QE UC-NRLF B M 175 727 MOIES OP THE GEOLOGICAL SOYEY. ENGLAND AND WALES. THE ERUPTIVE ROCKS OF BEEXT TOE AND ITS NEIGHBORHOOD, INCLUDED IX SHEET 25 OF THE ONE-INCH MAP OF THE GEOLOGICAL SURVEY; WITH SOME INTRODUCTORY REMARKS OX THE APPLICATION OF THE MICROSCOPE TO PETROLOGICAL RESEARCH. BY FRANK RUTLEY, F.G.S. PUBLISHED BY OEDEE OF THE LOEPS COMMISSIOXEB3 OF HEE MAJESTY'S TEEASUEY. LONDON: HINTED FOE, HER MAJESTY'S STATIONERY OFFICE, AND SOLD BY LONGMAN & Co., Paternoster Row; TRUBNER & Co., Ludgate Hill; LETTS & SON, 33, King William Street; EDWARD STANFORD, 5o, Charing Cross; and J. WYLD, 12, Charing Cross ALSO BY Messrs. JOHNSTON, 4, St. Andrew Square, Edinburgh ; HODGES, FOSTER, & Co., 104, Grafton Street, and A. THOM, Abbey Street, Dublin. 1878. Price Fifteen Shillings and Sixpence. I BERKELEY LIBRARY UNIVERSITY OF CALIFORNIA EARTH SCIENCES LIBRARY EXCHANGE Geological Survey of England Sc Wales Frontispiece K SIP t ^'C. \~LJfj .y/jr.-' , .--, />/ 'V |k^\ /3$ /'' I ^Sife^l A i ^ *'-i l ->7\ '^, /^^ H * i l ^ ' \ $ 1 1! If A j. f l|l ^ I V \ "3 / ./ / s r. ft x x / / / 5 j%\ \ mt\ I fv \ Vpb i is T5 k i i i vi 1 1 ' ! ! / til MEMOIRS OF THE GEOLOGICAL SURYEY, ENGLAND AND WALES. THE ERUPTIVE ROCKS OP BRENT TOR AND ITS NEIGHBOURHOOD, INCLUDED IN SHEET 25 OF THE ONE-INCH MAP OF THE GEOLOGICAL SURVEY WITH SOME INTRODUCTORY REMARKS ON THE APPLICATION OF THE MICROSCOPE TO PETROLOGICAL RESEARCH. BY FRANK RUTLEY, F.G.S. i___ PUBLISHED BY OBDEB OP THE LOBM GOMMISSIOSBBS OP HBB JGAJESTY'S TRBASTTBY. LONDON: PRINTED FOE HER MAJESTY'S STATIONERY OFFICE, AND SOLD BY LONGMAN & Co., Paternoster Row; TRUBNER & Co., Ludgate Hill; LETTS & SON, 33, King William Street; EDWARD STANFORD, 55, Charing Cross; and J. WYLD, 12, Charing Cross : ALSO BY Messrs. JOHNSTON, 4, St. Andrew Square, Edinburgh ; HODGES, FOSTER, & Co., 104. Grafton Street, and A. THOM, Abbey Street, Dublin, 1878. Price Fifteen Shillings and Sixpence. LIBRARY THE following Memoir by Mr. Rutley is an important contribution to the history of Brent Tor, and the sur- rounding igneous and metamorphosed rocks of that part of Devonshire. His remarks on the microscopic analyses of the minerals which compose the rocks, and the beauty of the coloured plates prepared by the author, will be appreciated by all who are interested in such subjects. It is not uncommon in modern memoirs on petro- graphical subjects, for authors to criticise the nomen- clatures of old authors, as if they had been possessed of all the knowledge that the microscope, polariscope, and analytical chemistry bring to bear on petrology. If on our national Geological Maps all the mine- ralogical variations in the constitution of igneous and metamorphic rocks were to be classified on such prin- ciples, the number of colours and shades of colours avail- able would probably be insufficient to compass all of the stratigraphical and petrological distinctions which are and will be made the number at present employed on the Geological Survey being 103. It is therefore satis- factory to those engaged in the broadly geological work of mapping to find, as Mr. Rutley remarks, " that the " views entertained by the late Sir Henry De la Beche " are in the main correct, and represent a vast amount " of truth derived simply from observation in the field.'* This opinion is also entertained by Mr. J. A. Phillips, in a late memoir in the Journal of the Geological Society, in which he states that notwithstanding mineralogical differences in many of the igneous rocks of Devon and C 31. Wt. B 409. a 2 313038 IV Cornwall, the fact that their ultimate analyses is almost the same, forms a new and additional justification for Sir Henry De la Beche having often adopted one colour and one general name for rocks, the mineral peculiarities of which are, indeed, sometimes only due to subsequent metamorphic action. AND. C. RAMSAY, Geological Survey Office, Director-General. 28, Jermyn Street, S.W. 27th June 1878. NOTICE. This, the first special petrographical work issued by the Geological Survey, has been written by Mr. E/utley after a personal inspection of the rocks in the field, followed by a microscopical examination of specimens carefully selected by himself. The district treated of was surveyed nearly half a century ago, by Sir Henry De la Beche, who recorded his observations and deductions in his report on the geology of Cornwall, Devon, and West Somerset, pub- lished in the year 1839, to which work Mr. Rutley's in- vestigations may be regarded as a sequel. At that time the microscope (except in the form of pocket-lens), was little used in geological research, and it is only comparatively lately that the importance of determining the mineral composition of eruptive rocks, and of accurately investigating their mineral structure, has become fully recognized, as the light which such investigations throw upon conditions under which those rocks were formed has become more and more apparent. In the present case the old geological boundary-lines, as originally surveyed by Sir Henry De la Beche, and now engraved upon the maps of the Geological Survey, hold good as a rule, or, if defective, only require very slight medication . The present Memoir will, I believe, prove an important and valuable addition to the other publications of the Geological Survey, from the beauty and fidelity of Mr. Rutley's microscopic drawings, as well as from the clear and masterly way in which he has treated the subject. The eruptive rocks, which are now collected during the progress of the Survey in mapping new districts, are subjected to microscopic examination, where such exami- nation seems needful prior to the publication of the maps and memoirs on the districts in which they occur C.31. A VI and the specimens thus examined will eventually be placed in the Collection of Rocks when re-arranged in the new room which will shortly be assigned to it in the Museum of Practical Geology. The results of these examinations will also be embodied in future editions of the printed Catalogue of Rock Specimens, which will henceforth have additional scientific as well as practical interest in its bearings on a right interpretation of the geology of the British Isles. H. W. BRISTOW, Geological Survey Office, Senior Director. 28, Jermyn Street, S.W. 19th June 1878. PREFACE. SINCE the days of Sir Henry De la Becne, little has been done to elucidate the geological relations and the mineral composition of the Eruptive Kocks, treated of in this memoir ; and I have now endeavoured by inspection of these rocks in the field, and by microscopic examination of carefully selected specimens, to make some addition to the knowledge so laboriously amassed, and so well considered by the founder of the Geological Survey. The addition is perhaps a small one, but, so far as it goes, I trust I have dealt honestly with the subject. In many instances I have expressed doubt as to the origin and mineral composition of these rocks rather than shield my ignorance by definite statements upon obscure and difficult questions. To the Director General and the Director of the Survey I am deeply indebted for the facilities which they have afforded me for prosecuting this work ; and I would also here record my obligations to Professor A. Rdnard, of Louvain, for the assistance he has given me in determining several of the mineral components of the rocks now described. The landscapes are from sketches taken in the field, and I have endeavoured to make the microscopic drawings truthful render- ings of the actual sections, rather than diagrams. FRANK RUTLEY. A 2 TABLE OF CONTENTS. INTRODUCTORY REMARKS ON THE APPLICATION OF THE MICROSCOPE TO PETROLOGICAL RESEARCH. Advantages derived from the use of the pocket lens and the application of simple tests before having recourse to high magnifying powers. Value of high powers in the examination of fine-grained and compact rocks. Use of polarized light. Microscopic characters of felspars. Alterations which rock- forming minerals undergo. Use of reflected light in examination of opaque minerals. Dichroism. Its use in distinguishing Hornblende, Augite, &c. Microscopic characters of Olivine, Schorl, Nepheline, Apatite, &c. Use of the term Viridite. Importance of mapping eruptive rocks on a large scale. Practical application of petrological knowledge. Inferences with regard to the origin of rocks. Instances of the use of the miscro scope in the deter- mination of the mineral composition of rocks. Action of acids upon smooth surfaces of rocks. Microscopic drawings. Importance of examining foreign rocks, when already described - - Page 1 PART I. EXAMINATION OP ROCKS IN THE FIELD. Brief description of the area treated of in this memoir. Description of the chief exposures of eruptive rocks shown on the one-inch map, with notes on their mineral composition and mode of occurrence, and observations on the boundary lines. Volcanic rocks in the immediate neighbourhood of Brent Tor. Elvan Dykes. Margins of granitic masses - 14 PART II. MICROSCOPIC EXAMINATION OF THE ROCKS. Brent Tor series, Lower Grenofen, Shilla Mill, Gunnis Lake, Brazen Tor, Greston Bridge, Churlhanger, W. Langstone, Indescombe, Cock's Tor 32 PART III. DEDUCTIONS. Boundary between the Devonian and Carboniferous Rocks. Division of the rocks formerly coloured as Greenstone on the one-inch map. Consideration of the stratigraphical relations of the rocks of Brent Tor. Why Brent Tor has not been removed by denudation. Smear Ridge and White Tor probably connected. Lava flows at Greston Bridge. Eruptive rocks around Exeter not connected with those of Brent Tor. Geological changes which the district has undergone. - 45 POSTSCRIPT - - 49 APPENDIX LIST OF MINERALS ----- 50 LIST OF PLATES. Page I. MAP OF THE ERUPTIVE ROCKS IN THE NEIGHBOURHOOD OF BRENT TOR (scale ^ inch to 1 mile) - - Front. II. BRENT TOR. Seen from Hart's Hall near Morwellham (a distance of 6 miles to the south) - - - 19 III. BRENT TOR. View taken from about half a mile to the south-west, showing the dip of the beds composing the tor - 20 IV. COCK'S TOR. Sketch taken from Great Staple Tor on the east, showing the feature formed by the Amphibolite and the valley in which the junction between this rock and the Granite exists. The gently rising foreground is Granite - - 29 V. GREAT STAPLE TOR, on the western margin of Dartmoor, showing the mural jointing in the Granite, which imparts a very distinctive character to some of these Granite tors - 30 VI. Fig. 1. Elvan Quarry at Lower Grenofen, showing parallel master- joints - - 31 Fig. 2. Brent Tor from the north, showing the craggy weathering of the rocks on this side of the tor. Fig. 3. Brent Tor from Smear Ridge, about 3 miles to the S.E. Fig. 4. Greston Bridge. Section shown in the Quarry. Displaying two flows of altered greenstone resting upon and parted by altered shales. CHROMOLITHOGRAPHS OF MICROSCOPIC SECTIONS. Plate VII. Fig. 1. Brent Tor (S. side near foot of tor) Rhyolitic Breccia. Fig. 2. (N. side) Pumice-Breccia, x 25 Plate VIII. Brent Tor. N.W. side of foot of tor. Basalt with glassy magma, mainly devitrified. x 55. Ordinary transmitted light. Plate IX. Fig. 1. Greston Bridge, altered Greenstone. Fig. 2. ,, quartz vein in altered slate. Fig. 3. ShillaMill. Fig. 4. Top of Brazen Tor, Crystal of Biotite enclosed in Quartz of Granite. Fig. 5. Top of Brazen Tor, fluid cavities in Quartz of Granite, containing bubbles and cubes of rock-salt, x 400. Fig. 6. Churlhanger, Amygdaloidal Schist (Schalstein). Plate X. Fig. 1. Brazen Tor, Amphibolite. x 55. Fig. 2. Schorl, in Granulite near its contact with Gabbro. x 25. Fig. 3. Cottage Inn, near Tavistock, cleavage of pyroxenic mineral in the Gabbro. Fig. 4. Cock's Tor, altered titaniferous iron in Gabbro. LIST OF WOODCUTS. Fig. 1. Diagram to illustrate the augmentation of bands by overlap of lamellae in obliquely cut sections of plagioclase when seen under the microscope by polarized light - - 2 Fig. 2 and 3. Diagrams to illustrate the apparent graduation of imbedded fragments into the surrounding matrix of rocks - 8 Fig. 4. Ideal section from the Dartmoor granite, to Ridge, near Petertavy, showing the probable connexion of the outcrops of Gabbro and Amphibolite - 15 Fig. 5. Vesicular bands in Ash at Langford Farm, near Tavistock - 22 Fig. 6. Jagged surface of Amygdaloidal Schalstein exposed in roadside at Churlhanger - - 24 Fig. 7. Mural jointing in the granite of Brazen Tor - - 27 Fig. 8. Blocks of Gabbro at Horndon _ 27 Fig. 9. Imbedded vesicular fragments in the rock at Hardwick Quarry - 36 Fig. 10. Cavities in Schistose Ash, Hardwick Quarry - 47 INTRODUCTION. THE present Memoir being the first special petrographical study hitherto published by the Geological Survey, it may be well to make a few remarks upon the use of the microscope in this branch of geological research. Until within the last few years the minute structure of rocks was comparatively unknown, and very great uncertainty existed as to the mineral composition of many, while that of the fine- grained varieties was a matter of mere speculation. The pocket lens in the hands of the older geologists was the sole means which they possessed of gaining an insight into the mineral composition and structure of rocks which presented no definite character to the unaided eye ; for although chemistry afforded them a rough idea of the minerals which a rock might contain, still it failed to indicate definite mineral species; and structural peculiarities too minute to be recognised by the pocket lens remained unknown. In spite, however, of the advance which has been made of late years in this branch of petrological knowledge by the help of the compound microscope, it must not be supposed that the pocket lens is a comparatively useless instru- ment. Much may be done by its aid, and often with better results than would be attained by the use of higher magnifying powers. This may be partly attributed to the greater ease with which we recognise minerals when seen by reflected light, and when showing actual crystalline faces and planes of cleavage, than when seen by transmitted light and presenting usually only the boundaries of crystals, and the little vestiges of cleavage which remain between the abraded planes which limit a thin microscspic section. The compound microscope is undoubtedly a most valuable aid in determining many of the minerals which compose rocks, but it should always be borne in mind that when the component minerals of a rock are sufficiently well developed to be re- cognised under a pocket lens by their crystalline form, cleavage, lustre, or by other tests, such as hardness, colour of streak, or blowpipe characters, it is better to rely more upon such evidence than upon the appearances seen under high magnifying power, especially when the latter do not happen to be very characteristic. It is, however, often impossible to arrive at any sound conclusions respecting the mineral composition of some rocks by the simple method just indicated. Thus, for example, in the Obsidians and ERUPTIVE ROCKS OF BRENT TOR, Pitchstones many int'eresting structures occur, which in some instances serve to indicate points connected with the solidification of those rocks, while in other cases microliths of augite, &c., crystals of felspars and micas are seen to constitute by no means an unimportant part of some of them. In fine-grained aphanitic rocks also, microscopic examination is the only method by which we can form any definite notion of the different mineral species of which they are composed, and even in rocks whose coarsely crystalline structure permits a ready determination of many of their components, the microscope often reveals the presence of large numbers of minute crystals of other minerals, the existence of which would otherwise have eluded notice, or have rendered analytical results difficult of explanation. The employment of polarized light in microscopic examination is also of very great assistance in determining the components of rocks. In many cases the crystalline systems to which felspars may be referred may be ascertained with considerable certainty, those belonging to the triclinic system exhibiting parallel bands when viewed by polarized light, the bands being due to the con- tiguous development of twin lamella. These bands are often very numerous in plagioclase crystals, but unless the section examined be cut at right angles to the planes of composition, the overlap of adjacent lamellae may give rise to colourless stripes, as shown in Fig. 1, where A and B represent twin lamellae polarizing in com- plentary colours, the colourless bands produced by the overlaps of the different lamellae being indicated by the unshaded stripes in the lower figure, and an estimate of the number of lamellae by the number of stripes would be liable to error. Fig. 1. A BABA8 AB X \ s \ \ \ \ \ A R A \ R A R In the Orthoclastic felspars the twinning usually takes place upon what is known as the " Carlsbad type," viz., in a plane parallel with the clino-pinakoid, and I believe that no instances are known in which an orthoclastic felspar exhibits more than two such lamellae in an individual crystal. Orthoclase crystals are also twinned at times upon another plan known as the " Baveno type." In this case, the twinning plane extends between the alternate edges formed by the basis and the clino-pinakoid, but microscopic orthoclase crystals twinned upon this type are seldom INTRODUCTORY REMARKS. 3 or never seen. In some examples of massive Orthoclase, and occasionally, but rarely in crystals, a very peculiar cross-hatched structure is shown by polarized light. The true nature of this marking is not yet satisfactorily ascertained, but it is very characteristic of Orthoclase. By means of the microscope many felspars are seen to be any- thing but homogeneous ; patches, and, as in the case of Perthite, lamellae of one species being intermixed with another. Not merely does the microscope demonstrate the presence of impurities in felspars, but in many, if not in most, other minerals. It is also interesting to note, by its means, the way in which alteration products or minerals of secondary origin are distributed in rocks ; how augite and hornblende crystals have been replaced by serpentinous matter ; how at times an augite crystal includes little patches of hornblende, which shade away into the surrounding mineral, and apparently represent an incipient stage of the development of uralite. The alterations which ferruginous minerals undergo are also well shown at times in microscopic sections. Magnetite becomes altered into hydrous peroxide of iron ; the titaniferous iron minerals also show curious alterations, and pyrites and limonite are common as secondary products in many rocks. The examination of these minerals can only be effected, as a rnle, by means of reflected light, since they are, in nearly all cases, too opaque to present any definite character by transmitted illumination, other than the outlines of crystals. These outlines enable us at times to discriminate between magnetite and titaniferous iron; but to distinguish magnetite from pyrites it is needful to employ reflected light, and even then a satisfactory determination can often only be effected by blowpipe or other analysis. Felspars, both monoclinic and triclinic, frequently undergo a considerable amount of change, a granular condition supervening, which, if far advanced, obliterates all internal structure, so that the felspars belonging to the one system cannot be distinguished from those of the other. This change is generally attributed to weathering ; but upon examining a section of basalt from Debdon near Rothbury, Northumberland, cut so as to display the pale grey weathered crust and the dark subjacent rock, I was much surprised to find that the plagioclase crystals pre- sented just as fresh an appearance as those in the umveathered portion of the rock. If this be an exceptional instance I do not know how to account for it ; certain it is, however, that in the majority of cases where the weathering of a rock is far advanced and minerals of secondary origin are plentifully developed, the felspars very frequently exhibit the clouded and granular texture just alluded' to. This is especially the case with the felspars in diabase. The method first pointed out by Tschermak of distinguishing between the minerals hornblende and augite by means of the dichroism of the former, and the absence of that character in the latter mineral, appears up to a certain point to suffice ; still some 4 ERUPTIVE ROCKS OP BRENT TOR. augites, especially if the sections be thick, show slight dichroism when a Nicol's prism is rotated beneath them, while in some exceedingly thin sections of hornblende the dichroism is very feeble. The test, as a rule, however, seems to hold good, while when well-marked transverse sections of these minerals occur the great discrepancy between the angles of the oblique rhomb'c prism afford a ready means of distinguishing between them. The dichroism shown by hypersthene and enstatite, and the absence or almost total absence of that character in cliallage and bronzite, also serve to some extent to facilitate the recognition of these minerals. Considerable difficulty may, however, be experienced at times in distinguishing between hornblende and magnesian mica when these minerals are disseminated through a section in excessively minute flecks, and when no definite crystalline form or cleavage is discernible, the colours of the two minerals being often much alike, and their dichroism when the mica is cut at right angles or obliquely to the basal plane being of about equal intensity. Under these circumstances it is well to search carefully over hand specimens of the rock with a strong magnifier, in the hope of finding a moderately well- developed crystal. The characters by which some minerals are recognised under the microscope are comparatively simple. Olivine, for example, may generally be known by the roughened appearance which the ground surfaces of its sections present, and by the rounded angles usually shown when the crystals are definitely formed. Its want of dichroism, its comparatively weak chromatic polarization, and its freedom from inclosures of other minerals also serve to dis- tinguish it. When unaltered, the crystals appear homogeneous in composition and structure, but when they have undergone change the result is usually serpentinous matter, which under polarized light breaks up into feebly polarizing and irregular patches. Schorl may usually be distinguished from hornblende by the trigonal transverse sections which sometimes occur in a thin slice of rock, and by the bluish tint which it frequently exhibits when seen by ordinary transmitted light, especially when it occurs asso- ciated with quartz. The hemihedral terminations of crystals are not often seen in rock sections, and consequently this characteristic is not, as a rule, to be observed. Nepheline and apatite may be recognised by their hexagonal transverse sections, and by the total extinction of light which takes place in such sections between crossed nicols. The former mineral usually occurs in larger crystals than the latter, the apatite crystals frequently being developed upon a very small scale, so that they appear as mere spicula? or microliths under tolerably high magnifying power. Crystals of apatite often form small colonies in certain spots in a rock, their distribution being by no means regular. Apatite is a mineral of very common occurrence in British eruptive rocks, but as yet the only known instance of the occurrence of nepheline is in the Wolf Rock, off the coast of Cornwall.* The longitudinal sections both of nephe- * Vide paper by S. Allport, F.G.S., Geol. Mag., Vol. VIIL, page 247. INTRODUCTORY REMARKS. 5 line and apatite are rectangular in form, and polarize in more or less vivid colours when the crystals are of moderate size. Leucite is a mineral as yet unknown in British rocks. If, how- ever, it should occur developed only in very minute crystals it might easily be overlooked or mistaken for some other mineral, since very small crystals of leucite, such as those which sometimes occur in the leucitophyrs near Kome, do not display any of the characteristics of the larger crystals, and their boundaries are often so ill-defined that they look merely like rounded singly refracting granules. In the absence of definite crystals, or of characteristic crystal- line aggregates, it is often difficult to distinguish between many of the hydrous alkaline silicates, which occur so plentifully as secondary products in many rocks which ,have reached different stages of decomposition. These mostly present a green colour when seen under the microscope by ordinary transmitted light, and, owing to the difficulty in referring them to definite species, Vogelsang proposed the use of the term viridite by which to designate them, until further study should enable the observer to assign them with precision to their respective species. The scaly forms of viridite are usually considered to be some variety of chlorite, while the fibrous kinds are regarded as serpentinous matter. Opacite and ferrite are also provisional terms ; the former applies to a black, opaque, amorphous substance, which often occurs pseudomorphous after other minerals. It may represent either earthy silicates or amorphous metallic oxides. The latter term is given to earthy matter, probably oxides of iron, hydrous and anhydrous. From the foregoing remarks it will be seen that at the present time there are several terms in use in mico-petrology which have a very broad signification and are merely provisional, and it seems desirable to retain these terms and to employ them in cases where doubt exists, rather than run the risk of encumbering petrology with descriptions apparently precise, really worthless. Unquestionably the microscope is a most valuable assistant when judiciously used, and its application to this branch of natural science promises to give a rapidly increasing knowledge of the mineral composition and minute structure of eruptive rocks. Many interesting facts connected with the paragenesis of minerals are disclosed by this method of research, and as our knowledge becomes more extended we should look upon these points not merely as questions of purely scientific interest, but we should carefully amass and group our facts in the hope that hereafter they may elucidate some general laws in nature, of which we are not yet cognizant. It would for example be a work of highest interest to map the eruptive rocks and those which have under- gone well-marked metamorphism on a large scale, and to note their most important variations in mineral composition. Sub- sequently the metalliferous lodes might be accurately laid down, and their characters compared with those of the adjacent rock through which they pass. A map carefully constructed upon such 6 ERUPTIVE ROCKS OF BRENT TOR. a plan would possess considerable scientific interest and might prove to be of great value in mining, since the strongholds of many of the metalliferous lodes are the eruptive and metamorphosed rocks, and if in time to come it should be found possible to establish a definite classification of the association of certain metalliferous lodes with rocks of a particular mineral com- position, the microscope will have done as good work for the cause of mining enterprise as for that of scientific truth. It is well also to bear in mind the fact that the mineralogical composition, the state of aggregation of the components, and other physical characters of a rock, serve to indicate its utility or worthlessness as a building stone ; and although it may be argued that nature already demonstrates this in the weathered surfaces of rocks, still we must remember that the agencies most de- structive to building stones in large towns do not at all fairly represent the purely natural work of decay which takes place round about the quarry. A good instance of this is afforded by the mineral apatite which is of common occurrence in many erup- tive rocks. In these, when naturally weathered, the apatite scarcely seems to undergo any change, yet we know that if acids were present in any quantity in the atmosphere (as is always the case in large towns), it would suffer considerably and in time become decomposed. So far as the origin of rocks is concerned, the microscopic examination of their sections tends at times to throw some light on questions of this nature. The deductions of Sorby, Renard, and others upon the temperature and pressure under which certain rocks have solidified, are the result of calculations based upon the relative size of bubbles and crystals contained in the fluids which occupy minute cavities in quartz, and other mineral components of certain rocks. With regard to the numerous rocks which from time to time have been described as volcanic ashes, tuffs, &c., great difficulty exists in reconciling some of these descriptions with the micro- scopic appearances which many of them present. In these questions the microscope opens out a large field for scepticism. The fragments of different minerals and rocks are of course to be looked for in deposits of this class. The angular or rounded surfaces of such fragments do not, however, afford us much help in coming to any satisfactory conclusions, since, when the sur- faces of the fragments are rounded, we may attribute that rounding to attrition against other fragments, either in the throat of a crater, or to the attrition which water-worn fragments have undergone. Again, the angularity of the fragments will not always serve as evidence of the history of the formation of the rock which is composed of, or which contains them, since we meet with angular fragments in unquestionable volcanic ejecta- menta, and also in rocks whose origin has clearly been an aqueous one. In the latter case it is true that the component fragments of the rock are usually water- worn and rounded ; but in instances where the materials composing the rock have on\y been transported INTRODUCTORY REMARKS. 7 a, short distance and deposited near the source from which they were derived the rounding consequent upon severe and protracted attrition is not to be expected. Again, the fact, that the fragments are those of minerals which compose eruptive rocks is no criterion that the rock containing them has also had an eruptive origin. The felspars which occur in the millstone grits and in other sedimentary rocks afford good examples of this, and it needs no proof to show that in many such cases the development of these crystals has not been due to any metarnorphic action ; moreover the felspars and other minerals of eruptive rocks when occurring in sedimentary deposits are usually in a fragmentary condition, and this would not be the case if they had been developed in situ. In some instances volcanoes eject showers of fine dust, in which scarcely any trace of definite minerals can be discerned, and the mud thrown out from certain craters appears also to be devoid of fragments or granules which can be referred to any of the minerals which constitute crystalline eruptive rocks. Here again a difficulty arises, for we have apparently no means of discrimi- nating between these unquestionably volcanic products and ordinary sedimentary matter. Lamination again is an equally useless character, for we may have volcanic ashes deposited as sediment in seas or lakes, and intermixed with or containing intercalated strata of ordinary sediment, brought into those seas or lakes by rivers. These are facts well worthy of consideration, and although in this Memoir I have had occasion to speak of certain rocks as volcanic ash, and although evidence in many cases seems to favour such a supposition, I have done so partly from deference to the opinions of Sir Henry De la Beche, partly from a want of evidence to disprove that they are composed of volcanic ejecta- menta, and partly from my own belief, or bias. At the same time I am also prepared to believe that these " ash beds" are often in great part composed of ordinary sediment, while a similar opinion is indicated in Sir Henry's own writings. Before quitting the subject of clastic rocks, it may perhaps be well to point out what may occasionally prove a source of difficulty to the micro- scopic observer. In some instances, and notably in parts of sections of some of the Brent Tor rocks, it appears that patches of rock, different in character from that surrounding them, shade off gradually into the adjacent matter of which the section is composed, or, in the case of very finely vesicular fragments, they seem to break up into little flecks and granules of irregular form, suggestive of the partial disintegration of these lapilli. This in many cases would I believe be an erroneous interpretation of the phenomenon, which is probably due to the planes of section including only portions of the outer crusts or surfaces of irregularly shaped fragments, the separate patches becoming so thin at their margins that they seem to shade off gradually into the adjacent rock, while, when the lapilli are vesicular and spongy, actual separation of the outlying intervesi- 8 ERUPTIVE ROCKS OF BRENT TOR. cular matter occurs. The accompanying diagrams, Figs. 2 and 3, will, I think, sufficiently explain what is meant, the parallel lines in the upper figures representing the planes of section and the diagonally shaded portions indicating the possible forms of the imbedded fragments before the sections are cut. Fig. 2. Fig. 3. Speaking of some of the schistose ashes of Cornwall, Sir Henry De la Beche observes : " They consist of schistose beds strongly reminding us of the substance of greenstone, finely comminuted and permitted to settle in water, in which calcareous matter was occasionally present. The principal mineral substance has, as Dr. Boase observes (Contributions towards a Knowledge of the Geology of Cornwall ; Trans. Geol. Soc. of Cornwall, Vol. IV., p. 175), an external character between hornblende and chlorite. These rocks form part of a general series of variable appearance, being sometimes slightly coherent, reminding us of the volcanic ash of modern times ; at others vesicular, or even graduating into greenstone and other trappean rocks, some of which seem to have been in a condition of pumice, prior to the infiltration of carbonate of lime and other substances into the cells which has now rendered the whole a solid mass." Rep. Geol. Cornwall, Devon, and W. Somerset, p. 57. IXTRODUCTOEY REMARKS. 9 And again : " There is in fact so intimate a mixture of compact and schistose trappean rocks with the argillaceous slates, that the whole may be regarded as one system, the two kinds of trappean rcoks having probably been erupted, one in the state of igneous fusion, and the other in that of ash during the time that the mud, now forming slates, was deposited, the mixtures being irregular from the irregular action of the respective causes which produced them ; so 'that though the one may have been derived from igneous action, and the other from the ordinary abrasion of pre-existing solid rocks, they were geologically contemporaneous." Eep. Geol. Cornwall, Devon, and "W. Somerset, p. 57. I have, in this introduction, only dwelt briefly upon the micro- scopic characters of a very few of the most common minerals which help to form eruptive rocks, and of a few of the most interesting questions relating to those rocks themselves. To give anything beyond this very bare sketch of the characters of the rock-forming minerals of frequent occurrence, would entail the conversion of this introduction into a small text-book of micro- petrology, while it would add greatly to the bulk of the Memoir, and prove most wearisome reading to anyone not specially interested in this branch of study. I have endeavoured very briefly to point out how far the evidence to be derived from a microscopic examination of rocks can be relied on, and how far the older methods of determination formerly, and now, at the field geologist's disposal, can assist and in some cases rival the work performed by more elaborate appliances. The microscopic examination of rocks not unfrequently subverts preconceived notions regarding their mineral composition, in some cases indicating differences where, to unassisted vision, no diffe- rences are apparent, in others showing that rocks very dissimUar in aspect have an identical or similar mineral composition. As instances of the latter I may cite the quartz porphyry, or granitic dyke occurring in Shilla Mill Quarry near Tavistock, the rock although varying considerably in colour and structure in different parts of the_ quarry being identical so far as the component minerals are concerned. Again, the dark iron-grey and patently micacaeous dykes of the Lake District are composed of the same minerals as the brick-red dykes which are so common in the Upper Silurian rocks of that country. In the latter, however, the magnesian mica is but poorly represented, so that while the one rock would be described as minette or mica-trap, the other would almost be designated micaceous felstone. Eocks intermediate in character between basalt and trachyte occur in several parts of the Lake District and Wales, associated in both instances with sedimentary deposits of silurian age. The microscopic characters of those pertaining to the former localities have been described by my colleague Mr, J. Clifton Ward, while those of the latter 1 have only recently investigated by order of Professor Ramsay. Mr. Ward's conclusions and my own coincide very closely, and indicate a community in the mineral composition of these approximately coeval but now widely separated lavas. 10 ERUPTIVE ROCKS OF BRENT TOR. Various forms of goniometers have from time to time been devised for measuring the angles of crystals when seen under the microscope. Of these that perhaps known as Schmidt's gonio- meter is the simplest in construction ; but, when the section in which the crystals occur is rather thick or dark, it is often difficult to see the cobweb with sufficient distinctness to enable the observer to adjust the instrument with precision. It is, moreover, well to remember that we can seldom be sure that the sections of crystals imbedded in thin slices of rock are precisely at right angles to the faces whose included angle it is requisite to measure ; so that even at the best we cannot obtain more than approximate measurements as a rule, and these measurements are only available for purposes of distinguishing between minerals when their res- pective angles vary to a considerable extent. To effect crude measurements of this kind it often suffices to draw two lines on paper, by means of the camera, corresponding with the outlines of the faces seen in the section, and then to measure off the angle formed by the intersection of these lines with a protractor. This simple method, however, will only give results to within 30 minutes, or at best 15 minutes, when a protractor of ordinary size is used ; and even if a more perfectly graduated instrument were em- ployed it is doubtful whether better results would be obtained, owing to the difficulty experienced in drawing the two lines with any great amount of precision, as the boundaries of the crystals are often rough and irregular Indeed when this method is adopted it is usually desirable to draw the same lines several times until they give similar readings with the protractor. Where very precise measurements are requisite the observations should be made with an ordinary reflecting goniometer upon small isolated crystal?, i.e., if it be possible to isolate them from the rock in which they occur. In some cases it is desirable to etch the smoothly-ground sur- faces of chips of rock with acids. By this means information may occasionally be acquired, not merely with regard to the solubility of certain mineral components, but at times evidence may be procured of a somewhat different nature. For example, a rock from Wrington Warren, near Bristol,* was found, after under- going such treatment, to contain fragments of limestone in which portions of minute crinoid stems were imbedded ; the limestone fragments forming little angular patches in a basalt. The application of acids to thin sections under the microscope is rather a difficult process, especially when the rock contains carbonates, and it is needful to examine the deportment of other adjacent minerals with acids. The difficulty in such cases consists in restricting the drop of acid to any particular crystal or frag- ment. Of course in the case of mounted sections covered with glass it is necessary to remove the cover and clean off the super- jacent balsam before any reagent can be applied, and this * Described in the Appendix to the Geological Survey Memoir on the Bristol and Somersetshire Coalfields 1876, p. 208. INTRODUCTORY REMARKS. 11 frequently entails the destruction of the section. The best way is to apply the acid before the section is reduced to its ultimate degree of thinness ; the acid can then be washed off, the process of grinding continued, and the section finished in the usual way ; and even in this case there is risk of affecting a greater thickness of the section than might be anticipated. A few words may here be said on the preparation of micro- scopic drawings. In all cases it is desirable to make the outline by means either of a neutral-tint glass reflector, or by an ordinary camera (Wollaston's prism). The former entails either a reversal of the object or a transfer of the outline, the latter involving additional labour, while the former necessitates the filling in of the detail of an object seen through a thick glass slide. The Wollaston's prism is therefore best adapted for drawings of rock sections. Some observers dispense with the use of any instru- ment of this kind and make the outline by eye, placing the paper on a level with the stage of the microscope. The detail should always be filled in by eye alone, a moveable index placed in the eye-piece of the microscope serving to record the precise spot where the work is being carried on. Some draughtsmen are in the habit of rendering their drawings diagrammatic, but this practice, although it may have its advantages in forcibly depicting the views of the observer, detracts from the value of the drawing as a truthful delineation of the object. In this Memoir I have tried to make faithful copies of the rocks as seen under the micro- scope, so that, should my conclusions be doubted, my drawings may at all events be beyond question. In most cases the sections examined and described in this Memoir have been prepared by Mr. Cuttell, of Ne\v Compton Street, Soho ; a few I have cut for myself. Machines constructed for this purpose may now be seen at the Loan Exhibition of Scientific Apparatus at South Kensington, while descriptions of how to prepare sections of rocks and minerals will be found in the following works : (( How to Work with the Microscope," by Dr. Lionel Beale ; " The Microscope in Geology " (an article in the Popular Science Eeview, No. 25, Oct., 1867, p. 355), by the late David Forbes ; and an article by J. B. Jordan, in the Journal of the Quekett Microscopical Club. It is greatly to be regretted that at present our literature upon micro-petrology is very limited, and for this reason it is most desirable to examine eruptive rocks from foreign localities, to compare them with the descriptions given of them by different continental petrologists, and to ascertain as far as possible in what respects they may be correlated lithologically or otherwise with the rocks which occur in our own country. In investigations of this nature it does not suffice to rely implicity upon microscopic research alone. The labours of the field geologist, the crystallographer, the chemist, and the physicist should all be impressed into the service, and it is for this reason that the work of an individual observer is not likely to be based upon a thorough knowledge of all these different branches of science ; while unless C.31. 12 EKUPTIVE ROCKS OF BKENT TOK. the sum of such special investigations can be added up, and the deductions which are based upon them duly weighed, we can only regard our petrological conclusions as provisional, although in many cases they may ultimately prove to be close approximations to the truth. Petrology is but a branch of geology, yet petrology is itself a composite science, while to thoroughly follow it out in all its ramifications is a labour which of necessity exceeds the capabilities of any single observer. It represents a large field for inquiry. Its great aim an elucidation of the origin of rocks by careful study of their mode of occurrence, their mineral composition, and their structural characters, both gross and minute. It is in fact the inanimate history of the globe based upon the anatomy and histology of the earth's crust. Apart from its scientific interest it has practical bearings of great industrial importance, but the scientific knowledge must be accumulated and arranged before it can be applied successfully to the interests of the community at large. The following is a rough explanatory table of the mineral com- ponents, &c. of the rocks mentioned in this work : BASALT (Dolerite, Aname- Plagioclase. site, Melaphyre). Augite. Magnetite (Titaniferous iron), Apatite, Olivine. Plagioclase (Oligoclase according to Dathe). Augite, generally represented by pseudomorphs of serpentine, &c. Magnetite, Titaniferous iron, Apatite, Olivine. Chlorite. Plagioclase. Hornblende. Magnetite (Titaniferous iron), Apatite, Quartz. Plagioclase. Diallage. Magnetite (Titaniferous iron). A term used of late years as a synonym for Diorite, but formerly embracing all varieties of Basalt, Diorite, and Gabbro. In this work it is used in the earlier sense of the term. Any fine-grained varieties of the preceding rocks which, owing to the imperfect or minute development of the individual mineral com- ponents, cannot be determined with precision. HYPERSTHENE ROCK - The rocks formerly described under this name are now, for the most part, regarded as Gabbros, the Diallage or Enstatite which they contain having been mistaken for Hypers - thene. AMPHIBOLITE (Horn- Hornblende, blende Rock.) Quartz. DIABASE - DlORITE - GABBRO - GREENSTONE APHANITE INTRODUCTORY REMARKS. 13 GRANITE - GRANULTTE QUARTZ (Elvan). PORPHYRY Orthoclase (Plagioclase). Mica (Potash or Magnesian, or both). Quartz. Orthoclase (Plagioclase). Quartz. Orthoclase. Micas. Quartz. Felsitic magma. PITCHSTONE (Rhyolite in Vitreous rock, variable in composition, but part). approximating to felspars and usually con- taining water, the vitreous condition of rocks of this class being due to rapid solidification. The representatives of these rocks at Brent Tor have undergone considerable physical if not chemical change, and are now in great part devitrified by the development of Microliths. A term which embraces Quartz-trachyte, Ob- sidian, Pitchstone, &c. The Rhyolites are classed as granitic, felsitic and hyaline. In this work rocks pertaining to the two last- mentioned classes are alone implied. These include volcanic ashes and sedimentary deposits composed wholly or in part of frag- ments of rocks and minerals. A rock which is possibly related to Diabase in composition, has a schistose structure, and is frequently amygdaloidal. ALTERED GREENSTONE- This expression is used to denote eruptive rocks so far decomposed that their normal mineral constituents can only be partially recognised. They were probably once Basalts or Diorites. RHYOLITE CLASTIC ROCKS - SCHALSTEIN -B 2 14 EKUBTIVE ROCKS OF BEENT TOK. PART I. EXAMINATION OF THE ROCKS IN THE FIELD. The area embraced in this Memoir has for its boundaries the River Tamar on the west, and the western margin of the Dart- moor granite on the east, while lines drawn east and west, about a mile south of Lidford and a mile north of Calstock, represent the northern and southern limits. These purely arbitrary boundaries comprise about 60 square miles, the area including only portion of a much larger district in which beds of volcanic ash and bosses and dykes of other eruptive rocks occur in great number, and often represent con- tinuations of the same beds, or spurs from the same deep-seated intrusive masses. Within the small district of which this Memoir treats, there occur many highly interesting and peculiar eruptive rocks, some intruded through, and others interbedded with, the Carboniferous and Devonian strata which lie on the west of Dartmoor. The object now in view is an elucidation, to some extent, of the mineral composition and structure of these rocks, not merely by examination of them in the field, but also by more minute investigation of carefully selected specimens ; thus supplementing the work of the late Sir Henry De la Beche by methods of research unknown to petrologists in the days when he surveyed this district, and rendered it classic ground by the careful obser- vations which he made and published in his Report on the Geology of Cornwall, Devon, and West Somerset. The physical features of this district need but a brief description here, since they are admirably dealt with in the work just alluded to. On reference to the Geological Survey Map, Sheet XXV. (1-inch scale), it will be seen that the culm or carboniferous series and the Devonian series of rocks abut abruptly against the western flanks of the granite which constitutes Dartmoor, and that from the line of contact a light, graduated wash of the colour, used to denote granite, is carried for a little distance over the adjacent culm and Devonian rocks. From this, however, it should not be inferred that these sedimentary deposits approximate either in mineral composition or in lithological character to the granite itself; in other words, it must not be supposed that there is a gradual passage from these rocks into granite, since there is little evidence, so far as I have seen, at all events near the present denuded surface on the east of Brent Tor, to show that such is the case. In the absence of opportunity for more detailed investi- gation it is, however, only right to quote the following state- ment which occurs in Sir Henry De la Beche's Report, p. 267 " In numerous localities we find the coarser slates converted into rocks resembling mica-slate and gneiss, a fact particularly well exhibited in the neighbourhood of Meavy, on the south-east of Tavistock." Where exposures of culm measures or Devonian EXAMINATION IN THE FIELD. 15 rocks occur near the margin of the granite, appearances usually indicate a slight physical change rather than a mineralogical differentia- tion. Approximations to this latter phase are, however, perceptible in places in the form of im- perfectly developed crystals, which may in some cases represent staurolite. About half a mile to the S.W. of Cock's Tor the grits have under- gone some alteration and hornblendic crystals have been developed in them. It happens, unfor- tunately, that where lines of actual contact exist, they are generally masked by superficial accu- mulations ; but it is often possible to procure samples of the rocks within a few yards of the contact, and if they then exhibit no alteration, it is evident that it is either of very trifling extent or that the demarcation between the two rocks is a comparatively sharp one. On referring to the Geological Survey Map it will be seen that at two localities, Brazen Tor and Waspworthy, there are small patches, mapped in as greenstone, which are in actual contact with the granite. These are Hornblende Rock (Amphibolite), and the line of demarcation between it and the granite is a very sharp one, and corresponds exactly with the line drawn on the map at the former locality ; but at the latter the evidence is not so clear, as there is very little rock exposed. A little further to the south-west two large intrusive masses of somewhat similar character constitute the features known as Smear Ridge and White Tor. These, together with the Brazen Tor and Waspworthy patches, are probably connected at some little depth beneath the present surface. The ideal section Fig. 4. may serve to illustrate this. Cock's Tor situated a little further to the south is composed of a rock very similar in appearance to those just men- tioned, but under the microscope it is seen to contain little or no free silica, and to be com- posed of diallage, plagioclase, titanic iron, and apparently a little apatite. It must therefore be regarded as a Gabbro. In support of this view, I may add that Professor A. Renard of Louvain has also examined this rock, and states that many unquestionable Gabbros which occur in Belgium, resemble it in every respect.* This rock extends due west on the map, but is much obscured by superficial accumulations on the west of the tor. * Height of Cock's Tor, 1,472 feet; Brent Tor, cir. 1,100 feet ; Black Down, 1,160 feet above the sea level. 16 ERUPTIVE ROCKS OF BRENT TOR. At Sowtentown it is represented as bifurcating and spreading westward in two broad and long strips. I ana inclined to think that it does certainly cross the valley of the Tavy, but some parts of the southern prolongation yield rocks of a totally different character, and although Sir Henry de la Beche, when describing them in his Report,* points out many of their changes ; he appears to have entertained some doubt about the exact relation which these rocks bear to one another, although he has included them within the same boundary lines on the Survey Map. To ascertain their precise relations to one 'another, so as to map them separately, would certainly be a somewhat difficult task, and the work would have to be executed upon a larger scale than one inch to the mile. At the Cottage Inn, on the main road north-east of Tavistock, there is a good exposure of Gabbro, succeeded further along the road by black gritty shale much veined and containing little cubes of pyrites, but the contact is not visible. Beyond this, close to a bend in the road, what appears to be a slaty breccia occurs, in contact with a schistose rock which resembles some of the beds described as ash. I believe that Gabbro, similar to that occur- ring at the Cottage Inn, extends as far as Indescombe, where another exposure is visible. Near this point the belt mapped as greenstone is constricted, while further on it becomes schistose, 'and may in many cases represent consolidated ash beds. I am even disposed to think that the constriction shown on the map may amount to an actual separation of these rocks, which ap- parently have nothing in common to justify their being mapped in the same patch, the one being intrusive, whfie the other is inter- bedded and schistose in character, nor can I conceive a passage between rocks so dissimilar. That they may inosculate seems possible as the two strips trend in the same east and west direc- tion, and it may have been from some such inference that they were" originally mapped together. The difficulty of ascertaining the boundaries is considerable, the exposures not being very numerous. There are, however, one or two things to be said in defence of this piece of mapping : First. That the schistose rocks were called greenstone ash and were mapped with the greenstones, while the broad signification which the term greenstone possessed some years ago, embracing as it did diorites, basalts, &c., serves to disarm criticism. Second. The small scale of the map rendered the insertion of much detail inadvisable. Third. The details relating to lithological differences are given in the "Report on Cornwall, Devon, and West Somerset," * " Where greenstones approximate or come into contact with the granite, the " crystallization has become large near it, and even a modification of the arrange- " ment of the component particles seems to have been produced, so that the horn- " blende resembles hypersthene." Geological Report on Cornwall, Devon, and West Somerset, p. 268. EXAMINATION IN THE FIELD. 17 although such information is not embodied in the map on account of the small scale upon which it is constructed. Some of Sir Henry De la Beche's remarks upon these rocks will be cited further on, together with a few additional obser- vations. The northern branch of the Cock's Tor fork extends westward for a distance of over seven miles, and although in some parts the exposures are few and unsatisfactory, still the evidence tends to show that the belt is of a more or less schistose and ashy character throughout a considerable portion of its length, and it therefore becomes a question whether this strip should be represented as actually joining the spur of gabbro which passes westward from Cock's Tor. That the two strips proceeding from this spur are denuded portions ol the same beds I have little doubt, as in both cases they are in great part composed of schistose ash with some local variations in structure, texture, and mineral composition, while from the paucity of exposures in some places it is scarcely possible to note the gradual passages which may take place between them. North of these strips outliers of very similar character occur, and on reference to the Geological Survey map it will be noticed that the principal ones assume a somewhat annular disposition around the mass which constitutes Brent Tor. The strip which lies immediately south of Brent Tor, and which passes west from Burnford Farm through Pillands and Churl- hanger, is shown on the map to branch in a northerly direction a little north of Foghanger, and about a mile further north it bends again into a western course extending slightly beyond the Tamar and ending abruptly a little south of Greston Bridge. From the dips recorded on the Survey map, and from one or two additional ones, it seems probable that the three strips of ash which cross the main road from Tavistock to Brent Tor are but repetitions of the same beds, while their tolerably persistent character in no way tends to invalidate such a supposition. The great doubt expressed by Sir Henry De la Beche about the boundary between the culm and the Devonian series also serves to render this question an open one. Towards Dunterton the ash seems to give place to a compact rock resembling basalt, and at Greston Bridge the isolated patch marked on the map is shown to consist of two beds of a similar rock, parted by and resting upon black shales approximating to Ljdian stone, PI. VI. fig. 4. The strips of ash on the north and east of Brent Tor, although showing local variations, may also be regarded, with a fair amount of probability, as being continuations of the beds on the south and west. Brent Tor itself differs considerably from these ash beds which encircle it. On ascending the Tor one is struck with the compactness of some of the rocks and with the slag-like vesicular appearance of others, as compared with the fissile schistose character of the neighbouring ashes. Imbedded fragments and lapilli, fre- quently rounded, project from the weathered surfaces, and although the rocks which compose the Tor appear at first sight to be slightly varying conditions of the same kind of rock, yet they 18 EEUPTIVE ROCKS OF BEENT TOE. form as it were a group by themselves, and may be clearly dis- tinguished from the ash beds which environ them. We shall subsequently enter into some considerations connected with the granite of Dartmoor and Kingston Down and the Elvan dykes, which proceed from the granite masses, but at present it will be better to restrict our attention to Brent Tor and the ash beds which surround it. Brent Tor is a small hill with a rather gradual slope upon the eastern, and more abrupt ones on the northern and southern sides, while on its western or north-western aspect it is craggy and precipitous. (Plate VI. page 31, figs. 2 and 3.) It is a con- spicuous object for many miles around, but this is due to its occupying an elevated situation, the ground from which the tor arises sloping gradually, but almost constantly, downwards in the direction of Tavistock, while on the three remaining sides it also declines, slightly on the north, but on the east and west into valleys of considerable depth. Viewed from a distance, as from some spots on the Morwell Downs, it appears to stand as an isolated hill upon a plateau (Plate II.), and I am disposed to think that this is the case, and that Brent Tor is a hard mass which has offered considerable resistance to degrading agents, and that it is surrounded by on old plane of marine denudation. Viewed from the south-west a decided dip is apparent in the beds which constitute the tor, as shown in Plate HI., but the dip varies, appearing to curve upwards towards the northern side. This dip is more perceptible at some distance from the tor than when the observer is in its immediate vicinity. At the base of the tor, on the N.N.W., an aphanitic-looking rock occurs, which is probably the " greenstone " mentioned by Sir Henry De la Beche (Report on Devon and Cornwall, p. 121). From microscopic examination I am inclined to regard this rock as a much decomposed basalt, with an originally glassy magma which is now devitritied. On ascending the tor this is succeeded by another rock very similar in appearance, from the weathered surface of which angular and sub-angular fragments project. The latter are mostly fine-grained and resemble the matrix, which is here and there vesicular. Still higher on the north side the rock seems persistently fine-grained and aphanitic in appearance, the weathered surfaces showing in places a brecciated structure, but with the fragments resembling the matrix. At one spot at the base of the tor, on the north-western side, small slaty-looking patches are imbedded in the fine-grained rock. About half way down the south side of the tor, one or two of the beds appear to be scoriaceous on the weathered surfaces. When this weathering has more completely decomposed the contents of the vesicles the rock resembles a cinder, but where less weathered the fresh fracture merely shows a light spotted aphanitic-looking rock, somewhat similar in matrix to those already observed. In one exposure of rock on the lower part of the south side of the tor, strong vertical joints are visible running about north and south, and 15 south of west. PLATE II. 19 BRENT TOR FROM HART'S HALL, NEAR MORWELLHAM. 20 ERUPTIVE ROCKS OF BRENT TOR PLATE III. EXAMINATION IN THE FIELD. 21 In the field on the south side of the tor slightly schistose rock occurs, with projecting nodules on the weathered surface. These nodules are sometimes fine-grained and compact like the matrix, but they are usually scoriaceous, the vesicles often being filled with a white zeolitic mineral. The sandy-looking nodules frequently contain irregular scoriaceous patches, and lower down in the field a rock occurs with included fragments resembling slate and red jasper. The appearance, however, is deceptive, for by microscopic examination the rock is seen to be a partially devitrified pitchstone, while the imbedded fragments are finely vesicular lapilli resembling pumice. Still further down the hill, close to South Brentor village, there are blocks which contain slaty-looking fragments and scoria. ' . Some of the rocks which constitute the tor present, under the microscope, a decidedly rhyolitic aspect and approximate in character either to the felsitic or the hyaline rhyolites. They are for the most part micro- felsitic ; but it is possible that they were originally hyaline and that their felsitic character is due to devi- trification. Upon first examining Brent Tor it appeared to me probable that the rocks of which it consists might merely represent part of an old bed or series of beds of sediment into Avhich scoria and lapilli had been showered, but further inspection, coupled with a microscopic examination of the rocks, seems conclusively to show that the statement of Sir Henry De la Beche was a perfectly correct one, viz., " That in the vicinity of Brentor a volcano " had been in action, producing effects similar to those produced 26, 42 Granulite, mineral constitution of - - ".* - - 13 Great Staple Tor, granite of - " - 28 mural jointing in granite of - 28 Greenstone name how applied altered, at Greston Bridge I/, 44 ash __-_-- -16 broad signification of term - - 16 coloured, on sheet 45 - - 45 Greston Bridge, altered greenstone at - 1 7, 44, 47 Lydian stone at - - 17 ,, Quarry, altered slate at - 17>43 Gunnis Lake Quarry, granite of - 26, 42 H. Haplite of Brazen Tor - - 43 Hardwick Quarry, schistose ash of - ,23, 36 Hematite - - - - . *- - - - -48 High magnifying powers, employment of - Kingston Down, granite of - 26, 48 elvan course in granite - - - 42 Holl, Dr. H. B. - - - 49 Hone-stone at Kilworthy - - - 23 Hornblende, dichroism of - 3 Hornblende rock, mineral constitution of - - 12 of Brazen Tor - 15,38 Horndon, blocks of gabbro at - 27 Hydrous alkaline silicates - - - 5 Hypersthene - - 4 -rock - - - 28 I. Ilmenite, decomposition of- - 39 Imbedded fragments in rocks of Brent Tor - 35 microscopic appearance of 7 Indescombe, basalt of 16. 39 J. Jordan, Mr. J. B., section-cutting machine of - 11 K. Kelly College, stone used for building of - 23, 37 Kilworthy, balls in schistose ash at schistose ash at Kingsford Corner - - 28 54 ERUPTIVE ROCKS OF BRENT TOR. Page Lamerton, decomposing rock at - - 23 Langford Farm, section shown at - - 22, 37 Lead ores - - - 48 Leucite - 5 Limestone fragments in basalt - 10 List of minerals from Tavistock neighbourhood - - 50 rocks and their constituent minerals - 12 Literature, micro-petrol ogical - - - 11 Lower Grenofen Quarry - * -41 Lydianstone at Greston Bridge - 17, 43 M. Machines for cutting sections of rocks - - 1 1 Magnesian mica, determination of, under the microscope r 4 Magnetite, alteration of - 3 Manganese mines, capel in - 35 oxides - 23, 35, 37, 48 Margins of granite masses - 25 " Master rock " at Horndon - 27 Meavy, alteration of slates at - 14 Microscopic characters of Apatite - - 4, 6 of Leucite - 5 of Nepheline - 4 of Olivine - - 4 Microscopic drawings - - 11 Micro-structure of felspars Mineral constitution of rocks, table of - 12 Minerals, list of, from neighbourhood of Tavistock - 50 Morwell down tunnel, section in - - 25 Morwell rocks, altered slates - 25 Mount Tavy - 22 Mural jointing in granite - - 27 - N. Nassau, schalstein of - 36 Nepheline -- _ - _ - - - 4 Neutral-tint reflector - - - - - - -11 o. Olivine - - - 4 Opacite _-- -----5 Origin of rocks - - 6 Orthoclase, large crystals of, in granite of Brazen Tor - - 42 twinning of - - 2 P. Perthite, inter-lammellation of 3 Petertavy - - 42,49 Pillands - Pitchstone, definition of - - 13 Plagioclase in basalt of Debdon - - 3 twinning of 2 INDEX. Page Pocket lens, use of - 1 Polarised light, employment of -2 Porphyritic granite of Brazen Tor - - 42 Pumice-breccia of Brent Tor - - 32 Q- Quartz of Brazen Tor Granite, inclosures in - 42 Quartzite of West Langstone - - - - - 35 Quartz-porphyry, mineral constitution of - - - 13 " of Lower Grenofen Quarry - 41 of Shilla Mill Quarrv - - - - - 41 R. Reagents, application of under the microscope - - - - 10 Reflected light, employment of in microscopic examination - 3 Reflector, Beale's neutral tint - - - - - 11 Renard, Prof. A. - - 6, 15, 38, 39, 40 Rhyolite, definition of- - - - - - -13 devetrified, of Brent Tor ... -33 Rhyolitic-breccia of Brent Tor - - - - - - 32 Ridge, gabbro at ------ 23, 40 Rocks, origin of - - - - - 6 Rock-salt in fluid cavities in granite of Brazen Tor - - - 42 Rosenburch, Prof. H. ... -38,40 Schalstein ......... - 24 definition of - - - - - - 13 Schistose ash of Kilworthy - - 36 rocks -- ....-8 Schorl in granite of Brazen Tor - * 26, 42, 43 microscopic characters of - 4 Schorlaceous granite of Brazen Tor - - 34 Scoriaceous lava of Brent Tor - - - 34 Shilla Mill Quarry - 9, 41, 42 Shortabura ... .23 Smear Ridge, gabbro of - - 15, 46, 49 Sorby, Mr. H. C. - - - 6 Sowtentown - - - - 16 Stiles wick, diorite at - 22 T. Table of mineral constituents of rocks - - - - 12 Tamar Granite Quarry - - - 26 Tinstone - - - - - - - - -48 Titaniferous iron, alteration of - - 39 in gabbro of Cock's Tor - - 38 Trachydolerites of Wales and Lake District - - - 9 Tschermak, method of distinguishing hornblende from augite - - 3 Tuffs . . 6 Twinning in felspars - ... 2 Twowell 23 56 . . ERUPTIVE R.OCKS OF BRENT TOR. V. Page Vesicular fragments in schistose ash - - 36 Viridite - - 5 Volcanic ashes - 6 tuffs - 6 W. Ward, Mr. J. C. 9 Waspworthy, amphibolite of 15, 38 Weathering of felspars - - 3 Week, quartzite of - 35 West Langstone - - 35 Wheal Friendship - ..... 45 White Tor 27, 56 Wolf rock, nepheline in 4 Wollaston's prism - - 11 Wrington Warren, limestone in basalt of - - - 10 z. Zirkel, Prof. F. - 40 Jeological S-urv&y c England. & Wales vn. Brent Tor. (South, side, near the ba6&.J*25. 2 Brent Tor (North, sidej *25. Vinseut Brook* Davi Son, Lith Geological Survey oc England. & Wales 400 1. Decomposed Greenstone, Greston Bridge. 2. Slate beneath, lower Greenstone, Greston Budge. S.Etvan S. end of Shilla. Mill Quarry, (Polar d ) 4 Ma^MicsL&cavities in Quartz of Granite, Brazen Tor 5. Fluid cavities contl cubes of NeuCLBraLzenTor. 6. Amygdaloidal Sdialstem,Churlhan|ern r Leunerton Vincrat Brooka.Day &Son.Lith. Geological Survey o England & Wales N?l Homblcnde.Pjmtfls and Quartz m Amphibolite, Brazen tor, Devon 2. Schorl in the Granulvtic margin of the Granite in contact with N 1. 3. Pyroxene fprohabty- Diallage) in&abbro^ Cottalehin.MainRoaxitD Majytayy 4. Titaiuferous Iron, partly altered, fGabbro) Cocks Tor.near Tavistock. Nl, *S&. N os 2,3 and 4*25. Ordinary transmitted Ught, Onthe surface of N94atittU Ught was also reflected Vaent Brook.. Dv& Son.Uth LONDON; Printed by GEORGE E, EYRE and WILLIAM SPOTTISWOODK, Printers to the Queen's most Excellent Majesty. For Her Majesty's Stationery Office. [B. 409. 250. 8/78.] RETURN EARTH SCIENCES LIBRARY TO* 230 McCone Hall 642-2997 LOAN PERIOD 1 2 HOURS 2 3 4 5 6 2 HOUR RESERVE BOOKS CANNOT BE RENEWED BY TELEPHONE DUE AS STAMPED BELOW FORM NO. DD8A UNIVERSITY OF CALIFORNIA, BERKELEY BERKELEY, CA 94720 038 iNiiP EVL1BR III! THE COAL-FIELDS OF THE UNITED KINGDOM ARE ILLUSTRATED BY T| FOLLOWING PUBLISHED MAPS OF THE GEOLOGICAL SURVEY. COAL-FIELDS OF UNITED KINGDOM. (Illustrated by the following Maps.)' Anglesey, 78 (SW). Bristol and Somerset, 19, 35. > Coalbrook Dale, 61 (NB & SE). Clee Hill, 53 (NE, NW). Denbighshire, 74 (NE & SE^, 79 (SE). Derby and Yorkshire, 71 (N'W, NE, & SE), 82 (NW & SW), 81 (NE), 87 (NE, SE), 88 (SE). Flintshire, 79 (NE & SE). Forest of Dean, 43 (SE & SW). Forest of Wvre, 61 (SE), 55 (NE). Lancashire, 80 (NW),8i (NW), 89 (SB.NE, NW, & SW), 88 (SW). (For corresponding six-inch Maps.see detailed list.) Leicestershire, 71 (SW), 63 (NW). Newcastle, 105 (NE & SE). North Staffordshire, 72 (NW), 72 (SW), 73 (NE), 80 (SE), 81 (SW). *8outh Staffordshire. 54 (NW). 62 (SW). Shrewsbury, 60 (NE), 61 (NW & SW). South Wales, 36, 37. 38, 40, 41, 42 (SE, SW). Warwickshire, 62 (NE & SE), 63 (NW & SW), 54 (NE), 53 (NW). Yorkshire, 88, 87 (SW), 93 (SW). SCOTLAND. Edinburgh, 32, 33. * Haddington, 32, 33. Fife and Kinross, 40, 41. IRELAND. auturk, 174, 175. *Castlecomer, 128, 137. jllienaule (Tipperary),146. ff or Sections illustrating these Maps, see detailed list.) * With descriptive Memoir. GEOLOGICAL MAPS. ^ Scale, six inches to a mile. ^Rie Coalfields of Lancashire, Northumberland, Cumberland, Westmorland, Durham, Yorkshire, Edinburgh shire, Had- dington, Fifeshire, Renfrewshire, Dumbartonshire, Dum- friesshire, Lanarkshire, Stirlingshire, and Ayrshire are surveyed on a scale of six inches to a mile. Lancashire. 47. Clitheroe. 48. Colne, Twiston Moor. 49. Laneshaw Bridge. 55. Whalley. 56. Haggate. 6s. 57. Winewall. 61. Preston. 62. Balderstone, &c. 63. Accrington. 64. Burnley. 65. Stiperden Moor. 4s. 69. Layland. 70. Blackburn, &c. 71. Haslingden. 72. Cliviger, Bacup, &c. 73. Todmorden. 4s. 77. Chorley. 78. Bolton-le-Moors. ,79. Entwistle. 80. Tottington. 81. Wardle. ' 89. Rochdale, &c. 92. Bickerstafl'e, Skelmers- dale. 93. Wigan, Up Holland, &c. 94. West Houghton, Hind- ley, Atherton 95. Radcliffe, Peel Swinton, &c. 96. Middleton, Prestwich, &c. 97. Oldham, &c. 100. Knowsley, Rainford, &c. 101. Billinge, Ashton, &c. 1<>2. Leigh, Lowton. 103. Ashley, Eccles. 104. Manchester, Salford, &c. 105. Ashton-under-Lyne. 106. Liverpool, &c. 107. Presoott, Huyton, &c. 108. St. Helen's, Burton Wood. $4. Ormskirk, St. John's, &c. 109. Warwick, &c. 6s. 85. Standish, &c. 111. Cheedaie, part of Stock- 86. Adlington, Horwick, &c. port, &c. 87. Bolton-le-Moors. 112. Stockport, &c. 4s. 88. Bury Hey wood. 113. Part of Liverpool, &c. 4s. Durham. Scale, six inches to a mile. Sheet. Sheet. 1. Ryton. 4s. 8. Sunderland. 2. Gateshead. .., . 1865, 2s. 6d. 1866 to 1876 2s. each.