, and all this within a space only of thirty feet from ignited granite, which is capable, according to the theory, of effecting greater changes on beds far removed, and notwithstanding the intervention of an immense mass of bad-conducting substances. It is not impossible that in this case, since the granite at the junction is soft and not perfectly crystalline, it may be a derivative rock, as at Caithness in Scotland, gradually passing into the unaltered and untransported granite : but it is, perhaps, more probable that, as in similar cases, the nature of this rock may have been misunderstood; indeed, Studer has already ob- served that, " les apparences de Predazzo et de Valle Rab- biose, sortent pour ainsi dire de la ligne de celles qu'on peut appeler ordinaires, parce qu'elles se lient aux eruptions du porphyre pyroxenique, et il reste toujours quelque incertitude pour savoir si le granit de Canzacoli n'est positivement qu'une forme particuliere de la roche pyroxenique." * The appearance of the stratified rocks, next the granite of Cham- poleon, is more analogous to that which is exhibited in the vicinity of dykes of porphyry and trap, than to that of gneiss, mica-slate, and similar crystalline rocks. We are, however, informed by Hugi that, in the Alps, granite overlies fossiliferous limestones and slates, equivalent * Bulletin dc la Soc. Gcol. du France, tome ii. p. 55. Ch. XIV.] OF THE STRATIFIED ROCKS. 327 to the more recent beds of lias, or the oldest members of the oolitic group ; and that it has sometimes changed them into gneiss and mica- slate. This is more in point, and if satis- factorily proved would certainly be decisive of the question now under consideration ; but, before the evidence can be received, it must be substantiated by the most minute and circumstantial details, in order that the train of reasoning by which this conclusion has been arrived at may be carefully scrutinised. We have already endeavoured to point out how little dependence can be placed on an apparent transition between the primary and secondary rocks, even when these are well exposed to view : and when it is remembered what different conclusions have been drawn from phenomena clearly displayed in the cliff-sections of the Cornish coasts, how difficult it often is to detect the real state of things, even under the most favourable circumstances, we may form some conception of the allowances that ought to be made for Alpine investigations. It may, perhaps, be considered by some that we are only starting a groundless and vexatious objection : but hear what Studer has said of this same geologist, when describing the connection of gneiss and lime- stone in the Bernese Alps : "II est'singulier que M. Hugi ait mal represente les faits dans ces lieux ; car ni sa descrip- tion, ni ses profils, ne coincident avec la realite, qui confirme cependant si bien ses observations dans ce Rothal." Again, De la Beche, after describing the phenomena of Jungfrau, as recorded by Studer, adds, that " we must here remark the uncertainty of many sections of this kind, however clearly the lines of separation may be exhibited on the face of a huge precipice, as this is ; for it is well known to all accustomed to examine disturbed districts, where inconformable rocks have been tilted up together, that natural lines of section often cause a large mass of inferior rock to appear included in the superior beds, when, in fact, such appearance is entirely de- ceptive. This arises from a portion of the older rock pro- jecting into the newer rock, having been accidentally cut Y 4- 328 ON THE IGNEOUS METAMORPHOSIS [Oh. XIV. through in the line of section." * "I spent some weeks," says Bakewell," almost close to the Jungfrau mountain, in the canton of Berne, and studied its structure with particular attention; and I have no hesitation in expressing a decided opinion that the section of M. Studer, representing two cone- shaped masses of limestone penetrating the granitic gneiss near the summit, is fallacious. The part represented as penetrated by the limestone is concealed by a covering of eternal snow." f If we might be permitted to venture an opinion on the appearances at the Jungfrau, founded on the descriptions of the above-mentioned geologists, we should say that they are perfectly analogous to the gneiss formations of Scotland, Norway, and other countries. This gneiss abounds in mag- nesian minerals belonging to the talcose formation, which is often characterised by the peculiar granitic rock called protogine : it is generally lamellar, but in some parts massive, and there presents irregular concretions and veins of granite ; so that, according to the proportion of these parts, the mass oscillates between granite and gneiss, though the latter pre- vails. The adjacent limestone abounds in talc and other minerals of this nature, and sometimes, by the intimate union of magnesia, it becomes dolomite ; but the latter is not always in contact with the gneiss, so, as in the case of the dolomites of the Tyrol in contact with augite porphyry ; it cannot be attributed to the influence of the accompanying rock on common limestone whilst in a state of ignition. Now, this crystalline limestone is the same as often occurs within gneiss, it only differs in position ; and is in like manner subject to the same interference of veins or layers of gneiss, either lamellar or granitic. That this limestone graduates into the secondary rocks of a calcareous nature, so that it is difficult to say where the one begins and the other ends, is only in * Geological Manual, 8vo. p. 457. f Introduction to Geology, fourth edition, p. J01. Ch. XIV.] OF THE STRATIFIED ROCKS. 329 accordance with numerous facts which show that all the crystalline rocks thus pass into the mechanical or secondary deposits which have been formed of their detritus ; and not only so, but even the unstratified and igneous granite exhibits a similar transition into stratified conglomerate and sand- stone, without the intervention of any crystalline slate: facts which teach us that too much dependence must not be placed on this evidence as a proof of identity of formation. The same reasoning is equally applicable to those cases in which granite is supposed to have converted the secondary strata into primary. The particulars of Hugi's examples, referred to by Lyell, are not at hand : but the instances of Glen Tilt in Scotland, and of the Eastern Pyrenees, detailed in the sixth chapter, may serve for the present. In the first place, let us consider the metamorphosis which the strata are supposed to have undergone at Glen Tilt, in consequence of their coming in contact with granite. These strata are, various kinds of schist, limestone, and quartz- rock. The schists are clay-slate, hornblende-schist, talc-schist, and mica- slate, which in some places approaches near to gneiss ; but these are not disposed in any fixed or determinate order : the hornblende -schist is often next the granite, but not always ; the magnesian varieties are generally at some distance there- from ; and the mica-slate and gneiss, which ought as the most crystalline to be nearest to the agent of these changes, have been produced where the influence of heat must have been less energetic ; whilst portions of slate near the granite have been, to use the prevailing phrase, altered in a less degree. The limestone at a distance from the granite is very crystalline, and either perfectly calcareous, or in some parts more or less blended wifh and variegated by magnesian minerals : the presence of magnesia is now generally attributed to the inter- position of igneous rocks ; but here, as frequently happens elsewhere, this is not always dependent on, or in proportion to, the proximity of the granite. On the contrary, at the junction of the limestone and granite, the former resembles 330 ON THE IGNEOUS METAMORPHOSIS [Ch. XIV. hornstone or compact felspar, effervesces slowly, and contains a large portion of silica : how can this change be ascribed to caloric ? It cannot be admitted that the limestone has been fused and combined with the adjacent granite, because it is stated that the lines of stratification are still preserved, though the strata are much dislocated and disturbed : and the chemist will not grant the possibility of carbonate of lime being converted by heat into silica. Again, the quartz-rock is in many positions nearly pure quartz even adjacent to the granite, whilst at a considerable distance therefrom, where the schistose beds are straight and undisturbed, this rock not only contains felspar, but is so disposed in conjunction with mica or hornblende as to form gneiss and even a perfect granite, and when this is the case the adjacent slate is inter- mediate between gneiss and mica-slate. How could granite have produced this change on a remote mass of quartz-rock, when that in its vicinity remains unaltered ? How could the presence of granite cause the introduction of magnesia, alkali, and other elements, into distant strata, through rocks which are similar, and which may therefore be presumed to have an equal affinity for such elements. Let us now turn to the crystalline limestones of the Pyrenees, described by Dufrenoy. He has concluded that the elevation of their strata, and their apparent gradation into the succeeding fossiliferous rocks, prove that they belong to the same mass, and that they have been altered and elevatgd by the adjacent granite. The changes here indicated are, a more crystalline texture, the presence of magnesia in the beds next the granite, and the abundance of iron-ores : the con- dition of its texture is only a character which this limestone enjoys in common with most primary rocks. The nature of the proof afforded by magnesia has been already considered ; and the presence of metallic ores near the junction of granite is certainly in itself no evidence of the manner in which the containing rocks were formed. The questions of elevation and transition have been already discussed : as regards the Ch. XIV.] OF THE STRATIFIED ROCKS. 331 latter, some might consider the line of disunion between the ferruginous limestone and the incumbent bed to form the boundary of the primary limestone : but much stress ought not to be laid on this fact ; for it is more than probable that, though it occurs at many points it may not hold good throughout its whole extent. The beds of granite appear to this distinguished geologist to afford positive proof that the main mass of granite is more recent than the limestone. How can we otherwise comprehend, says he, the intercalation of granite between beds of dolomite, if the granite was not intro- duced after the manner of veins ? According to the prevailing theory it cannot well be otherwise; and he therefore has concluded that these layers of granite are veins : but if this be admitted as a justifiable conclusion, then all the layers of granitic rocks found in the strata of gneiss, mica- slate, and clay-slate, and parallel therewith, are also veins, though we know by experience that these beds, in some cases, have no connection with the adjacent granite. Under all these con- siderations, it appears to us less objectionable to regard these limestones, with their granitic beds, and the main mass of granite, as members of the same primary formation, and con- nected with the fossiliferous limestones by mechanical transi- tions ; and, although the line of union may be at present obscure, it may be hereafter detected by more minute ob- servations, and nicer discrimination. It must not be omitted to remark, that secondary rocks have been frequently found adjacent, and even in contact with granite, without exhibiting an altered or metamorphic appearance ; and yet at the same time, by the inclination of their strata, indicating, according to the principles of the Plutonic theory, that they have been elevated by the pro- trusion of granite. Sedgwick and Murchison have recorded many examples of this nature, in the Transactions of the Geological Society, and have anticipated this objection, by supposing that the granite has been forced through the strata in a solid state. But in the north of Scotland, the phenomena 332 ON THE METAMORPHOSIS OF STRATA. [Ch. XIV. are so various at different points, that, as already stated in a former chapter, sometimes the strata are concluded to have been broken, sometimes both the strata and the gr anite whilst in other cases the granite itself is shattered into frag- ments, the strata remaining uninjured ; and, lastly, in some places the granite has produced crystalline slates, penetrated by granite-veins, indicative of interior ignition. These various appearances, however, are not so contiguous but that they might be imputed to numerous and distinct eruptions of granite : but even admitting this to hold good in Caithness and Sutherland, the same indulgence cannot be extended to the Isle of Arran, where the red sandstone reclines near the granite, in one place in an unaltered state, whilst, imme- diately adjacent, primary slates abound, containing granite- veins. And even if this objection should be met by sup- posing the same mass of granite to have experienced suc- cessive protrusions under different conditions, still it must be remembered, that these are only ingenious conjectures, sug- gested by the perplexing necessity of the case, and unsup- ported by any known effects of the internal igneous power now in operation. In conclusion, we contend that the analogy between the primary and trap rocks is not perfect, the strata next the former exhibiting a chemical as well as a mechanical dif- ference ; and admitting the sedimentary strata to have been changed by the contact of trap in a state of ignition, a most perplexing difficulty arises, in the fact that the alterations are only partial : what could possibly have been the peculiar controlling circumstances which prevented a like cause from producing the same effects ? Ch. XV.] ON THE DISPLACEMENTS OF STRATA. 333 CHAPTER XV. DO THE PRIMARY ROCKS AFFORD PHYSICAL EVIDENCE THAT THEY HAVE EXPERIENCED FISSURES, DISLOCATIONS, AND OTHER MECHANICAL MOVEMENTS ? The apparent indications of displacements. The curvatures of rocks. Alter- nations of curved and straight strata. Veins often tortuous in straight beds. The colouring ingredients of rocks arranged in undulating and contorted lines. Similar appearances in agates and alabaster also in igneous rocks. Sir James Hall's explanation of curved strata. The moving power imaginary. The spheroidal structure of rocks. Primary conglomerates not fragmentary the secondary, sometimes mechanical, at others concre- tional. Igneous and aqueous rocks also conglomerated. Granite- veins their structure and composition incompatible with theory analogous to slate-veins and to certain arrangements in crystalline rocks. Mineral veins different kinds of veins of segregation in Cornwall. Definition of true veins objected to. Intersections of veins no criterion of their relative ages. Remarks on the heaves and other supposed movements of veins. Conclusion. THE subject of this chapter has been in part anticipated in the discussion on the supposed elevation of the primary strata ; and, certainly, on the determination of that question the nature of some of the phenomena about to be considered in a great measure depends; for these are generally pre- sumed to be lesser movements, the concomitants of those vast catastrophes by which the stratified deposits of all ages were upraised from a horizontal position. The effects, however, of these supposed subordinate move- ments present such varied appearances, and require to be combated by such different arguments, that their individual consideration cannot fail to elicit some instruction ; and, indeed, unless they are passed under review, both by the supporters and the opponents of the prevailing theory, the discussion will not be satisfactory. 334> ON THE APPARENT DISLOCATIONS [Ch. XV. These subordinate movements are said to be indicated by the curvatures and contortions of the strata, and by the various phenomena exhibited by granite-veins, by dykes of porphyry, trap, and similar rocks, and by mineral and metalliferous veins. In the narrow limits of this work, these important topics cannot be fully detailed ; it must therefore suffice to notice those facts only which bear more immediately on the question. It is generally admitted that the layers of rocks may attain a certain degree of curvature, by deposition on an undulating surface s such as the exterior of the earth actually presents. But this cause has its limits, and is in no wise applicable to cases of complicated flexures and contortions, which so frequently occur in every country. These appear- ances were once thought to be confined to the older or pri- mary strata ; but it is now known that, though most frequent in gneiss and mica-slate, they do exist not only in every secondary group, but also in the tertiary formations. The precise manner in which these curvatures have been pro- duced has given rise to much difference of opinion, though most geologists of the present day are agreed that, however modified by circumstances, they are ultimately to be attri- buted to the application of some moving power. There are certainly many instances of curved strata which might possibly have originated from mechanical vio- lence, provided these strata were, when bent, in a plastic or yielding state ; but what evidence have we of the existence of such ductility in these rocks coeval with the application of a moving power ? and, in examining this question, let us confine our attention to the primary rocks, a restriction which is most favourable to the supporters of this opinion. It is presumed that the advocates of the Plutonic theory will not select the period of deposition, when gneiss and mica-slate are supposed to have existed as unconsolidated sediments, but would rather adopt the metamorphic stage, whilst the primary schists were in a state of intense ignition Ch. XV.] OF THE PRIMARY ROCKS. 335 by the contact of melted granite ; the protrusion or lateral extension of which is the adopted cause of the bendings and flexures of the gneiss and mica-slate. Now, without stopping to enquire which mode of motion is the more probable, whether that previous to, or that attendant on, the elevation of these strata from a horizontal to their present elevated inclination, let it be admitted that these concurrent con- ditions of plasticity and motion (which we have already contested) have effected the curvatures of these rocks. If this be the case, how then has it happened that one stratum, or a series of strata, is regularly curved at one point, and yet is broken off abruptly at another ; or, to speak more cor- rectly, terminates abruptly. This is of frequent occurrence ; but, what is more to the purpose, the apparent fracture and flexure may be sometimes witnessed at the same point of curvature ; several examples of which have been recorded by Macculloch, but one remarkable instance, in particular, which he saw in Lunga, one of the Western Isles of Scotland, and which he has attributed to the modified action of the moving power, operating on strata of a different nature, the associated rocks consisting of alternating layers of argillaceous schist and quartz- rock. " Where the flexure is very acute, the quartz-rock is broken, while the schist is only bent; and, in some extreme cases, fragments of the former are separated and entangled among the latter; facts speaking a language that cannot be misapprehended." * A different view, how- ever, may be taken of this phenomenon, and it may be interpreted into evidence unfavourable to the conditions of plasticity and motion ; for if the whole were softened, how came it to pass that the quartz -rock was " urged beyond its power of flexibility ? " Because, it may be answered, the quartz-rock and the argillaceous schist possessed different degrees of hardness. But this is a mere assertion ; we do not know that the latter is more fusible than the former, on * System of Geology, vol. i. p. 115. 336 ON THE APPARENT DISLOCATIONS [Ch. XV. the contrary, indeed, many kinds of this schist are equally, if not more refractory in the furnace : but supposing the quartz -rock to be less susceptible of being softened, how does it happen that in other parts of the same series of rocks, the quartz-rock partakes of every degree of flexure and con- tortion ; and even pure quartz, in the form of veins of various size, is very commonly seen traversing not only clay-slate, but also gneiss, mica-slate, and other primary schists ; and " fol- lowing the sinuosities of these rocks, however intricate, without any fracture or breach of continuity." Surely, then, if quartz-rock can be bent and contorted, at such a tem- perature as attends the metamorphosis of sedimentary de- posits into crystalline slates, or as is required to render the whole mass of these slates soft and ductile, it caunot be admitted that the appearance under consideration is a proof of fracture. But even if this objection be not allowed, another immediately presents itself, which is indeed of an analogous nature : what we have just seen to occur in the layers of a single curve composed of different rocks, we find, on the large scale, to extend over considerable tracts ; thus, immense beds of contorted rocks alternate with those which are comparatively straight and regular, and this holds good whether the whole mass of strata is highly inclined or nearly horizontal, that is, according to the prevailing opinion, whether it has been broken through and tilted up at a con- siderable angle, or has been undisturbed. The description of an occurrence of this kind in the Pyrenees, by Duhamel, as quoted by Greenough from the Journal des Mines, clearly shows the different manner in which parallel strata have been arranged : " Compact felspar, trap, and limestone there form an immense mass, composed of a prodigious number of alter- nate beds of no great thickness, and inclined to the horizon at a high angle : some of them are plain and regular, while others are twisted in a thousand different directions, without disturbing the parallelism of the beds above or beneath." * * The First Principles of Geology, p. 62. Ch. XV.] OF THE PRIMARY ROCKS. 337 We have also noticed a similar fact in the northern part of Cornwall ; the strata are arranged in regular undulating curves, but parallel with each other ; some are composed of straight, others of curved and twisted laminae, exhibiting the most intricate contortions. How can these facts be explained on the grounds of mechanical violence ? for how is it possible that some strata could be soft and others not, some bent and twisted, whilst others give no indications of any forcible derangement : and yet all these strata (if metamorphic) have been exposed to the same igneous action, and all (whatever their nature) have been subject to the same moving power, since they alternate with each other in parallel beds ? This is no new objection ; the same has been often urged by those who have contem- plated the various anomalous arrangements of contorted strata, arrangements which " cannot be explained by sup- posing the beds to have been regularly deposited in the first instance, and shifted afterwards by subsidences, shocks, or convulsions." In addition to the anomalies above mentioned, it may be stated that the contemporaneous quartz-veins (which sometimes follow all the intricacies of the curved strata, and at others traverse these curves in some places, and are parallel thereto, in others, as if of a later origin) are occasionally found run- ning a most serpentine and tortuous course through mica- schist and other primary slates which are regularly arranged ; for instance, at Tremearne, near Trewavas Head, in Cornwall, in the vicinity of the junction of the slate and granite, the former rock is traversed by quartz-veins under these circum- stances ; and Macculloch has observed " that veins of quartz sometimes occur in gneiss, which are most intricately con- torted, although the laminae of the including rock show no corresponding indications of flexure." And he adds, " this is a difficulty which, among a thousand others, must remain for future explanation ; and how far its solution may modify 338 ON THE APPARENT DISLOCATIONS [Ch. XV any of our theories, it is impossible at present to foresee."* The same indefatigable geologist has furnished us with another anomaly in the exhibition of curvatures on the surface of an artificial cliff in the slate, at the back of the gun-wharf, Plymouth ; these curvatures are formed by a number of dove- coloured stripes of unequal thickness, which traverse a faint brown-red ground in very irregular curved lines, bearing a sort of parallelism to each other ; in short, resembling strongly a piece of marbled paper. " The continuity of the lines of colour precludes all possibility of a succession of deposited layers otherwise than in those very lines, and affords at the same time a proof, if any were wanting, that the fissile pro- perty of the slate has not been the result of stratification."! And appearances of the same nature are very common on the surface of the laminae of several kinds of Cornish slates, and more particularly of those which belong to the intermediate or transition series, and which abound in veins and deposits of manganese ore. Some varieties of shorl-rock (both in the granite and in the slate), of actynolite-rock, and of other unnamed rocks belong- ing to the slate group of Cornwall, abound in contorted appearances which somewhat resemble the preceding, but are not perfectly analogous, depending on the manner in which their constituent minerals are respectively combined : thus, in the shorl-rock, the quartz and shorl intimately united, ex- hibiting various shades of black and dark blue, alternate with portions in which the shorl is wanting, and consequently con- sist only of white quartz ; and these differently coloured parts present, throughout the whole rock, and in every direction, the most intricate contortions, perfectly resembling those so common in mica-slate. These rocks are generally compact or massive, and their cleavage may be effected with equal facility either across or in the direction of the curves : but * System of Geology, vol. i. p. 11 9. f Geol. Trans., vol. iv. p. 399. Cb. XV.] OF THE PRIMARY ROCKS. 339 when this contorted structure occurs in schistose rocks, the laminae are often found to correspond with the flexures, and consequently they are fissile in the same direction. It is, therefore, no easy matter to determine where the line is to be drawn between these different kinds of curvatures, so as to separate those which are supposed to have been pro- duced by the application of force to rocks in a soft and ductile state ; for, indeed, they are so connected one with another, and assume such identical forms, that one is a perfect repre- sentation of the other in miniature. If this be admitted, another step will lead us to the waved and marbled delineations in agates and alabasters : and then the conclusion irresistibly forces itself on us, that all these internal contortions of every degree and size, whether of the whole or any portion of a rock, may be referred to a peculiar mode of concretionary structure, how difficult soever it may be to conceive by what means it was produced. On this view of the subject, the same kind of structure might be expected to occur in granite and other igneous rocks, that is, in the very agents themselves to which the curvatures of the stratified rocks have been attributed : for it has been shown that the same laminated, schistose, and tabular structures, are common to rocks both of igneous and of aqueous origin ; and, if these different modes of structure are independent of the nature of the rock's origin, and only to be ascribed to the modified operations of the attraction of co- hesion, by which the incoherent particles of the rock were consolidated, the igneous rocks ought also to exhibit curves and contortions. And so they do ; this theoretical deduction is borne out by an appeal to nature. Wherever a good sec- tion of granite is exposed in a cliff or quarry, we shall find that the parallel lines by which this rock is intersected, though having a regular bearing, like the curved strata when viewed along a line of considerable extent, are very seldom perfectly straight, but are more or less curved ; and this obtains not only with the horizontal, but also with the per- z 2 340 ON THE APPARENT DISLOCATIONS [Ch. XV. pendicular joints of the granite : and all the large veins which traverse this rock generally run in a serpentine course. These appearances are beautifully displayed near Mousehole at the Logan Rock, and in other parts of the granitic districts of CornwalL No advocate of the Plutonic theory would offer to explain all these phenomena by a mechanical agency : but they ap- pear to be so nearly allied to each other, that it would be no easy task to distinguish those that have been violently formed from those which could not have had such an origin. Take, however, the most favourable of these examples, the primary slates, and, even admitting that granite has been pro- truded, how and when were their convolutions effected? These are generally referred to the elevation of the granite. Sir James Hall advocated this opinion, though he candidly confesses, that he " has not been able to discover any case in which it has performed this function, all the junctions bear- ing marks of an infusion of the liquid granite into hard and brittle strata." " The scenes in Galloway," he adds, " prove that the granite has been more recently formed than the contorted slate; but they prove over-much in one respect, since they show the arrival of the granite at its present place to have been posterior, not only to the formation of the strata, but also to their convolutions when in a state of soft- ness, and to their subsequent consolidation." He therefore concludes, that the existing granite has not produced this mechanical effect, but some unknown mass of granite, acting at some unknown time antecedent to the elevation of the strata.* By such arguments as these the violent formation of con- voluted strata is still maintained : their fallacy, however, is too obvious to need farther comment. The curvature of the layers of the unstratified rocks, both on the large and small scale, appears to be intimately con- * Royal Trans, of Edinburgh, vol. vii. p. 98. CH. XV.] OF THE PRIMARY ROCKS. 34-1 nected, if not identical with, the spheroidal structure so generally observed to prevail in granite, and in the various porphyries and traps. It is not always very evident, but is generally developed in the individual concretions of these rocks by disintegration ; and is often conspicuous on a large scale, in the round-backed form of hills which are entirely composed of such rocks. Now some may contend, that even these appearances may have been produced by force applied to these rocks whilst in a state of semi-fusion. If, however, any power has in- fluenced this arrangement, it is probably only such as might arise from the expansion and movement of the parts during crystallisation or consolidation ; a force very different from that which is supposed to have deranged the laminae and strata of the sedimentary rocks. On all these considerations, therefore, it may be concluded, that the various curved appearances which rocks present, are not certain indications of their having been subjected to mechanical violence ; on the contrary, that they are not incompatible with the effects of the power of cohesion by which the rocks were consolidated. This opinion has been entertained by many geologists ; and amongst others, by Mr. Weaver, who, after a careful examin- ation of the inflections and contortions of the rocks in the east of Ireland, considers that these peculiar arrangements have not been produced by violence; and concludes, that these appearances, as well as the stratified structure, in all its varieties, result from concretion on the large scale.* The spheroidal structure brings us to the consideration of the brecciated and conglomerated appearance of some rocks, which have by many geologists been regarded as sure evidence of a mechanical origin. It must not for a moment be imagined, that we are about to contend that all breccias and conglomerates are not frag- * Geol. Trans, vol. v. p. 175. Z 3 ON THE APPARENT DISLOCATIONS [Ch. XV, mentary and derivative rocks : such a position would be absurd, and contrary to positive evidence. But it is proposed to enquire, whether the resemblance to fragments and peb- bles observed in many primary, and indeed in some fossili- ferous rocks, may not be only a sportive imitation of nature, a coincidence more apparent than real, and capable of being referred to a cause distinct from that by which the materials of the mechanical rocks were produced. Let us begin with the more simple case of the conglomerated appearance: and it is necessary that the general reader should reconsider the details already given on the structure of rocks. In the preceding chapters we have endeavoured to show, that the crystalline strata are not sedimentary deposits, sub- sequently altered by Plutonic agency ; but the occurrence of these conglomerates, interstratified with such strata, may ap- pear to be opposed to this conclusion ; at least it is viewed in this light by the most eminent geologists of the day. We, however, appeal from these authorities to nature ; and main- tain, that the nodules of these conglomerates, generally, if not always, possess characters very different from those which occur in true detrital deposits. The latter often consist of various substances, some of one rock and some of another ; exhibiting different forms, according to the nature of the fragments from which they have been formed ; and all these occur in a basis which has seldom the same composition as the included nodules. The former, however, are of a similar nature in the same rock ; and in their constitution partake of the elements not only of the adjacent strata, but also of the basis in which they are imbedded ; and very frequently their component parts are arranged around compact or cry- stalline nuclei ; sometimes, indeed, so regularly, as to assume a perfect concentric structure. Thus, in the case of gneiss, it often happens that the nodules and the basis can only be distinguished from each other by the globular structure of the former ; or, if they differ in composition, as in the Isle of Ch. XV.] OF THE PRIMARY ROCKS. 343 Coll, where the central part of the nodules consists of horn- blende-rock, this difference can be explained without con- sidering that these nuclei have been derived from fragments of the latter rock ; for the characteristic mineral of this gneiss is hornblende; and that it has been attracted to certain centres, and there formed by its predominance a hornblendic rock, is in perfect accordance with the patches or concretions which occur in granite, and all the primary rocks: and it must be remembered, that insulated nodules, patches, short layers or veins, and other irregular forms of the hornblende- rock, are characteristic of the adjacent strata of gneiss ; the conglomerated stratum only differing from their concretions, by being so abundant as to prevail over the ordinary struc- ture. Again, turn to the description of the conglomerated mica-slate of Drontheim ; its nodules are also formed of the same constituent parts as the containing rock, but they ex- hibit a different texture, owing to their mode of aggregation : the mica is intimately diffused throughout the schistose basis ; but in the nuclei of the nodules it is in a great measure wanting ; thence their compact structure, and, as if to com- pensate for this deficiency in the nuclei, it predominates in the micaceous laminae by which the nuclei are concentrically enveloped. And the structure of the conglomerated chlorite- schist of Relistian is very analogous to that of this Nor- wegian rock. But it may be argued, that it frequently happens that these conglomerates are formed of quartz-no- dules, as is also the case with similar rocks in secondary formations. It does so : but if the former be minutely ex- amined, it is very probable that their nodules will exhibit characters incompatible with a mechanical origin, such as the penetration of them by one or other of the constituent minerals of the containing rock ; as in the case of the schist in the Cotentin, where talc is blended with the quartzose concretions, as described by Brongniart ; who considered it a proof that the nodules were formed contemporaneously with the strata in which they are enveloped : or the nodules them- 34-4? ON THE APPARENT DISLOCATIONS [Ch. XV. selves, by their occasional irregularity of form, will indicate an intimate connection with the basis, as in the rock of Crone- bane, recorded by Weaver, in which they are extended in thin edges between the laminae of the stratum ; a disposition and form certainly not indicative of violent attrition. It may, however, be farther urged by the advocates of the Plutonic theory, that the secondary rocks also exhibit examples of nodules and bases possessing a similar com- position, and in which the nodules have the same kind of concretionary structure. Such cases have been recorded : we have noticed one at Crackington Cove *, on the north coast of Cornwall; its nature is not positively determined, but it probably belongs to the same series as the dunstone and dark limestones of Devon, and is therefore a fossiliferous formation of the oldest description. But we cannot admit this as evidence that these rocks are of mechanical origin ; it does not follow, because this conglomerate is situated in a sedimentary deposit, that its lenticular concretions have been produced by transportation and attrition : it only indicates that such nodules may sometimes result from the peculiar aggregation of the particles of rocks during their consolid- ation, whether of aqueous, igneous, or any other origin, just as the same cuboidal, prismatic, lamellar, schistose, or other kinds of structure may have originated in all rocks under similar circumstances: all that is requisite for such an ar- rangement is, that the integrant particles have the necessary degree of motion among themselves during the lapidification ; but the manner in which this is attained does not appear to be material. We are borne out in this conclusion by the fact, that supposed igneous rocks possess the same kind of conglomerated structure as stratified rocks; as shown in Macculloch's sketch of the decomposing granite in the Isle of Manf, and also by the occurrence of the same arrange- * Geol. Trans, of Cornwall, vol. iv. p. 178. t Western Isles of Scotland, pi. xxviii. fig. 3. Ch. XV.] OF THE PRIMARY ROCKS. 345 ment in trap. In Pembrokeshire, says De la Beche, " besides the above-mentioned varieties of trap, there occurs one which has the appearance of conglomerate. The base is a black and very compact trap, which contains grains of quartz, and seeming fragments of crystallised felspar and of compact fel- spathic cornean. It is probably, however, a pseudo-con- glomerate, or concretional rock, originating in a peculiar and contemporaneous aggregation of its constituent parts." * And of the same nature are the orbicular granite and porphyry of the island of Corsica, and the trap rocks around Edinburgh and in other parts of Scotland; and surely the globular structure of neither can be attributed to mechanical action. So likewise the spheroidal and concentric arrangement of the laminae in the sandstone at Dunbar, which is referred to the influence of ignited trap, but which, however effected, has probably been superinduced since the accumulation of the original incoherent materials of this sandstone ; for we cannot suppose that these spheroids have been formed by attrition ; and such an origin is still less applicable to the concentric globules in the travertin of Trivoli, and in similar tufaceous rocks, which are in a state of constant and progressive de- position from calcareous waters. The brecciated appearance of some primary rocks can rarely, if ever, be confounded with that of true fragmentary formations; for one or other of their ingredients will be found assuming such delicate or intricate ramifications as cannot be reconciled with a mechanical origin. It is true that this may not be visible in every specimen, but it cannot fail to be observed when these rocks are examined on a large scale: and farther, the ingredients will commonly be found, here and there, gradually passing into each other by mineral transitions. It is very probable that cases have been observed in which the same characters are possessed by se- condary rocks ; but this would not be any objection ; it would * Geol. Trans. (New Series), vol. ii. p. 3. 346 ON THE APPARENT DISLOCATIONS [Ch. XV. only render it probable that the brecciated, like the con- glomerated, structure may occur in rocks of every descrip- tion, being perfectly independent of the modes in which the materials of such rocks were produced and accumulated. Having enquired into the nature of the angular and glo- bular concretions of rocks which impart a fragmentary ap- pearance, we are the better prepared to enter on the examin- ation of the phenomena at the junction of granite with slate : here also we shall find occurrences of a similar nature in the detached portions of granite and of slate which are respectively enveloped in the main mass of each of these rocks, but they are farther complicated by the presence of granite- veins traversing the slate; a convincing proof, ac- cording to the prevailing theory, that the latter rock has been rent and fissured by one or more convulsions. Granite-veins are supposed to belong to different periods, because they sometimes intersect each other ; and it is pre- sumed that the oldest must be those that are connected with and are in continuation of the main mass of granite, because they appear to have been formed at the time of the protrusion of the granite through the slate in a state of igneous fusion ; whilst the newer ones must be those which not only traverse the slate, but also penetrate the granite in a distinct and regular course, indicating that they have been injected since the consolidation of the granite. But even on this simple view of the subject, and admitting that the slate, in a soft and yielding state of semi- fusion, could have been broken and separated in every possible direction by a mass of melted granite, so that in some instances, as at Cape Wrath, in Scotland, the veins should far exceed in quantity the stratum that is penetrated ; yet, even after every allowance of this kind, it is difficult to explain the fact, that the individuals of each set of veins not only intersect and heave each other in the most promiscuous manner, but the same thing happens between the veins of distinct kinds. Again, we not only find granite-veins within granite, intimately connected with Ch. XV.] OF THE PRIMARY ROCKS. 347 this rock, at some points, by mineral gradation, but also, as at the Logan Rock, in Cornwall, united by large crystals of felspar, which are imbedded partly in the granite and partly in the vein ; and, in like manner, the granite-veins in the slate, though frequently divided on each side by open lines or seams, resulting from the action of the elements, yet, in very many instances, the vein and the slate adhere so strongly together that they cannot be accurately separated by the hammer ; and appear to pass so gradually into each other, that a difference of colour is the only distinguishing character; and even this is sometimes, as at the junction of the main mass of granite and of slate, so indistinct that we cannot see where the one begins and the other ends. Not only so, but the vein and the slate exhibit the same phenomena at Bunawe, in Scotland, as the vein and granite do at the Logan Rock. " Although the granite-veins," says Macculloch, " sometimes run through the schist in a distinct form, just as they do in the junction of Loch Ranza, yet in many places they are intermingled with it in the most remarkable manner; crystals of hornblende may be observed shooting far into the body of the schist, so as to render it often difficult to assign the limits of each rock, and in a less degree, the quartz and felspar of the veins exhibit the same appearance." * Now these facts do not proclaim the disturbance of a sedimentary rock by another of a totally different nature; they rather suggest the idea that these rocks are of a similar composition, and have been formed in the same manner. In the case of granite and gneiss this suggestion would not appear so startling, but in the case of granite and slate it carries an air of absurdity, according to the notions generally entertained of clay-slate. Let us, however, instead of using a term so indefinite as clay-slate (and so inappropriate, as far as the Cornish schistose rocks adjacent to granite are concerned), rather regard these slates as compact felspars, * Geol. Trans, vol. iv, p. 120. 348 ON THE APPARENT DISLOCATIONS [Ch. XV. united with small and variegated portions of mica, shorl, or hornblende, which are commonly equably diffused, imparting colour to the slates ; and the compact felspar, since it con- tains more silica than crystallised felspar, may be looked on as an union of felspar and quartz. On this view, both the granite and the slate have a similar composition, differing from each other only in the proportions and in the mode in which their constituents are combined, so that it matters not whether the strata be gneiss or clay-slate in contact with the granite ; it is equally easy to conceive these various appearances to have arisen from the peculiar arrange- ment of the constituent parts during combination. Let us turn to the various felspathic rocks which exhibit light and dark-coloured stripes, and these will often be found to display the most intricate flexures and wavings ; and also sometimes the light, and at others the dark parts, will be seen ramifying and intersecting each other precisely in the same manner as the granite and the slate ; and in the variegated marbles and serpentines appearances somewhat similar may also be ob- served ; and yet in none of these cases can it be supposed that the veins and the bases have had a distinct origin. It is true that in these examples the rocks are of a more simple composition, and therefore we do not find such great contrasts as in the associated granites and slates; but this cannot be a serious objection, since we not only find the different kinds of granite in a state of union equally abrupt, and also the various crystalline slates similarly circumstanced ; but in the middle of a block of granite, far removed from the schistose group, the constituents are oftentimes so arranged, as not only to resemble angular portions of hornblende-rock, shorl-rock, and porphyries, but also of gneiss, mica-slate, and felspar-schists. Are we not taught by this, that what has been produced on a small scale may have happened over more extensive tracts. These small rays of light, however, do not alone illuminate this interesting page of nature, but others, gradually increasing in splendour, conduct us step by Ch. XV.] OF THE PRIMARY ROCKS. 349 step, until the characters become perfectly legible, though they may be misinterpreted. Thus, in the mountains of Ben- na-vear, near Balahulish, at Loch Ranza and Loch Spey, and in other parts of Scotland, described by Macculloch, angular portions of various kinds of slate (as micaceous, hornblende, and argillaceous schists, gneiss, and quartz-rock) occur in the granite, insulated and far apart in some places, and in others equalling in quantity the granite, which seems only to form a connecting paste or basis ; and this again is succeeded by the respective slates, in which the granite is wanting excepting in the state of veins and subordinate masses. Perhaps it may be contended that this fact indicates that these slates were shattered and detached into angular portions by the invasion of melted granite : admitting, however, that such fragments of slate could be thus detached, and preserved in insulated positions by the rapid cooling of the granite in the interstices, yet, when we leave the junction of the granite and slate, and proceed within the granite, numerous difficulties assail us at every step. How could the small portions of slate which there occur retain their accurately truncated and angular forms when involved in intensely heated granite far exceeding in bulk the included fragments ? By what coun- teracting principle have like causes been prevented from pro- ducing like effects? Why did these slates escape fusion when exposed to heat equal to the melting point of granite ? since one of them, the hornblende-schist, will fuse before the jet of a common blowpipe; as will also the primary clay-slate, if more felspathic than quartzose. Again, it may be enquired how these portions of slate could have escaped the perfect metamorphosis which is attributed to the most elevated tem- perature ? Gneiss, this theory asserts, is more schistose and distinctly stratified at a distance from the ignited mass of granite, but, as it approaches thereto, it becomes more crys- talline, until at last the lines of stratification are nearly ob- literated, and it becomes granitic gneiss not to be distinguished 350 ON THE APPARENT DISLOCATIONS [Ch. XV. mineralogically from true granite. If this state of things be dependent upon the heat to which these strata have been sub- jected, how came it to pass that the angular portions of gneiss could have escaped fusion and an assimilation to perfect gra- nite, when placed not only in contact with, but absolutely enveloped in a mass of melted granite ? In like manner, how could the clay-slate preserve its identity, when we are taught that it is converted into hornblende-schist by exposure to an elevated temperature ? We will not, however, enlarge on this topic : many other inferences are so obvious that they will readily suggest them- selves : it remains to know whether these objections can be satisfactorily explained. To return to the granite-veins. Do they always proceed from the granite, gradually tapering away in such a manner as fissures might be expected to do if produced by the forcible protrusion of a large mass through the stratified rock ? It has been stated that they exhibit no uniformity either in their size or mode of arrangement ; they proceed from the granite in every direction, underlie at every angle, and, in their course, become larger and smaller in the most disorderly manner; and not only so, but even when comparatively regular, and a foot in thickness, they are often found to terminate in minute strings before they reach the exposed part of the mass from which other veins are given out. These veins also may be frequently seen, not only in the granite, but also near the junction, and even in the slate entirely, at a considerable distance from the granite, varying both in length and thickness, and terminating at both ends either abruptly, or gradually diminishing to a mere thread. When veins of this description occur wholly in the schistose rock, far removed from the granite, they can scarcely be supposed to have proceeded from the latter: thus, the innumerable granite-veins that are completely enveloped in gneiss and mica-slate, could not possibly have had such an origin ; nor the short venous portions and irregular masses of granite Ch. XV.] OF THE PRIMARY ROCKS. 351 which are similarly situated in clay-slate, and which, as in the gneiss and mica-slate, are so closely blended therewith as to pass imperceptibly into each other. Sometimes, indeed, these rocks are variously intersected by quartz-veins, which every one would pronounce to be contemporaneous, and yet these often assume a compound crystalline character, and in some parts perfectly resemble the granite-veins. And it must also be observed, that when granite-veins attain a foot or more in thickness, the open seams traversing the containing rock, dividing the mass into concretions, often pass through the veins, giving rise, when the concretions are detached, to fragments or blocks which consist partly of slate and partly of granite : precisely as happens at the junction of the main masses of granite and slate, when these have not been separ- ated by seams of structure coincident with the line of union. In short, the granite-veins put on such an endless variety of appearances, as to size, length, and number ; and sometimes interlace and reticulate the schistose rock in such a com- plicated manner and so profusely, that the whole mass appears to be a congeries of these veins, the basis, as the gneiss at Cape Wrath, forming but a small portion of the whole : so that it can be compared only to the venous appear- ance of a thickly mottled and variegated marble. This fact alone ought to lead us to suspect, that probably these veins have not resulted from the injection of granite into the fissured slate. We have, however, been so long accustomed to consider that all veins were once fissures, and that all portions of rock that have an elongated form or structure are veins, that it requires no little effort to prevent ourselves from jumping immediately to this conclusion, whenever we meet with such appearances. And yet we know that it has been frequently stated, that even sandstones, gypsums, and other sedimentary deposits often exhibit forms which, in the primary rocks, would be denominated veins. Professor Jameson has repeatedly pointed this out ; and Dr. Macculloch has also recorded some appropriate examples. After having 352 ON THE APPARENT DISLOCATIONS |Ch. XV. described the mutual interference of limestone and schist with each other at Gow's Bridge, in Glen Tilt, he remarks, that " it is evident, that both the prolongation of the limestone and that of the schist bear a great resemblance to the veins which, in the case of granite, may be traced from a mass of that substance into the neighbouring rocks : yet there is no doubt that both these rocks are stratified. I have observed the same appearance in clay-slate, and it is frequent in the islands of Scarba and Jura, where this substance alter- nates with quartz-rock, and where both rocks are greatly contorted. A more remarkable example occurs in a rock which constitutes one of the numerous beds of which Schi- hallion is composed. This rock is a mica-slate, containing imbedded fragments of granite and of quartz-rock, often of considerable magnitude. The larger fragments of quartz- rock are sometimes partially split at right angles to its lamellar structure, and these fissures are filled with the sub- stance of the mica-slate, putting on the same pseudomor- phous appearance of a vein. I have found similar veins of red sandstone in the limestone of Arran, and they have also been seen in trap : thus, at Kinnoul, greywacke schist is pro- longed into ramifying veins in the interstices of a trap rock by which it is broken and disturbed. * And, lastly, the con- tortions of argillaceous schist in Isla are very great, and sometimes they are elongated into the body of the quartz- rock : " The appearance is so like that assumed by veins, that it is not surprising if it has sometimes misled observers." f Now we frankly confess that, in this case, as in that of dis- criminating between the layers of granite and the strata of primary slates, we have not only lost our way, but cannot discern it even when thus pointed out ; for we have looked again and again, and cannot perceive that Nature has made any distinction between granite-veins and slate-veins ; we can only see, in this peculiar form of matter, elongated por- * Geol. Trans., vol. iii. p. 279. f Western Isles of Scotland, pi. xxii. fig. 5. Ch. XV.] OF THE PRIMARY ROCKS. 353 tions of the respective rocks, which may be either con- nected therewith, or entirely insulated in the traversed mass. That slate-veins have not been more frequently observed, may be attributed to their generally possessing the same colour as the containing rock, on which account they are not easily distinguished ; and also to such veins having a compact structure, similar to the massive varieties of rock abounding in each primary slate, and which, as has been already stated, only differ from each other in structure, being species of the same genus. The apparent movements which intersecting granite- veins exhibit have been adduced as a proof of the mechanical origin of these veins: we will not, however, consider this subject at present, as we shall shortly discuss the nature of mineral veins, in which precisely the same phenomena are more clearly developed. We will, therefore, conclude this topic with Dr. Macculloch's very interesting remarks on the arrangement of the minerals which constitute the granite of Portsoy ; he considered it necessary to apologise for entering into such minutiae, but we do not hesitate to quote the passage at some length, as it appears to bear with no inconsiderable weight on the subject under discussion. " The singular disposition and mutual relations of the crystallised substances which form the granite of Portsoy are known to have afforded to Dr. Hutton an argument for its igneous origin ; and its peculiar character (called graphic) has been supposed to arise from a simultaneous, or nearly simultaneous, crystallisation of the several substances con- tained in it. Thfe specimens about to be enumerated are such as not only throw considerable doubt on this explanation, but are in fact sufficient to prove a sequence of epocha even in this limited space, and to show that the compound rock in question has been formed by successive operations, the nature of which, however, I fear we shall not easily de- termine." " In one of the specimens a detached crystal of shorl, of a flattened and irregular figure, has been broken into A A 354 ON THE APPARENT DISLOCATIONS [Ch. XV. four parts, by transverse fractures, which have again united without the intervention of any intermediate substance. Pre- vious to this reunion, however, they have been slightly shifted, in such a way that the several parts of the fractures project, and the whole crystal has undergone a slight deviation from its original straight line. If it be alleged that this appear- ance could arise from a disturbed crystallisation, the next specimens will remove any doubt on this head. In these, the crystals have not only been fractured in the same way, across their axes, but the fractures are filled by the quartz and felspar, which constitute the body of the rock. The granite-veins of Arran do not show more clearly the ramifica- tion of a central substance through the fractures of the neighbouring rock, than these specimens show the veins of quartz proceeding from the mass, and penetrating every fissure which had been formed in the crystal. It is perfectly evident that whatever is true of the above cited granite-veins must also be true of this rock, that the shorl has been crys- tallised, then broken, and penetrated by quartz in a state of fluidity. Nor is there any intermixture of the matter of quartz with the matter of shorl, but the line of separation is most accurately drawn between them. It follows then from these circumstances, that the rock in question is not a simul- taneous formation from a state of fusion, nor can we readily understand how it can be the effect of fusion at all, con- sistently with the chemical principles we acknowledge. Had such a mass of fused quartz invaded the minute fragments of shorl which the specimen exhibits, the latter must either have been formed into a shapeless mass, or at least the asperities of fracture could not have remained in a substance whose fusibility is so much lower than that of quartz. To those who attribute the formation of this rock to aqueous solution, it may be objected that the crystals of shorl are sometimes bent without fracture, so as to form considerable curvatures. The noted fragility of shorl will not allow us to suppose that it could be bent without breaking, unless it had been pre- Ch. XV.] OF THE PRIMARY ROCKS. 355 viously rendered flexible by some chemical agent possessed of powers which we have not hitherto discovered in water." * We shall hereafter revert to this curious subject : but it may be now observed that this interference of one crystal with another, in a compound mass, is not uncommon, though seldom so distinctly displayed as in the Portsoy granite ; in- deed, any large grained variety of granite, if minutely ex- amined, will be found to afford some examples of this phenomenon. The general opinion concerning the nature of veins is, that they are of three kinds : 1st. Those which are referred to an igneous origin, such as injected and sublimated veins. 2d. True veins, or such as were originally open fissures, but subsequently filled by chemical or mechanical depositions from water. 3d. Contemporaneous veins, or such as seem to be coeval with the aggregation of the component par- ticles of the rocks in which they occur. The last mentioned are now often called veins of segregation : a term introduced by Professor Sedgwick, at the suggestion of Mr. Whewell, to express that they have been formed by a separation of parts during the gradual passage of the mineral masses into a solid state. Now if any veins of this character occur in Cornwall, we may commence with them as a fixed point with whicn our subsequent observations may be compared. " In all the crystalline granitoid rocks of Cornwall," says Professor Sedg- wick, " there are many masses and veins of segregation. Such are the great contemporaneous masses and veins of shorl- rock ; and some of these are metalliferous. The decomposing granite of St. Austle Moor is traversed, and sometimes en- tirely superseded, by innumerable veins of this description. Upon these lines of shorl-rcck, there is often aggregated a certain quantity of oxide of tin, which sometimes diffuses itself laterally into the substance of the contiguous granite. * Geol. Trans., vol. ii. p. 432. A A 2 356 ON THE APPARENT DISLOCATIONS [Ch. XV. After examining this district with Professor Whewell during the summer of 1828, we left it, in the conviction that several of the neighbouring tin works were opened, not upon true lodes, but upon veins of segregation" * These remarks of the Professor were called forth in noticing Mr. Weaver's ob- servations on the metalliferous veins of the south of Ireland : viz. that the copper ore sometimes occurs in true veins ; and, at other times, is very generally diffused either in separate particles, or in strings, veins, and filaments, more or less connected with each other, but not continuous, and therefore contemporaneous with the rocks to which they are subordin- ate. Now this is precisely the condition of both copper and tin ores in Cornwall ; for they are never in that country arranged " in great vertical dykes of metallic ore," as seems to be the common opinion, but in grains and in globular and angular portions of various forms and dimensions, or in layers, veins, and strings, completely insulated either in the quart- zose part of the veins, or in the rocks, whether granite or slate, whether enveloped in angular masses, or horses, in the veins, or connected with the main mass in which the veins are situated. If, then, such a disposition of the ores be an indication of segregation, all the metallic minerals of Cornwall are referrible to this mode of production. And since the metalliferous veins of St. Austle Moor are admitted to have had a similar origin, in what respect do they differ from those in other parts of the county ? Some may say that they are shorter than true veins, and may be generally seen terminat- ing in length and depth : this may appear very plausible, but will not bear the test of a rigid examination. It is true, that innumerable veins in this district answer to this description ; but it is no less true, that, between the smallest of these, and the large regular lodes of the adjoining mines, which have not been outivorked in length or depth, there is a regular gradation as to size : so that no characters have been detected * Philos. Mag. or Annals, vol. ix. p. 284. Ch. XV.] OF THE PRIMARY ROCKS. 357 by which they can be distinguished from each other. Nor does the difficulty terminate here. It has been asserted, that such veins of segregation abound in granite ; and it is like- wise well known, that veins, having precisely the same appear- ance, are frequently found in the slate ; and when they are very numerous in a given layer, the whole collectively are considered as a lode by the miner, whether they occur in a schistose or granitic rock, as in the mines of Lanescot, and of Balleswhidden. In fact, these depositories of ore in slate are as well entitled to the name of stockworks as when situated in granite. So that, we have also veins of segrega- tion in slate ; and who can draw any line of demarcation be- tween such veins, and the largest veins which have been explored by mining operations? or, indeed, even between the same description of veins which traverse both the granite and the slate at the junction of these rocks ? Now according to the prevailing theory, veins cannot be contem- poraneous when they intersect granite and slate, because these rocks are supposed to have been formed at different periods; and we have heard this circumstance used as an argument against the possibility of the vein being of such a nature ; but surely this is not correct reasoning : the question is whether they are or are not contemporaneous, without any reference to the nature of the origin of the containing rock. We have frequently and carefully examined numerous parallel quartz-veins, running through both rocks, and differing from each other in size from a mere thread of a few inches in length, to a large lode extending beyond the limits of observa- tion, but have not been able to detect any other distinction, all bearing the same relations to the containing rock, and exhibiting the same phenomena when interfered with by veins which have a different course. Even Mr. Carne, though he founds his distinction between true and contemporaneous veins on their difference of dimensions, observes, that some veins, which are very short, belong, perhaps, to the former order ; and that others, of considerable length and width, to A A 3 358 ON THE APPARENT DISLOCATIONS [Ch. XV. the latter. Since, therefore, the systematic characters of these orders are so indefinite, and since Nature does not appear to have made any discrimination between them, we think that we are justified in concluding that the veins which traverse the primary rocks are all of the same nature ; and if so, it follows, that all are contemporaneous, since some are indisputably of this character. Thus furnishing another argument in addition to those already advanced, that the granite and the slate are likewise contemporaneous, being modifications of the same mass, only differing from each other in structure, the result of the same constituent particles variously aggregated. In confirmation of this view, we will proceed to enquire whether the appearances of veins which are supposed to indi- cate a mechanical origin, furnish satisfactory evidence in support of this opinion. True veins, or veins of fissure, as they are sometimes termed, are defined to be "the mineral contents of vertical or inclined fissures, nearly straight, and of indefinite length and depth/' * This is Mr. Game's definition ; and, as far as we are aware, it has not been objected to by geologists : indeed, it accords with the idea long entertained, that veins were originally open fissures. In the first place, we object to this statement because the Cornish veins are never straight throughout their course, and their sides are never parallel for any considerable distance, since the veins are constantly varying in their width : for, as stated in the ninth chapter, veins undulate both in the direction of their course and descent, forming curves, which in general appear to be very irregular. That veins are very complicated in their course is proved by the fact that no inconsiderable skill is requisite in dialling ; whereas, if they were nearly straight, this oper- ation might be easily performed, if not dispensed with altoge- ther. As regards the indefinite length and depth of veins, * Geol. Trans, of Cornwall, vol. ii. p. 51. Ch. XV.] OF THE PRIMARY ROCKS. 359 we repeat that they do sometimes terminate ; and although the miner often finds that veins which have tapered away into a mere string again expand into their original dimensions, yet they are not always worked to this conclusion ; for, if they continue very small for some distance, they are generally abandoned : and that they are not of indefinite length may be inferred from the fact that the continuations of profitable veins, belonging to adjacent mines, have been sought for in vain within the limits of probable dislocations. Indeed, if veins were of this indefinite length, mining speculations would not be so precarious. If the term indefinite means any thing, true veins have no terminations : we will not, however, lay much stress on this point; and, perhaps, it was only meant to express that the extent of the length and depth has not been ascertained. Again, it is said that veins were originally fissures; first, on account of the resemblance in their forms ; and, secondly, because they contain mechanical deposits such as have been ob- served in actual fissures, now communicating with the surface. The noted case of Relistian mine has been often adduced as a proof of the latter ; but we have already attempted to show that the masses of conglomerate enclosed in the lode are not of a fragmentary nature, but owe their peculiar appearance to a particular structure, not uncommon to some rocks, and which is participated in by a chlorite-schist in the vicinity of the lode. As regards the resemblance between the form of veins and fissures, this cannot be considered conclusive ; if so, the spaces occupied by the small elongated portions of slate were also once fissures, as indeed even the strata themselves, for they might be equally compared with chasms such as are produced by earthquakes. On the supposition that veins were originally fissures, Cornwall must have been subject to innumerable convulsions : and in some districts, the total thickness of these openings must have borne no inconsider- able proportion to the whole mass. In St. Just, for instance, within a single mile, how numerous the parallel lodes are A A 4 360 ON THE APPARENT DISLOCATIONS [Ch. XV which have been worked ; and these are few compared with those that have not been examined, and still more so with those veins that are not metalliferous. Now these parallel veins are supposed to be of the same age, and, being indefi- nite, they, of course, reach as far as Mount's Bay, a distance of seven miles ; conceive, therefore, the comparatively narrow peninsula of the Land's End district, intersected by thousands of open fissures, inclined at a great angle, and yet the severed parts not falling together according to the known laws of gravity. But, perhaps, it may be argued that, although these veins belong to the same epoch, they were not absolutely co- existent as fissures, having been successively formed at various intervals during this period. Then we must fancy that con- vulsions followed each other for a certain space in one direc- tion, and afterwards operated in other lines, in order to explain the position of the different series of parallel veins, described by Mr. Carne : but we will shortly endeavour to show, that the intricate interference of veins with each other cannot be easily reconciled with this notion. We must not, however, leave this topic of fissures without noticing the nature of the union which subsists between veins and the containing rock, both as regards their respective structures and composition. It has been stated, in the ninth chapter, that large or true veins are often so situated as to occupy, as it were, spaces parallel with the joints or seams of the traversed rock ; that is, they are parallel with one or other of the extended surfaces, resulting from the aggregation of similar concretions. We will not, however, from these facts venture on any generalisation, but repeat what we have already stated before the British Association, that the subject is worthy of careful consideration, in order to ascertain to what extent, and under what particular circumstances, this arrangement obtains. The same condition is of very common occurrence on the small scale ; for though the line of contact between veins and the rock, as between different kinds of rock, granite and Ch. XV.] OF THE PRIMARY ROCKS. 361 slate, for instance, is often distinct, and marked by an open seam or joint, yet it is no less true that such a separation does not always exist, the vein and the rock forming indivi- dual concretions, in which both are firmly and intimately united : and this is best observed between the included por- tions of slate and the substance of the vein ; as in such posi- tions decomposition is not so energetic, the percolating water generally following the line of junction between veins and the main mass of rock. Now it may be conceded, that fissures would probably be effected in the direction of one or other of the great lines of structure, in preference to a disruption of the solid rock : but it cannot be so readily allowed, that the substance of veins introduced into such fissures could unite with the already consolidated materials of the rocks, so as jointly to compose the same concretions ; that is, that one portion of a concretion has been formed at one period, and another at a different time long subsequent to the former. Concerning the composition of veins, if it be admitted that quartz, carbonate of lime, and some other minerals, can be introduced into veins in a state of solution ; yet it will not, we presume, be supposed a possible event, that granite, slate, and other rocks could be deposited in fissures by a similar pro- cess : and yet the true veins of Cornwall generally contain no inconsiderable proportions of the adjacent rocks ; and as these vary in nature, so do the rocky parts included in the veins, as well as the other mineral contents. Oh ! but, say the advocates of the mechanical origin of veins, these portions of rocks are neither exudations nor depositions, but fragments detached from the adjacent mass, which explains their re- semblance in composition. Can any one who has maintained such an opinion possibly have examined the position of these insulated portions of rock, or even reflected on their nature ? for it appears to be a most self-evident proposition, that these pieces, whether detached during the disseverment of the solid rock, or during the violent injection of the matrix into the 362 ON THE APPARENT DISLOCATIONS [Ch. XV. supposed fissures, would immediately fall to the bottom of the empty chasm, or subside through the injected fluid, whether igneous or aqueous, and not remain in positions which com- monly correspond with the bearings of the adjoining mass. And let it be also remembered, that these insulated portions also abound in veins of various sizes, sometimes composed entirely of metallic minerals, and at others of quartz ; at one time intersecting likewise the matrix of the large vein, and at another being blended therewith ; and, moreover, these subordinate lodes (or veins within veins) exhibit all the phe- nomena of true veins, not even excepting those commonly referred to movements. How and when were such veins formed ? When they occur in granite, they are called con- temporaneous. According to the prevailing theory, however, this cannot be their origin, because the fragments of rocks, and the matrix of the vein, cannot be supposed to be of the same formation. How then could the different series of inter- secting subordinate veins be produced? How could the successive fissures and injections be accomplished within the consolidated vein? These objections equally apply to all veins in rocks, which are known to terminate in length and depth, as indeed they would also to true veins, if we would only admit the analogy of the known to apply to the unknown limits, instead of concluding that the largest veins must be of indefinite depth, because they extend lower than 1 500 feet below the surface ; a measurement utterly insignificant com- pared with the probable thickness of the earth's crust, and therefore not an adequate datum for such a deduction. Analogy is surely, in this case, the safer guide of the two. We find veins descend to a certain depth, retaining their average breadth till they terminate ; others continue their course lower than the deepest mines, but they likewise may not extend beyond limits, which may be of moderate dimen- sions compared with the diameter of the globe, though pro- bably extending beyond the reach of mining operations. It is time, however, to turn our attention to the inter- Ch. XV.] OF THE PRIMARY ROCKS. 363 section of lodes, a class of phenomena which has been more studied than any other on the subject of veins, and which is still considered to establish the axiom laid down at a very early period, that a vein which traverses another is more recent than the one intersected. Mr. Carne, in the paper already alluded to, has attempted, on this principle, to ascertain the relative ages of the Cornish veins, or rather we should say of the true veins ; for his other two orders, the contem- poraneous and doubtful, have not been included in this con- sideration. But we do not propose to walk in the same track, preferring our free and heterodox mode of enquiry, by bringing all kinds of veins, without any exception, to the test of this rule. In the first place, contemporaneous veins, or veins of segregation, very frequently intersect each other, giving rise to the usual appearances of dislocations and displacements. This fact could not escape notice, and Mr. Carne attempts to explain it away, by asserting that " these phenomena are only apparent ; as it is, in general, easy to perceive that what seem to be separate parts of the same vein, are different veins, which terminate at or near the cross vein." We would ask, do not the intersected portions of true veins often possess the same character ? We are aware that the point in question has not been very extensively investigated, for the obvious reason, that the miner is not much interested in it ; and such an examination, indeed, would be generally rendered very difficult, as the points of intersection are commonly obscured by a blending together of the matrices, and more especially by decomposition. The determination of this question is not at present very material, since it is admitted that this mark of dis- tinction cannot always be detected in contemporaneous veins. But how do these veins comport themselves, when they interfere with doubtful veins ? When contemporaneous quartz- veins (that is, such as are short, crooked, and of irregular size) meet granite-veins, whether in granite or in slate, they are sometimes intersected thereby, at other times they are 364 ON THE APPARENT DISLOCATIONS [Ch. XV. themselves traversed ; ancTexamples of both kinds may not only be seen within a few feet of each other, but if the granite-vein be branched, each ramification may be differently affected. But granite-veins, whilst within granite, are said to be con- temporaneous, but in slate, or when equally traversing both rocks, they are then doubtful. On this view, the contempo- raneous quartz-veins are sometimes older and sometimes more recent than some granite-veins which are of the same age, or than others which are of doubtful origin : so that if there be any value in this principle, it involves the dilemma that the veins existed before the containing rocks, that the latter were broken and fissured previous to their formation. The same arguments apply to elvan-courses, for they are similarly cir- cumstanced, as respects contemporaneous quartz-veins, both in granite and in slate. The prevailing opinion is, that gra- nite-veins and dykes have been produced at every possible period since the protrusion of the granite ; and that they are posterior to the stratified rocks in which they occur ; but inasmuch as they are sometimes intersected by shortj crooked quartz-veins which are as old as the slate, the latter must, ac- cording to the rule, be more recent than the igneous rock. Again, what is the result of the intercourse of these quartz- veins with true veins ? It is said " that they are always tra- versed by them," but we do not think that this generalisation has been deduced from observation ; we rather incline to the opinion that it is of theoretical origin. Wherever small irregular veins are contiguous to large metalliferous veins, they are regarded as strings or branches ; generally they are fairly entitled to this denomination ; but we have seen the lesser veins extended into the substance of the larger, just as granite veins are often elongations of the mass, but sometimes con- tinue their course through the granite in as distinct a manner as through the slate : such minute characters, however, are not very visible under ground, even when an object of research ; but on the cliffs and on the sea-shore, they may be easily distinguished. Ch. XV.] OF THE PRIMARY ROCKS. 365 How are doubtful veins affected by true veins ? Granite- veins are sometimes traversed by contemporaneous veins, and also by true veins, if it be admitted that the large quartz-vein, intersecting both granite and slate, at Mousehole, is a true vein, and, if so, a parallel vein of the same nature is crossed by this granite vein at the distance of a few feet. Elvan- courses are commonly traversed by true veins : sometimes the latter are included in the former; but, in one instance, as at Polgooth, the elvan continues uninterruptedly through several metalliferous veins. The general opinion is that the elvans are veins of fissure, formed by igneous injections : Mr. Carne thinks that circumstances are in favour of their being true veins, and the miner sometimes considers them to be lodes; the facts, however, already brought forward, render this very doubtful, since they both traverse and are intersected by con- temporaneous as well as by true veins. We now proceed to show that the relative ages of the sub- divisions of true veins are of the same uncertain character as those of the doubtful order. In the first place, it may be observed, that the application of the rule of intersection has led to the formation of several classes which seem to be very natural arrangements, inasmuch as they differ from each other either in composition, or in their bearings, or in both respects. Thus, the two first classes are tin lodes ; the third, fourth, and sixth, copper lodes ; the fifth class, cross courses or veins, not varying more than 30 from the north ; the seventh, cross jlulcam, the same as the fifth, only entirely composed of clay ; the eighth and last, the slides, of the same nature as the seventh, as regards composition and bearing. Now the tin lodes are divided into two classes, because some are traversed and heaved by others ; and they have been termed the oldest and the more recent tin lodes : the former are said to dip generally towards the north, the latter for the most part towards the south. But there is no general rule ; indeed such veins, underlying to the same point of the com- 366 ON THE APPARENT DISLOCATIONS [Ch. XV. pass at different angles, cause intersections ; and, therefore, other classes ought to be instituted ; if, indeed, a character which can only be discovered by extensive workings, and is always subject to change, ought to be esteemed of any value in a classification. The directions of the tin lodes are com- monly within a few points of east and west, but there are numerous exceptions : those of St. Just run nearly north and south, as does also one lode in Polgooth mine ; and they must, therefore, be considered not as tin lodes, but as belonging to the class of cross courses. In Polgooth, also, occur veins of tin, pointing, not east and west, but north-west and south-east, and these contra lodes traverse and heave some regular east and west tin lodes ; so that, according to the law of intersection, contra lodes are sometimes more ancient than the first class or the oldest true veins. We must, therefore, add to the pre- sent class of tin lodes, contra, cross, and as many other classes as there are exceptions. Besides, it may be stated, that veins in some parts contain copper ore, and in others, these mi- nerals are intermixed with the tin ; and since tin and copper lodes have the same bearings, how is the class to be deter- mined when such veins do not interfere with those of an un- mixed character? In this manner, each of the classes might be considered in review, and without bringing forward other facts than those already on record, might be proved to be objectionable : thus, it might be stated, that the more recent are only separated from the oldest copper lodes, because cross courses are sometimes intersected by copper veins; if this additional class had not been instituted, the fifth and seventh classes would have been united ; indeed, the last of these is not distinguished by the miner from cross courses ; and these combined classes might also have comprised the eighth or slides, since they are only mineral veins which commonly run parallel with the metal- liferous. The only generalisations that can be made concerning the veins of Cornwall, in the present state of knowledge, are Ch. XV.] OF THE PRIMARY ROCKS. 367 that they have various dimensions ; that they cross each other in certain determinate directions ; that they are composed either of solid earthy minerals, of ores, or of loose matter, such as clay or friable quartz (which are probably only the first in various stages of decomposition), some or all of which are frequently united in different proportions. Having considered the nature of the intersection of veins as an indication of their having been formed at different times, we will next discuss the apparent movements by which these intersections are supposed to have been accom- panied. In the most simple form of such an occurrence, the prevail- ing opinion on this subject is very plausible, and seems to be confirmed by the fact, that in the mines, portions of the in- tersected vein have been observed to follow the side of the traversing one, so as to point out, as it were, the direction and the nature of the displacement : but, when we contemplate the curvatures, both in length and depth, of the supposed fissure, (now occupied by the substance of the cross vein,) the cor- responding convexities and concavities of its opposite sides appear to be perfectly incompatible with such movements ; for the fissure would have been obliterated thereby, in many parts where the vein now possesses its full dimensions. Now, as regards the indications of ore along the cross vein, inter- mediate between the discontinuous veins, we do not, on the supposition of both being contemporaneous, think it an un- warrantable conjecture, that the power of cohesion, electro- magnetism, or whatever force governed their arrangement, should have left at the points of interference some traces of the joint action of the intersecting currents. However, be this as it may, how can we surmount the difficulty, that one vein crossing another, without any movement in one place, in others at different depths, produces heaves of various extent, and vice versa, as in Dolcoath mine ? When more complicated instances of intersection are ex- amined, the imperfection of the theory is still more evident : 368 ON THE APPARENT DISLOCATIONS [Ch. XV for instance, when one vein crosses several others, which are of the same denomination. At Mousehole, two granite veins, of no inconsiderable length and regularity, are heaved by a quartz-vein ; one to the distance of three feet, but the other not more than six inches ; and this happens within a few feet of each other : how could one part of the rock be moved farther than the other? or, how could any displacement happen ? for not many yards off [the quartz vein terminates, and the joints of the slate continue without any derangement across the direction of the vein. The intersections of small contemporaneous veins, however minute, frequently afford similar examples ; and in the case of large veins, such as are explored in mining operations, this condition of the various disconnected parts is so common, that we need only refer to the instances already quoted in the ninth chapter. Nor must we omit to remark that if the lines dividing the layers or laminae of rocks be closely inspected, they will sometimes, especially when the rocks consist of al- ternating layers of different composition or colour, seem to be heaved either with or without the intervention of veins of any description : the opposite layers, however, have not always the same dimensions ; or, as in the instance of gneiss traversed by a quartz-vein, in Fudia, described by Macculloch, the laminae are heaved by the latter as far as it extends, but im- mediately beyond they are not disconnected. * Lastly, veins exhibit the same apparent phenomena of motion without the presence of cross veins or fissures, as in Balnoon mine : and the laminae of rocks likewise afford analogous appearances, as in the gneiss of the Island of Lewis, also described by the geologist just quoted, f Granite-veins, whether of different composition or not, and even the crystalline ingredients of many rocks, also interfere with each other in every possible manner, so as to give representations of all the phenomena of metalliferous veins. * Western Islands of Scotland, Plate xi. fig. 3. t Idem, Plate xvi. fig. 5. Ch- XV.] OF THE PRIMARY ROCKS. 369 In concluding our remarks on this important subject, we venture to express a hope that if the objections advanced are not altogether unanswerable, yet, that they are of sufficient weight to prove that the apparent movements which the pri- mary rocks are supposed to have undergone, are not esta- blished on a satisfactory basis : since it cannot be admitted that the curvatures and convolutions of their laminae and beds, their occasionally brecciated and conglomerated struc- ture, their elongated masses in the form of rock-veins, and, lastly, their mineral and metalliferous veins, do furnish phy- sical evidence of mechanical movements ; for such appear- ances, in most instances, if not in all, may have originated from peculiar modes of arrangement during the aggregation of the constituent particles of the primary rocks. B B 370 RECAPITULATION OF THE OBJECTIONS [Ch. XVI. CHAPTER XVI. MAY NOT ALL THE PHENOMENA OF THE PRIMARY ROCKS, BOTH STRATIFIED AND UNSTRATIFIED, BE SATISFACTORILY EXPLAINED ON THE SUPPOSITION THAT THESE ROCKS ARE OF CONTEMPORANEOUS ORIGIN? Recapitulation of the principal facts, and of the conclusions deduced therefrom. The primary slates not sedimentary deposits are in their original positions their contortions, fragment-like structure, and veins, no indications of vio- lence not derived from the metamorphoses of fossiliferous strata. The pre- vailing theory not founded on a correct deduction from facts. Another ex- planation of the phenomena of the primary rocks offered its consistency with the general opinions concerning the nature of the secondary and tertiary formations. Conclusion. IN the preceding chapters we have advanced objections against various opinions which are generally entertained con- cerning the nature of the primary rocks ; and we have, at each step, successively concluded that the phenomena which were under consideration might receive a more plausible solution, by supposing them to have been coeval with the formation of these rocks. It is now proposed to show that these several conclusions are consistent with each other, and that the theory of the primary rocks, resulting from their combination, may be con- joined with that of the secondary and tertiary formations which now prevails, and which has been deduced from a com- parison of their phenomena with analogous appearances, pro- duced by causes now in operation. But, before entering on these considerations, it is desirable to recapitulate the ob- jections which have been brought forward, and the leading arguments by which they have been supported, in order to Ch. XVI.] TO THE PREVAILING THEORY. 371 bring the matter into as small a compass as possible, so as to render the nature of the dissent more evident. In the first place, it is contended that the primary slates are not stratified, if it be thereby understood that they con- sist of sedimentary beds arranged by the agency of water after the manner of the fossiliferous deposits. The analogy between the arrangement of the beds, and of the laminae, in the rocks of both formations, is not disputed; but it has been argued that it furnishes no evidence that the primary slates are of a mechanical or aqueous origin, since not only granite, but also trap, and even lava, present the same appearances: in short, we maintain that arrangements which only mark the varieties of concretionary structure produced during the con- solidation of the once mobile particles of rock, and which do not characterise any particular formation, cannot be admitted to prove that the primary slates were originally sedimentary deposits. Since, therefore, the supposed indications of stratification are as applicable to granite as to the crystalline slates with which it is associated, it might be inferred that this unstratified rock is also stratified, and that the generally esteemed igneous rock is of aqueous origin. How can this dilemma be avoided ? Are there any other characters, common to the so called primary strata and the fossiliferous groups, by which they may be distinguished in all cases from the granitic rocks ? The form and structure of beds, and the alternations of rocks varying in mineral composition, as already stated, do not afford any criterion ; nor can we adopt the transitions between these two classes of strata as evidence of the primary slates being stratified, and of aqueous origin, for these again are connected with granite by a more complete bond of union, by that kind of transition which we have called the mineral, in order to keep it distinct from the mechanical, which obtains in the former case. A lengthened consideration of these difficulties has led us to infer that the prevailing classification, which divides rocks B B 2 372 RECAPITULATION OF THE OBJECTIONS [Ch. XVI. into the stratified and unstratified is very inaccurate, and more especially so since the term stratum has not been hi- therto satisfactorily defined. Again, we have objected on various grounds to the opinion that all inclined strata, and particularly the primary, were ori- ginally horizontal : 1. Because, with the exception of those secondary strata, which contain tubulites and similar fossils, the evidence in favour of such elevations is neither so decisive nor so copious as to warrant a general conclusion : on the con- trary, we know that rocks are deposited in beds which are inclined at considerable angles ; and that, when this happens, the beds are also fan or wedge shaped, being wider at their lower than their upper extremities, so that each succeeding bed, in the ascending order, becomes less and less sloped, until the cavity or hollow, in which the deposits were formed, is partially or even entirely obliterated ; a mode of arrange- ment which has its exact counterpart both in the secondary and tertiary formations, as has been observed in numerous instances in almost every country, but which, hitherto, has not been distinctly and unequivocally detected in the primary slates. 2. We also dissent from this opinion, because beds may not only be originally deposited in inclined positions, the limits of which have not been yet ascertained, but because the layers of rocks attributable to their concretionary structure, which are, for the most part, placed at great angles, have evi- dently been often considered as strata in the fossiliferous, and always in the'primary formations ; and, in the case of the latter, such beds have been described as dipping various ways, and as being sometimes parallel with the laminae and at other times differently inclined, in consequence of the mass being composed of several concretionary layers. Now, numerous examples have been adduced to show that such inclined layers, whether laminae or beds, are not confined to the primary slates ; but that they actually occur in every group of sedi- mentary rocks, whether these consist of pebbles, sand, mud, and similar mechanical deposits, or of precipitates from a state Ch. XVI.] TO THE PREVAILING THEORY. 373 of chemical solution ; and moreover, that they are also oc- casionally present in granite, trap, lava, and analogous rocks : facts which demonstrate that the so-called strata have not de- rived their inclined positions from elevation : and when to these instances are added those beds admitted to have been formed on inclined planes, there is a great preponderance of evidence against the generally received opinion that strata were ori- ginally horizontal and have been subsequently tilted up by some internal moving power. 3. We cannot subscribe to this doctrine of elevation, because the protrusion of granite, the supposed cause of such movements of the strata, is not justified by any known analogy. The internal fire, the pri- mum mobile of all the commotions and derangements of the solid crust of the earth, appears to be adequate to the up- raising of vast tracts of country, but this is effected without altering the previous position of the various rocks and uncon- solidated deposits. It causes fissures and chasms in the su- perimposed rocks, through which it forces aerial fluids, lava in a state of fusion, fragments of rocks, mineral substances, and other kinds of volcanic ejecta, which arrange themselves around the apertures so as to form conical hills or mountains composed of highly inclined and irregularly concentric layers. But in none of these cases have any appearances been observed which indisputably establish such elevations of the strata as are now the subject of consideration. And even granting that granite has been so protruded, such an elevation could not have been effected without causing fissures through the incumbent strata; for even the comparatively small effects of actual volcanic operations, which are unattended by local tiltings up of the beds, produce such openings : and if this were the case, how could the adjacent strata have escaped total destruction ? for immediately the expansion overcame the resistance, a dread- ful explosion must have inevitably followed. It may perhaps be argued, that the strata, when thus upheaved, were in a ductile state, in consequence either of their being intensely ignited, or of their not having been as yet consolidated : but BBS 374 RECAPITULATION OF THE OBJECTIONS [Ch. XVI. such an opinion, however well adapted to remove one diffi- culty, falls into another ; for it is opposed to the phenomena exhibited at the junction of granite and slate, and which, even Sir James Hall has allowed, indicate "an infusion of the liquid granite into hard and brittle strata." And we have attempted to show that the protrusion of this rock in a solid state is not only unsupported by analogy, but is at variance with the na- ture and arrangement of the granite and schistose rocks at their junction. On these grounds, therefore, we think that the theory of the elevation of strata by granite is not founded on correct and certain data : indeed, inasmuch as the nature of the term stratum is indefinite, and has been applied in an indiscriminate manner to very different kinds of beds, even by the same observer, no evidence concerning the direction and dip of strata can be esteemed, until it be determined in what sense the term has been employed. We have stated our rea- sons for not adopting Elie de Beaumont's theory concerning the relative ages of mountain chains : nothing farther, how- ever, need here be added, since many of the most eminent geologists have proved that it is not in accordance with the phenomena of inclined strata in different countries. In examining the other appearances of the primary rocks, which have been commonly referred to various movements, we have expressed an opinion that they are fallacious, and more easily explained on the supposition of their being con- temporaneous with the formation of the rocks in which they occur. We disagree with the general opinion, for the fol- lowing reasons: 1. As regards the convolution of rocks; because contorted beds alternate with other beds which are plain and regular; because the laminae of rocks are also simi- larly circumstanced ; because the colouring ingredients of rocks, whether on a large scale, as in strata, or on a small, as in laminae, are likewise subject to the same arrangement ; and, lastly, because granite and trap, travertin and other aqueous deposits, and agates and alabaster, also exhibit the most complicated curvatures and contortions, in all of which Ch. XVI.J TO THE PREVAILING THEORY. 375 the indications of forcible bendings can only be apparent. 2. Concerning the rounded and angular fragment-like portions of rock which occur in some members of the primary formation, we think them very different from some similar occurrences among the derivative rocks, but not from all ; for they some- times, like the primary, are to be attributed to concretionary structure, resulting from the peculiar manner in which their component parts were aggregated ; and in such cases the apparent fragments are always more or less intimately united with the bases by mineral transitions, or possess such frail and delicate forms as are not consistent with their supposed pre- vious attrition ; both of which circumstances are very dissimi- lar to the ordinary condition of the nodules and fragments of true conglomerates and breccia. 3. We object to the supposi- tion that granite veins were originally fissures subsequently filled by the injection of granite in a state of fusion on several grounds } but principally on three, relating to their structure, composi- tion, and interference with other veins. In form they are very seldom regular, as cracks or fissures generally are, but present an endless variety, both as regards their size and direction, and intersect each other, in the most promiscuous manner ; and, when sufficiently large to exhibit a concretion- ary structure, the lines by which this is denoted sometimes correspond with the sides of these veins ; but often, as at the junctions of granite and slate, indiscriminately involve a larger or smaller proportion of the adjacent rock. In composition, they always partake more or less of the nature of the inter- sected granite or slate, including portions of these rocks in their course, with which, as well as with their main mass, they are frequently united, by the most perfect mineral transitions ; and this is not only the case, but the prophyritic crystals, whether of felspar or hornblende, contained in the main mass or substance of the veins, are sometimes arranged across the line of junction, so as to be common both to the vein and containing rock ; a bond of connection, which has also been observed at the meeting of granite and slate, and which some B B 4 I 376 RECAPITULATION OF THE OBJECTIONS [Ch. XVI. may attempt to explain, by attributing it to the metamorphic period, when both rocks were in a state of complete or semi- fusion : but such a solution cannot be extended to granite- veins, which, according to the established theory, were injected into hard rocks, capable of being fractured, and of furnishing small angular fragments. Lastly, in their inter- ference with other veins, they intersect, and are themselves intersected by, quartz- veins, which possess both true and contemporaneous characters : thus indicating, according to the axiom, " that a vein which traverses another is more recent than the one intersected ; " that granite-veins have not only been introduced into the containing rocks, but also ex- isted prior to their formation. 4. Concerning the mineral veins, which occur in primary rocks, we contend that, so far from the present state of our knowledge on this subject being sufficiently advanced to justify any theory, it is so very imper- fect, that we have not even accurate descriptions of the objects themselves, the very elements on which the generalis- ation ought to be founded : and we have adduced numerous, and several of them new, facts, to show that, if any opinion be permitted, that of their contemporaneous origin is the more plausible. That mineral veins cannot be defined to be the contents of fissures, we have attempted to establish by the following objections: 1. No line of distinction can be drawn between veins of segregation and true veins, the former of which are said to be coeval with the consolidation of the traversed rock, and are admitted to characterise some of the metalliferous veins of Cornwall. Nor can we discriminate between large and small veins, or such as are supposed to be indefinite in extent, and those which are entirely comprised within the limits of our observation ; that is, between the commonly called true and contemporaneous veins. 2. As regards the origin of true veins in fissures, we contend that such variously inclined cracks and chasms, extending for considerable lengths and depths, are. incompatible with the laws of gravity ; that the great contortions of veins are not Ch. XVI.] TO THE PREVAILING THEORY. 377 consonant with the usual forms of fractures; and that the portions of rocks, similar to the adjacent mass, which are included within the quartzoze substance or matrix of these veins, and which are often of great size, and arranged con- formably with the adjacent strata, could not be maintained in such positions, within either empty fissures, or in the midst of igneous injections, but would fall to the sides of the veins, or even to their bottom, far below the present limits of mining operations. 3. The nature of their composition, and the mode of their connection with the adjacent rocks, both by mineral transitions, and a community of con- cretional structure, are facts which do not indicate the introduction of foreign and heterogeneous substances into fissures; and such an origin is rendered still more pro- blematical, by the occurrence of subordinate veins in large lodes, or what has been termed veins within veins ; for how could fissures be formed and filled in such situations, and not only in the matrix, but also continuously through the con- solidated portions of rocks ? and moreover, under such circumstances, how could these lesser veins exhibit all the phenomena of intersecting lodes ? 4. We do not subscribe to the axiom just quoted concerning the relative ages of veins as indicated by intersection ; because every description of veins, whether segregated, contemporaneous, or true, presents precisely the same appearances : this, in the second instance s has been asserted to be fallacious, but we have disputed the correctness of this view of the case at some length. Moreover, the different orders of veins interfere with each other in like manner, by which it seems to be proved that they are all of the same nature. Lastly, the division of true veins into classes of different ages cannot be maintained, for each class affords exceptions ; and not only so, but even the relative age of two veins, as in Dolcoath, may be inferred to be of a certain deno- mination in one part of the mine, and of a very different one in the other ; because, at a given level, A traverses B, and at another, B is the intersecting vein. 5. Although the heaves 378 RECAPITULATION OF THE OBJECTIONS [Oh. XVI. and throws of veins appear at first sight to confirm the me- chanical origin of these bodies, yet they will not stand the test of a rigid scrutiny ; for of whatever kind the veins may be, when several are traversed by one vein, the extent of the apparent movement is in general not uniform, and sometimes varies in the case of each individual vein : and it may be likewise added, that similar phenomena occur not only in veins, but also in the strata and laminae of rocks, when no cross vein, or even fissure, can be detected. Finally, we have advanced objections to the theory concern- ing the metamorphosis of sedimentary deposits into crystalline or primary slates. 1. We have expressed some doubts whether the transitions which have been observed between these rocks, can be regarded as a criterion of identity ; because, on this principle, the secondary and tertiary formations might be referred to the same epoch, and even granite, gneiss, and similar rocks might be confounded with adjacent conglomerates which are composed of their detritus : and it has been attempted to illustrate the fallacy of such transitions being considered as evidence of coeval formation, by the occurrence of beds of transported debris on the shores of Mount's Bay, which may, by such means, although overlying sand containing sea-shells, and a stratum of decayed wood or a submarine forest, be proved to be identical with the primary slates ; or, in other words, the latter would be demonstrated to be the metamorphic strata of sedimentary deposits, belonging to the modern geological epoch. 2. The facts by which the advocates of the Plutonic theory have attempted to prove the power of fire to convert secondary strata into primary, have been passed in review ; the effects of trap-rocks on sedimentary deposits have been compared therewith ; and then we have attempted to show, by referring to chemical analyses, that, with the exception of dolomite, and perhaps of porcelain-jasper and of analcime, the nature of which seems to be doubtful, all these changes are such as may be attributed to a mechanical alteration of the structure of these rocks, resulting from a Ch. XVI.] TO THE PREVAILING THEORY. 379 partial state of fusion. Whilst the primary slates next granite not only exhibit similar appearances, but differ from the more remote portions of these rocks in their chemical composition, which, if indicative of any alteration, points to the transmutation of one earth into another ; such as that of alumina, or of lime, into silica. But this is not the only difficulty ; for all the primary strata, excepting occasional pure beds of limestone and quartz-rock, if there be such, contain some alkali which does not exist in the sedimentary rocks : and many arguments have been brought forward to prove that the alkalies could not have been introduced by the granite ; for if so, it must have traversed some beds, without entering into combination therewith, though these occasionally consist of quartz, which, according to theory, must have been in a state of ignition. The principles of chemistry do not form the only grounds of objection to the so-termed metamorphoses, for these are also at variance with the laws of caloric : for how can it be conceived that heat could be so transmitted through such a vast extent of non-conducting beds of clay and sand, as to produce its greatest effects at a considerable distance from its source, without bringing the nearest beds into a state of perfect fusion, or, indeed, without causing any, or but very little alteration in several intermediate strata ? Such are the principal objections which, in the preceding chapters, we have advanced against the prevailing theory, and we venture to express a hope that they will be deemed sufficiently important to obtain a candid and patient exa- mination. For these various considerations, we therefore conceive that the prevailing theory is not in accordance with the phenomena of the primary rocks, and that it is not founded on a full and fair deduction from facts. We are informed that the modern or improved method of theorising " seeks an interpretation of geological monuments by comparing the changes, of which they give evidence, with the vicissitudes now in progress, or 380 RECAPITULATION OF THE OBJECTIONS [Oh. XVI. which may be in progress." * But the earlier enquirers (re- marks the same author whom we have just quoted) employed themselves in conjecturing what might have been the course of nature at a remote period, rather than in investigating what was the course of nature in their own times. Indeed, of late years, notwithstanding the repeated warnings of experience, the ancient method of philosophising has not been materially modified. " In our attempt, however," says Lyell, " to un- ravel difficult questions, we shall adopt a different course, restricting ourselves to the known or possible operations of existing causes." We have been induced to make these extracts for two reasons; first, in order to vindicate the character of the earlier enquirers ; and, secondly, to show that the Professor himself is a disciple of the ancient philosophy of which he has ex- pressed his disapprobation. That the speculations of geolo- gists, formerly, were entirely imaginary, cannot, we think, be maintained : it is true that they were not founded on such extensive data of the known effects of natural agents as they now are, but they always had some reference to actual oper- ations, though these were frequently but very imperfctly under- stood. But surely it is not fair to consider their intellectual exertions inferior to our own, by a comparison instituted be- tween the dawning twilight of the science, and the present time, when the whole horizon is illuminated by copious and brilliant rays of light : succeeding enquirers will possess a more extended knowledge of nature, and probably a more perfect mode of investigation than ourselves, but they must not on that account suppose that our science was not founded on the same rules of induction, though often misapplied, and sometimes extended beyond their legitimate limits. In the same manner, Lyell has (as we conceive) carried the analogy of the operations now in progress too far, in supposing that * Principles of Geology, vol. iii. p. 3. Ch. XVI.] TO THE PREVAILING THEORY. 381 all the phenomena of the primary rocks, or, indeed, of the secondary, can receive thereby a satisfactory solution. As regards the application of this rule to the tertiary rocks, he appears to have been eminently successful ; but when he touches on the department of the science which forms the subject of this work, his speculations seem to us to be as fan- ciful as those of the earlier geologists. The Neptunian spe- culations, originally suggested by the observed effects of aqueous agents, were pushed, he says, beyond the limits of analogy : an observation which seems to apply also to the modern Plutonic theory, since he has admitted that " nothing strictly analogous to the primary rocks can now be seen in the progress of formation on the habitable surface of the earth> nothing, at least, within the range of human observation." * This being the case, we think that, after the numerous objec- tions which have been advanced against this theory, we are justified in our dissent from speculations which are not sup- ported by analogy, and will not account for all the phenomena; and we may, perhaps, be permitted to attempt their explanation in a different manner, and to show how this may be conjoined with the prevailing opinions. It has been already stated, that the result of the preceding enquiry has led us to conclude that the primary rocks, both granitic and schistose, are of contemporaneous origin, only differing from each other in their component parts being so aggregated as to produce different kinds of structure. We do not contend that any particular agent, whether fire or water, has formed these rocks ; as it is immaterial to our views, how the fluidity of the mass was originally accomplished, provided its particles had such a degree of mobility as would permit their regular and elective arrangements. But as the general opinion inclines to the adoption of fire as the grand primeval cause which prevailed over all others previous to the consolidation of the earth's crust ; and as it appears to be * Principles of Geology, vol. iii. p. 11. 382 PROPOSED MODIFICATION [Ch. XVI. rendered probable by the phenomena of volcanos, the high temperature of deep mines and of mineral springs, supported moreover bj the approval of some eminent astronomers, it may be adopted as the first step towards the erection of a theory of the earth. At this stage of the speculation, we might indulge in va- rious fancies, as to the precise order and mode of consolid- ation, and as to the condition of the aerial and aqueous fluids during the secular refrigeration of the surface of the igneous mass. We have, however, been anticipated therein, by French as well as by British geologists ; and though we might suggest some modifications of their views, yet we prefer commencing at the period when the exterior of the earth was so far solidified and reduced in temperature, as to admit of the combined co-operation of igneous, aqueous, and atmospheric agencies, similar to those which now modify the form and condition of the superficial parts of our planet. At this early period in the history of the earth, we conceive that all the rocks belonged to that class, commonly called primary, which has been the subject of our consideration ; that is, they were not all granitic, as is generally supposed, but also schistose ; both kinds being intermixed, sometimes the one and sometimes the other predominating in certain portions of the earth, and associated together under precisely the same arrangements as those now observed in the primary formations. On this view of the contemporaneous origin of the granitic and schistose rocks, we may easily comprehend numerous phenomena which appear to be exceedingly com- plicated, on the supposition of these rocks being of distinct and diverse natures the one of igneous and the other of aqueous derivation. Thus, if the various kinds of granite and schist were formed at the same time, they might be ex- pected during consolidation to assume similar concretionary structures. And it is, therefore, not surprising that the layers or beds resulting from the extensive aggregation of such concretions, should often pass through both rocks, or Ch. XVI.] OF THE PREVAILING THEORY. 383 be arranged, side by side, in a regular and conformable order, as at the junction of large masses of these rocks, or, in the case of their association, in lesser or alternating beds ; nor, that the individual concretions should frequently contain both rocks in different proportions, and yet exhibit a distinct line of junction, as denoted by the colour or the texture of each mineral compound : for since the various parts of the whole mass graduate into each other, it can be easily imagined that the elective chemical attractions which regulated the com- binations of the particles of the cooling fluid mass, might be independent of the mechanical attractions of cohesion, by which the concretions were formed during its consolidation. Again, as regards the internal structure of these rocks, we have seen that it may be either compact or schistose, in all without exception ; though the former prevails in some places and the latter in others : these kinds of structure can, there- fore, only be considered as specific, not as generic distinctions : and much less as indications of different modes of formation, affording a proper basis for the division of rocks into the stratified and unstratified classes. We have, elsewhere, at- tempted to illustrate this subject by referring to the structure of an ingot of bismuth, which has been allowed to cool very slowly from a state of fusion ; it exhibits clusters of crystals, both rectangular prisms and octahedrons, surrounded by foliated laminae; affording a striking representation of the formation of the massive and schistose structures in one and the same mass, and that too in a simple elementary substance. When we descend to the composition of the granitic and schistose rocks it will be found to furnish additional evidence of their contemporiety. Thus, each group of these rocks shows that all its members, however dissimilar in their ap- pearance, repeatedly pass into each other by the most insensible gradations ; and not only so, but these groups have the same relations, wherever they meet together. This character is so strongly marked, that even stratified limestone, where it joins granite, as at Glen Tilt, partially loses its fissile structure, and 384 PROPOSED MODIFICATION [Oh. XVL becomes very siliceous, the granite at the same time being very quartzose ; and when a crystalline schist has a granular texture, and meets with a fine-grained porphyritic granite, so as to render the junction apparently abrupt ; then large crystals of felspar traversing both rocks have occurred, removing all doubt as to their identity. And this view of the primary rocks is farther confirmed, by the numerous instances that have been recorded, concerning the coincident composition of adjacent granitic and schistose groups ; an agreement which has not only been observed in several countries, but also, in various parts of the hornblendic, shorlaceous, talcose, and other formations. And lastly, the structure, composition, and modes of union with the containing rocks, render it exceedingly probable that the conglomerated and brecciated concretions, and the granitic and mineral veins, are not of mechanical origin, but have been formed during the cooling and consolidation of the primary igneous mass. To recapitulate the evidence in favour of this opinion is not necessary, since it may be readily gathered from the last chapter, or may be deduced from the objections enumerated in this against the prevailing opinion. Such, we presume, was the condition of the primary rocks at the time of their formation from the refrigerating igneous mass, and when the causes now in operation were first called into action. It may, perhaps, be deemed incumbent on us to explain how the bearings of strata, of beds or courses of granitic rocks, and of veins, always exhibit, in certain local- ities, a considerable degree of regularity, which the prevailing theory attributes to the expansive power of the internal fire fracturing and elevating the strata, in various but determinate directions. It appears to us that these conditions of the primary rocks have been effected by the same influence as that which caused the laminae diagonally arranged within strata, and contemporaneous veins, to assume certain positions ; occurrences which cannot be supposed to have originated in the application of any extraneous force. Is it not within the Ch. XVI.] OF THE PREVAILING THEORY. 385 bounds of probability that the chemical union of the elements of the fused mass, whence resulted such a vast body of definite minerals, should be accompanied by the evolution of numerous currents of electricity, or of analogous fluids ? for we know that the oscillations of the particles of matter, whether pro- duced mechanically, or during chemical combinations, will elicit streams both of common and galvanic electricity. If, then, it be acceded that the primary rocks may have been traversed by such currents during their formation, we have an explanation of the regular disposition of the granitic rocks, of veins, and of other crystalline substances ; and, indeed, not only of the subordinate parts, but also of the entire mass. This idea will remind the reader of Mr. R. W. Fox's expe- riments, from which he has concluded that the Cornish metal- liferous veins were formed by electro-magnetism. By such imaginary currents, crossing each other in different directions, we also fancy that the phenomena of intersecting veins might be accounted for, the more powerful ones having uninter- ruptedly continued their course, whilst the weaker ones ex- perienced various degrees of diversion, being either partially or altogether involved in the impetus of their stronger oppo- nents. But whether the bearings of the beds and veins of the primary mass originated from this source, or from any innate disposition of the constituent particles of its minerals to affect certain lines of combination, will probably remain a mystery for many ages ; but the time may possibly come, when, by accumulated experience, circumstances which are now inex- plicable may receive a more rational solution. At present, however, we cannot obtain any certain knowledge concerning the formation of the earth, earlier than that epoch when the granitic and their associated schistose groups existed, and fur- nished, through the operation of causes similar to those now in action, the materials of which the oldest derivative rocks are constituted. These groups are, therefore, fairly entitledj if this view be correct, to the appellation of primary, since they appear to have been formed before all rocks which are c c 386 PROPOSED MODIFICATION [Ch.XVI. analogous to modern productions. Notwithstanding, it would be preferable to adopt, in place of the term primary, some other, not conveying a theoretical signification ; for even that of granitiferous, which we have sometimes used, is not entirely free from objection, since some intrusive porphyries and traps mineralogically resemble the granitic rocks. Having stated our conjectures concerning the primary formations in their original condition, we now proceed to point out in what manner they maybe conjoined with the prevailing theory concerning those rocks which have been subsequently formed. The primary rocks, at the earliest period of the existing order of things on the earth's surface, appear to have been subject to the chemical and mechanical actions of the elements : by the former of which they have been, to a certain extent, successively decomposed and disintegrated ; and by the latter, degraded and converted into gravel, sand, clay, and like incoherent substances, capable of being transported into new localities. Such deposits have been derived from the primary rocks during every geological epoch, but not always in the same ratio, inasmuch as they have gradually covered up and pro- tected these rocks from the elements, and have in their turn become exposed to the same destructive influence; and thus it is, that recent transported deposits are seldom of an unmixed character, being wholly composed of the debris of primary rocks only in their immediate vicinity. In like manner, also, newer deposits are conjoined with the granite at the surface, either covering older ones or immediately resting on the pri- mary rock : the precise order of these depositions depends on the combination of various circumstances ; among the most important of which may be enumerated, the relative levels of sea and land, previous to and succeeding such formations, the degree of power possessed by the transporting agent, and the form and position of the surfaces on which the deposits took place. Ch. XVI.] OF THE PREVAILING THEORY. 387 Such successive masses of transported debris have been ob- served in every geological group, and appear to have been produced by extraordinary movements of water, occurring at distinct and remote periods, and which have been termed di- luvial currents, or debacles. These catastrophes are now dis- puted, the ordinary operations of water being deemed adequate to explain all the phenomena. Let us, however, try this opinion by applying it to the recent deposits resting on the primary rocks of Cornwall ; and they are well adapted for this purpose, because their materials may be traced to the parent rocks, which form the more elevated parts of the country. Rivers could not have brought down these beds of debris, because the extent of land between them and the summits of the hills is only sufficient to produce small rivulets ; the bursting of lakes could not be their cause, since the form of the surface is not in favour of such a view, nor to the collection of large bodies of water; the waves of the sea could not be the agent, because the deposits often rest on vegetable strata, the remains of trees and plants, which actually flourished on the places where they now repose. Perhaps it may be argued that they were formed during the emerging of the land from the ocean : if this be admitted, then Cornwall has thrice subsided, and again arisen above the water, and acquired a different sea-level at each catastrophe. But even such operations are surely abnormal, differing much from the constant and ordinary action of the sea; and we are only desirous of establishing that water, as a transporting power, has periodically acted with extraordinary violence. Now, between these debacles, we conceive that there were successive periods of comparative rest, in which the ordinary changes were gradually progressive. Thus, wherever solid rocks or loose deposits were exposed, atmospheric and aqueous degradations would be effected according to their situations : their general surface, however, would be protected ; the dry land by vegetation ; marshy places by the conjoint action of plants and sediments ; the hollows, containing lakes, c c 2 388 PROPOSED MODIFICATION [Ch. XVL by deposits, mixed with organic remains ; the bottom of the sea by the accumulation of gravel, sand, and marine pro- ductions. Since, therefore, these various occurrences, in each geological epoch, are contemporaneous, it is evident that vegetable, lacustrine, marine, and other formations may be the equivalents of each other, though greatly differing in nature : and, as one and the same district may exhibit all these formations, even resting on the same rock or diluvial deposit, so each of these, confined to an extensive tract or country, owing to its being entirely situated above or below the sea-level, would be parallel or equivalent to the others, although one would be characterised by terrestrial, another by lacustrine, and the third by marine organic remains. This is a most important consideration, for it teaches us that we must not only become acquainted with the order of succession, in whichjhe various families of plants and animals appear to have been called into existence, but we must also determine the precise genera and species of land and of fresh and salt water animals and vegetables, which flourished during the same geological epoch. The frequent intermixture of all kinds of deposits, in the same group, will, perhaps, ultimately lead to the full deve- lopement of organic equivalents ; and then the uncertainty concerning the analogy of remote series of rocks, which at present perplexes geologists, will be greatly diminished. This task, however, cannot be speedily accomplished; indeed, even the animated beings peculiar to every climate, and to various situations in the same country, during the modern or present epoch, have not as yet been ascertained. As regards the igneous or volcanic power, the remaining cause of changes now in progress, we may presume that it has been active, from the period of the formation of the primary rocks to the present time. It is very possible that, during the consolidation of these rocks, dykes and veins may have been formed, like those of lava in the cones of volcanos. But the cases appear to be very dissimilar ; one of these is the Ch, XVI.] OF THE PREVAILING THEORY. 389 effect of violent injections and protrusions, the other is sup- posed to have arisen from the gradual refrigeration of a fluid mass : and, besides, we have not been able to detect in the primary rocks any indication of such occurrences. We will, therefore, as during the consideration of atmospheric and aqueous causes, begin at the epoch when the formation of the oldest detrital deposits commenced. From that time, different parts of the surface of the earth have experienced successive alterations in the relative levels of sea and land, which may be fairly referred to the expansive force of the internal fire, since similar phenomena have been observed in volcanic countries. During each geological epoch, volcanos appear also to have existed : and that they communicate with or even below the primary rocks, is rendered very probable by the ejection of fragments of such rocks ; by their cones in some instances reposing on granite; and, lastly, by the com- position of lava, trap, basalt, and similar substances, which contain a portion of alkali, and which may therefore have been derived from the same source as the primary rocks. Lastly, each sedimentary group is traversed by dykes and various masses of crystalline and alkaliferous rocks, which possess all the characters of an igneous origin : these rocks in the oldest deposits often resemble the primary, both in composition and in external and internal properties, and can only be distinguished therefrom by their geological position and mode of union with the adjacent schistose rocks. Whe- ther such dykes or intrusive igneous rocks do actually occur in the primary at great depths, or whether they have traversed every part thereof during each sedimentary epoch, is a ques- tion which our present knowledge does not enable us to determine. The primary igneous rocks may be so cir- cumstanced with respect to the intrusive ones ; and such a condition would not be incompatible with the hypothesis which considers granite, and its associated schists, to have been contemporaneously formed before the existence of the c c 3 390 CONCLUSION. [Oh. XVI. fossiliferous strata : but we think it better not to admit this until it is clearly demonstrated. Having thus briefly stated our notion concerning the form- ation of the earth, and having pointed out in what manner this conjecture, although opposed to part of the prevailing theory concerning the primary rocks, may yet be combined with the general opinion regarding the nature of the secondary and tertiary formations, it only remains to apologise for thus obtruding another hypothesis into a science which is already overburdened with speculations. We might plead, in extenuation of our conduct, the very general bias to theorise in those who have been zealous cul- tivators of geology ; but we think that we have a better ex- cuse in the necessity of our case ; for we felt it incumbent on us, after such strenuous exertions to overturn the Plutonic theory, to offer another explanation of the perplexing facts. Indeed, if we had not attempted to supply the deficiency, it might be observed, in the event of our success, that an imperfect theory is better than none. We were also urged onwards in this course by other cogent motives the assur- ance that it would tend to render the work more complete, by throwing additional light on the nature of our dissent, and the sanguine hope that, by pointing out a different view of the subject, without perverting any known analogies, we might more readily induce a conviction that the phenomena of the primary rocks are not in accordance with the prevailing theory. We sincerely trust, however, whatever may be 'the issue of the approaching discussion, that, as fellow-labourers in a common cause, we shall be actuated by a mutual esteem, and only strive, in friendly competition, who can render the best interpretation of the great and glorious mysteries of Nature. INDEX. A. ABERDEENSHIRE, the hornblendic granite of, 25. Actynolite-rock, Cornish, the varieties of, 44. Aikin, Mr., on tubulites in inclined strata, 258. Alps, the altered rocks of the, at Champoleon, 326. , the talcose granitic formation of, 86. , the Eastern, the curved form of the primary rocks of, 122. Anglesea, the altered strata next basalt in, 302. Arran, the schistose granite of, 93. , the position of slate next granite in, 122. , the lamellar texture of the granite of, 234. , the dip of the slate next the granite of, 123. Austle, St., veins of segregation near, 355. Axis, anticlinal, of primary districts, 127. B. Bad-na-bae, the granitic conglomerate of, 261. Balahulish, schists in granite at, 147. Balnoon mine, the lode of, curiously affected, 191. Baltic, changes in the level of, 271. Beaumont, Elie de, the theory of, 215. 289. Beds, granitic, regular bearings of, 99. Ben-na-chie, the brecciated quartz-rock of, 117. Berger, Dr., on the induration of strata next trap, in Ireland, 303. Blanc, Mont, the talcose rocks of, 86. Botzberg, the primary strata of the, 299. Bone, M., objections of, to De Beaumont's theory, 289. Brecciated appearance of the primary rocks, 116. Brochant, M., on the rocks of Mont Blanc, 86. Brongniart, M. Alex., on the conglomerated steatitic-schist of the Cotentin* 115. Buch's, Von, description of the rocks of Norway, 74. Bunawe, granite-veins at, 158. C. Cadeen, the granite of, capped by mica-slate, 121* Caithness, the fan-shaped strata of, 262. c c 4f 392 INDEX. Caithness, the protrusion of solid granite at, 282. Cape Cornwall, granite-veins at, 149. r , description of a quartz-vein at, 174. Carne, Mr., on granite-veins, 150. 160. -, on the intersecting veins of Weeth mine, 185. Champoleon, the altered rocks of, 326. Cherbourg, the passage of slate into granite at, 138. China-clay and China-stone, Cornish, 16. in the granite of the Alps, 89. of Zinnwald in the Erzgebirge, 32. Chlorite-schist of Cornwall, 45. Clay-slate of Ireland, 63. Norway, 79. the Erzgebirge mountains, 85. , conglomerated, of Cronebane, 116. Cliffs and caves, ancient, inland, 272. Composition of some rocks associated with and belonging to the secondary class, 309. of minerals which enter into the formation of primary rocks, 320. Concretionary structure of primary rocks, 91. layers developed in amorphous masses, 250. , how formed, 253. Conglomerate, primary, near Drontheim, 80. , various kinds of, among the primary rocks, 115. Conglomerates, the nature of, 342. Contortions of lamina and strata, 334. Convolutions of strata, the theory of, 340. Conybeare, the Rev. W., on the altered appearance of strata next trap, in Ireland, 302. Cordier, M., remarks on elevation-craters, 270. Cornwall, the granitic rocks of, 15. , the primary schists of, 37. Cornubianite, description of, 39. Corsica, the primary rocks of, 89. Craters of elevation, 270. -. , LyelPs remarks on, 281. Cronebane, the structure of slate at, 106. Cruachan, porphyry in the granite of, 26. ' , granite-veins at, 157. Curvature of the Cornish slates, 108. Cyclopian Isles, elevated strata of, 278. D. Dartmoor, the metamorphic greywacke of, 222. Davy, Dr. John, on the granite-veins of Forth Just, 149. INDEX. 393 De la Beche, Mr., on the structure of the granite of Reville, 99. " trap-rocks of Pembrokeshire, 277. determination of the conformability of rocks, 287. , the remarks of, on dolomization, 305. -, on the primary slates and greywacke of Dartmoor, 322. Ding Dong mine, the lode of, curiously affected, 191. Dip of the slates of Cornwall, 106. next granite, 123. Dolcoath mine, Harriett's lode in, variously circumstanced, 184. , the interference of veins with an elvan-course in, 190. Dolomite, a supposed altered limestone, 305. Drontheim, the conglomerated slate of, 115. Dufrenoy, M., on the junction of granite and Umestone in the Pyrenees, 139. 330. Duhamel, on the alternation of straight and contorted strata, 336. Duntulm, the altered strata of, next trap, 301. E. Elevations of land above the sea-level, 272. Elvans, courses of, in granite, 22. , in the schistose group of Cornwall, 50. -, examples of heaves of, 54. Enys, Mr., on the cleavage of the Penryn granite, 99. Erzgebirge, the granitic rocks of, 31. , the primary slates of, 83. Euphotide of Cornwall, the varieties of, 45. Eurite, Cornish, the varieties of, 17. , beds of, in slate, near Bodmin, 150. associated with granite in Ireland, 30. . and granite, alternations of, in Corsica, 89. F. Felspar, compact, beds of, in gneiss, 70. Felsparite, Cornish, description of, 17. Pel spathic talc-rock or protogine of Mont Blanc, 87. Fire, central, its supposed mode of action, 213. Formo, in Norway, the brecciated gneiss of, 117. Fox, Mr. R. W., on the lodes of Dolcoath, 184. Fragmentary or brecciated schists of Cornwall, 48. Friendship mine, the lode of, traversed by cross courses, 186. G. Galloway, dip of the slate next granite in, 125. , the granite of, enveloping portions of slate, 146. -, contorted slate next granite in, 340. Geyersberg, alternations of gneiss and granite at, 84. Glen Tilt, junction of stratified rocks with granite at, 132. 394 INDEX. Glen Tilt, granite-veins at, 159. -, the supposed altered rocks of, 329. Gneiss of the Western Isles of Scotland, 67. traversed by granite-veins, 73. . of Norway, 75. . of the Erzgebirge, 83. -, transition of the, into mica-slate, 84. , alternations of, with granite in Norway, 76. , a general description of, 203. Granite, the ordinary composition of, 13. , Cornish, alternating layers of, 23. , beds of, in the slates of Ireland, 63. , in the gneiss of Norway, 76. and eurite alternating in Corsica, 89. , the^ schistose, of Arran, 93. and slate, the position of, at their junction, referable to their structure, 125. -. , detached masses of, in slate, 144. , a rock of all ages, 215. . , protrusions of, in a solid state, 216. possesses the characters of strata, 233. , the apparent stratification of, 242. [. . formation, shorlaceous, of Cornwall, 15. , hornblendic, of Scotland, 25. , micaceous, of Ireland, 28. , talcose, of the Alps, 86. Corsica, 89. Granitic groups, general remarks on, 194?. rocks, different kinds of, 11. , the Cornish varieties of, 15. 9 quartzose species of, 18. their relative arrangements, 20. - in the schistose group of Cornwall, 50. -, the structure of, 92. -, the cleavage of, 98. -, the strike of the beds of, 99. -, alternating beds of, 242. Granite-veins in granite, 21. in the schistose rocks, 148. Greenstone, porphyritic, of Aberdeen, 26. , primary, of Cornwall, 43. Greywacke, primary, of Ireland, 62. Gueymard, M., on the geology of Corsica, 89. H. Hall, Sir James, on the position of slate next granite, 125. , on slate in the granite of Galloway, 146. INDEX. 395 Hall, Sir James, on the convolutions of slate, 340. Happisborough, the inclined deposits at, 260. Hartz Mountains, the granite of the, 33. , the metamorphic greywacke of, 324?. Hawkins, Mr., on the elvans of Polgooth, 189. Heaves of veins, the nature of, 182. Helford, the conglomerate of, 260. 297. Henslowe, Professor, on Anglesea, 302. Kenwood, Mr., on the heaves and throws of veins, 183. Hibbert, Dr., on the basin of Neuwied, 289. Hornblendic rocks, primary, of Cornwall, 43. in gneiss of the Western Isles, 69. Huel Jewel mine, the lode of, curved in length and depth, 181. Virgin mine, the brecciated lode of, 116. Hugi on the altered rocks of the Alps, 326. Hutton, the prevailing theory a modification of that of, 217. Hypogene rocks of Lyell, 224. I.&J. Jameson, Professor, on the structure of sandstone, 240. , on rock concretions, 254. Igneous rocks, the formation of, 213. , division of, into three classes, 214. Intersection of veins at Mousehole, 153. Johnston, Mr., on the changes in the level of the Baltic, 271. lona, passage of granite into slate at, 139. Ireland, the granite of the eastern part of, 28. , the primary schists of, 60. , structure of the granite of, 100. , altered strata next trap in, 302. Junction of granite and slate, 126. , structure of rocks at the, 134. -, composition of, 137. Jungfrau, the union of the gneiss and limestone of the, 327. K. Killas next granite, the nature of the, 38. Kinnoul, slate veins in the trap of, 352. L. Land, elevations of the, above the sea level, 272. Layers of rock, diagonal arrangement of the, 238. Limestone, talcose, in Cornwall, 47. , masses of, in gneiss, 71. , traversed by granite veins, 74. , junction with granite at Glen Tilt, 133. 396 INDEX. Limestone, junction with, in the Pyrenees, 139. Little Bounds mine, metalliferous veins of, 174-. Lyell's, Mr., sketch of the Plutonic theory, 218. comparison of the secondary and primary strata, 233. M. Macculloch, Dr., on the granite of Aberdeenshire, 25. Western Isles of Scotland, 67. 209. spheroidal structure of granite, 95. term stratum, 231. -, on stratified appearance of granite, 234. -, on the trap of Skye, 275. arrangement of the constituent parts of Portsoy granite, 353. Magnesian rocks of Cornwall, 4-5. in the gneiss of Scotland, 70. 1 clay-slate of Norway, 80. of Mont Blanc, 86. Majendie, Mr., on the granite veins at Mousehole, 151. Metamorphic rocks of Lyell, 224. Metamorphosis of strata, 222. , the supposed cause of, 223. Mica-slate of Ireland, 60. in the gneiss of Coll, 69, of Norway, 77. of the Erzgebirge, 85. passing into clay-slate, 79. containing granite, 145. Mica, beds of, in the gneiss of Norway, 75. Monte Calvo, the shingle beds of, 264. Mount, St. Michael's, quartz-veins of, 171. Mountain chains, strike of strata various in the same, 289. Mousehole, granite-veins at, 151. , quartz-veins at, 172. Junction of granite and slate at, 127. Mull, junction of granite and slate at, 133. O. Oeynhausen and Dechen, on the granite-veins of Mousehole, 152, Ollareous serpentine, Cornish, 47. P. Palagonia, inclined strata of, 263. Pembrokeshire, the trap rocks of, 277. Peever mine (Huel), the intersected veins of, 188. Phillips, Mr. John, on the cleavage layers of slate, 248. INDEX, 397 Plutonic theory, a sketch of the, 212. Polmear Cove, granite veins of, 150. Junction of granite and slate at, 126. Porphyry, course of, 51. Portsoy granite, the structure of, 353. Primary, the term defined, 7. district described, 9. rocks, structure of the, 91. , the division of the, 36. 212. 221. , Ly ell's view of the nature of the, 218. slates, an enumeration of the, 37. , composition of, how ascertained, 40. , transitions of, into each other, 41. -, objections to the metamorphosis of the, 311. Proteolite, description of the, 40. Protogine, Cornish, 16. , of Mont Blanc, 87. Pyrenees, junction of limestone and granite in the, 139. 330. Quartz, beautiful layers of, in mica-slate, 79. Quartz-rock, brecciated, 117. in granite of Scotland, 28. Cornwall, 18. Ireland, 29. in gneiss, 72. Quartzose rocks, the brecciated, 116. , varieties of Cornish granite, 1 8. schistose rocks, 47. Rannoch, schist in the granite at, 147. , granite-veins at, 159. Relistian, conglomerate of, 1 15. Reville, the structure of granite at, 99. Rocks, the diagonal arrangement of, 238. , the structure of, confounded with stratification, 247. , on the relative positions of, 287. , primary, the apparent dislocations of, 332. 374. Rosemodris, junction of granite and slate at, 128. S. Schists, Cornish, the nature of their transitions into each other, 42. Schistose group of Cornwall, 36. of the Western Isles, 67, 398 INDEX. Schistose group of Norway, 74. rocks, structure of, in Cornwall, 104?. . . Scotland, 104. 109. . Ireland, 106. 112. -, position of, next granite, in Cornwall, 125. 129. 132. Scotland, 129. 132. -, dip of, next granite, 123. Scotland, granite of, 25. , Western Isles of, the primary schists of, 67. Sea-level, alterations of, in Cornwall, 272. Sedgwick, Professor, on the dip of slate next granite in Cornwall, 124. , on granite-veins of Wicka Pool, 151. , on veins of segregation, 355. Sedgwick and Murchison on Caithness, 261. 282. Segregation, veins of, in Cornwall, 355. Serpentine, Cornish, 46. in the gneiss of Scotland, 70. Shorl-rock, different kinds of, 15. , convoluted appearance of, 338. , beds of, in slate, 53. Shorl-schist of the Erzgebirge, 85. Sicily, trap -rocks in the strata of, 273. Skye, dykes of trap-rocks in, 275. Slate, angular portions of, in granite, not fragments, 349. Slate-veins analogous to granite-veins, 352. Strata, primary, the determination of the dip of, 106. , sedimentary and primary, compared, 233. , deposition of, on inclined planes, 259. , inclined, great thickness of, 265. , primary, the nature of, doubtful, 290. , not stratified, 371. , objections to the metamorphosis of, 311. , altered appearance of, next trap, 301. , primary, not raised from horizontal positions, 372. -, not altered sedimentary rocks, 378. , the apparent dislocations of, 337. Stratification, an enquiry into the nature of, 230. and structure often confounded, 247. Stratified and rounded masses of granite, 198. rocks, the alternation of, next trap, 222. Stratum, definitions of the term, 231. Structure of the primary rocks, 91. rocks of the same nature in every group, 247. Studer, M., on the granite and altered rocks of the Alps, 326. T. Talc-schist, Cornish, the nature of, 47. - in gneiss of the Western Isles, 70. INDEX. 399 Talc-schist of Mont Blanc, 86. Theory, the Plutonic, as applicable to the primary rocks, 212. , the prevailing, a summary of the objections to, 370. , a proposed modification of, 381. Ting Tang mine, intersections of veins in the, 184. Towan mine, South Huel, intersection of veins in the, 187. Transitions between rocks, the nature of, 296. of granite into schists, 129. Trap-rocks, associations of, with strata, 273. , the action of, on strata, 301. V. Veins, granitic, in the schists, 149. , the nature of, 346. -, the various phenomena of, 160. , mineral and metalliferous, 163. , small or contemporaneous, 167. , large or true, 170. , nature of their connection with rocks, 178. , not straight, but curved, 180. , the intersections of, 82. , heaves of, 183. , throws of, 187. , interference of, with el vans, 189. , tortuous in regular rocks, 337. , an enquiry into the nature of, 355. . ", on the relative ages of, 363. , on the apparent movements of, 367. Volcanos, the phenomena of, 267. W. Weaver, Mr., on the primary rocks of Ireland, 28. 60. 112. structure of granite, 100. -, on contorted strata, 341. Wicka Pool, Cornwall, granite-veins at, 151. Williams, Mr. John, on Huel Peever, 188. Z. Zincken, M. C., on the hornfels of the Hartz, 324. Zinnwald, alternations of granite and quartz-rock at, 32. THE END. LONDON : Printed by A. SPOTTISWOODE, New-Slreet-Square. 21 _-95'tf lr THE UNIVERSITY OF CALIFORNIA LIBRARY