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Lorsque le document est trop grand pour §tre reproduit en un seul cliche, il est film6 A partir de Tangle sup6rieur gauche, de gauche i droits, et de haut en bas, en prenant le nombre d'images n^cessaire. Les diagrammes suivants illustrent la m^thode. 1 2 3 1 2 3 4 5 6 ff'l I IN TtlK i [ ORIGIN AN!) CLASSIFICATION op ORIGINAL ROCKS. MX THOMAS MAC FA RLAX I{ M<).\ I Ri;.\I. : ■UI\Ti;i) \\\ MIK IIKI.I. AM) WILSON, Kl.' M . n II l; s lUl.ll. ( C () N T \[ N r s I . In I'I!(ii>i c iiu.v I I I. < 'l.ASSK.S IIK liUCKS ) Oiij-iiiiil or iLriiions rocks ,; liiiiviil or sciliinciitnry rocks 7 Alfcrcd and inctiiinoi|iliic rocks .s III Tknti UK (IF ("KicrN \i, HocKH :» AimlofiV lictwccii these (ukI tiinuicc shms ;i Viin'ilics of texture II I'llSSan'S li.tweeli these ];> I'roliillili' causes of these varieties II I \' 'I'llKlll ClIKVIlAI, ( 'OMFMISITION | 1 Ksseiitial aiiij acciileiital coiiipoiieiits | ;, XiMitral rocks I,; Silieic rocks I >^ Itasic rocks I ., Siliceous and liasoiis rocks lo Table (!) of the colli |)osit ion ot roik families jo \ . .MlNKIi.M.olilCAI. ("oNSrrn TION -j-^ Hot k minerals artilicinlly )irodi(ced jj I'lsseiii iai const it (lent s j.l i''els|iathic and has-c coiistitiic nts j;, 'I'm Me (II) sJK wiim' the const it nt ion of rock famiiiis . . . . 'ji; Talde (III) shewini; the constitnlioll of the s|ie. ies 'JS I'a.ssaj^'es anioii;^ the species •_>;! \'l. AerKssoiiiAi, CnssTrrrKNTs lio TUeir distriliution .inioii'^ oriLcinal rocks ;j \'ll. I)|:\ KIJU'KMKNT OF l{o(K ( 'oSsmi KM S :;:, I'"ractiircd ci\ .--lals ■;-, Enclosure of mini rals :;i; .Mitroscopieal investiyatioiis ;js Order of the formation of c (.nslitucnts lo IV. VIII. Si rnNTKSTS. I'llUfI' . 1 1 INlatioii liitwixt (liiisity and (oiiiiiositiini 41 Jlfiatidh liitwixt ilciisity and tcximc ll' Dcnsilits (»r Insrd n.< ks tJ IX. SlIII (11 l!K •'•"' I iriuin u| strnctnrr hari.lv defined in other .sciences? Are all systems perlect or natural? Why should litholo-y be an exception to other sciences, and its • Cottn; Die Gcstcinijlehri', pp. 1, 4. Btiidnits bo (li'iirivod of the advantn^os of a systpinntic orrarifrc- iiii'iit nf tlio oliji't'ts t") 1)0 studied ? A "iiatund " hy^'^'in 's not donrmdi'd. vwn wiTo such a tliiiij; pdHsiblc, in tliis or any other hciouco. Th.! nmrc rii,'id any nicthod of classitieation, and tlie more marked and unbending its divisional lines aro made, the nioro unnatural it becomes. It is exceedingly gratifyini; to find that, undeterred by the diiliculties of rock classification, such litlioloL'ists as \'on IIocli- stetter, Kjerulf and Zirkel, have been found wiliint,' to attempt it. Their bdmurs, and tlioso of other workers in the same field, have shed a flood of lij;ht upon a i>reviously obK-nre and unmterestinj^ subject. Altliduiih a jitrfiet system will, perhaps, never bo attained, still each attempt at propeily arrant^in;; i.ur knowledge of tho subject has its value. Chemical analysis and microscopical examinations (d" roeks have very much contributed towards render- ing such attempts snccos>ful. In the present paper it is proposed to give a systematic view of the various classes and species of crystalline roeks, in arranging which it is intended that their chemieal composilinn shall have greater prominence and weight th:in has been usual heretorore. However much it may seem desirable in this department of science, where all tho systems of elussilleation have been con- I'esscdly imperfect, to invent u system indei'cndent altogether of the ideas, more or less well founded, which prevail as to their origit) and age, and in which their physical and chemical charac- ters should only have consideration, it must not, on the other hand, bo forgotten that what is still more desirable in such a sy.^em is that it should re-arrange our knowledge of the subject in a clearer fdnn, render it more easy (if eomjirehension to the student, and be so dovetailed into the p.ist of the science as to bo uselul fur its advancement in the I'uturc. On this account it becomes impossible to neglect even the theoretical views of our ibreruniieis in this science of petrology, far less their arduous and often underrated geonostic labours. It al.-o becomes re- quisite to give a proper value to all the considerations which m.iy liave influenced their views, and to build upon the foundation which they have left us, the results of tho observations and research of the investigators of our own day. Considerations as to the manner of formation, texture, chemical 8 and luincraliiuioiil Odiniiosition, n;io aiul luciilities of rookp, liave nil, iiioru (ir less, iiifluciicoil j;i'nl()i.'i.^ts in iiaiiiiiiLr nnsi. lieation according to origin. limit's division ol'crystalliiK! mcks into indigenous and exotic, and Schetrcr's distinction ofplutonitcs and vulcanites arc both founded upon tluir real or sniiposed manner of formation. Lava and Uliyolito aieexaniples of special rocks similarly named. Tiien, with regard to texture, prohulily no other character possessed hy rocks has piven rise to a t;reater number of gtneiio teims. fc'chisf, hiato, porphyry, trachyte, aniygdaloid, conglomerate, and breccia, are examples of this, but of special names ibunded on texture only a few can bo instanced, tuch as granite and aphanile. The influence of chemical com))o- sition on u lithological nomenclature if, not, as yet, very markei\ for it is only recently that the analysis ol" rock>* has had much attention. Quite lately, however, Cotta has projiosi'd to dis- tinguish as basites tho.-e eruptive rocks containing less, and an acidites those c('ntaiiiing niore than sixty percent, of silica; and Scheeror, Kjorulf and lloth have each indicated methods of classiGcation, founded, to a very considerable extent, on general chemical composition. Ijy I'ar the greater number of special names in lithology arc based ujion mincralogiial characters. This is the case with pyroxonite, hornllende scliist, (juartzite, and many simple rocks, while among those ol' u compound nature where it was impossdjle to indicate their n-ineralcgieal com- position in one word, recourse was had to special names, with definite ideas attached to them as to mineralogieal constitution. Thus, dioritc came to denote a rock composed of iriclinic felspar nod hornblende; granulite, a schistose compound of quartz, ortlioclase and garnet; dolerite, a mixture of labradorito, au!L:ito and magnetite. As regards chissification, the mincraloi:ioal nature of rocks has always been abundantly considered. In this Way we have Hunt's orthosites and anorthositcs ; Senft's labra- doritps, and alabradoritcs, while Zirkel has made the nature of the dilFeient felspar species the corner stone of his system of classification, — crystalline or orii:inal rocks, being divided into ortlioclase rocks, oligoclase rocks, labradorife rocks, annrthite recks, and rocks void of felspar. The manner in which eon- BiilorntioiiH ns to ;;coIoj,Mcnl npe influence tlic namos of rocks mny be illiisfratcd liy the fullciwinf; ox!inii>l('H. Sninotinics cortniii )nrpliyiii'>» nml trncliyfos aro. in lirind spccinions, scircoly ili«- tin^^ui-ilialilt' from ciicli (itlicr. Wlit-n, howcviT, such rocks oi'ciir ninnnj^ carbonifrrmis or jirruvian strntn, fioolojrists liavo bocii iiicliiicil to torni tlicni pnrpliyritvs ; ami on the other linml, when tlioy arc of tertiary or recent ajie, the name tracliyfo is generally ^iviMi tliciM. Exactly tlio same mode oi" delormination, il'such it can 111' called, has been adopted in the cas(> of <:rceiistono and basalt, or rocks ol" such indistinct mineraloj;icvd e(im[»o>ition n« trap and ai»iianitr«. With re.'croiicc to locality it has ])iiiicipally occasioned .sj)ecial names, such as syenite, diinite and andc?ito, or caused varieties of certain other species to be indicated by such '\^ terms as baiiatite, sicvite, Jherzolitc, &c. From these considera- tions it woulil appear that, generally speakinj;, orij^rin has been allowed to determine the various divisions and subdivisions anioiii; rocks; that the majority of the generic names have refeieiicc to texture, while mineralot^ical composition and locality have had the greatest share in originating the special names of rocks. hi striving to attend to what has been indieatdl as desirable and nccessnry in any attempt at classifying rocks, it has appeared to us most judicious to attach greatest weight to their various characters in tlic following order: 1, origin; 2, texture; .'<, chemical cnnijiosition ; 4-, mineralogical composition ; and 5, locality. Ifa systeu) be required at all resembling those of other branches of science, th^se characters mi^'ht be allowed respectively to determine \\w. classes, orders, families, species, and varitics of rocks. II.— CLASHES Ul' I'tOL'lvS. If we, at the present day, look around us, and ascertain, from actual experience, what the methods arc which nature employs in l»roducing rocks, we fmd that they result from tljc operation of two very distinct agencies. On the one hand wo may see in different countries widely separated from each other, streams of melted matter issuing from volcanoes and solidifying to rocks on their sides, or at their feet, while on the other hand we may ol)scrvc, on every sea bench or river delta, sand nud c!ay, tch I dubris of pic-oxisting cry«tnlline masses or frnfrmootnry slratii boin^ gradunlly cnnsolidatod to now cock-*. Kxactly parallel to thesi' opcratidiis of nature arc ccrtiiiii iirtifici;il pmct.-scN at work nroiUKl u«, tbo proiliiots of which arc entirely aiiala^ous to tho two cla.iMos of rocks just indicateil. Wo may stand before an iron furnace and watch the (.toady stream of >l.i«; flowinix front the hearth Into a larire iron waj;on, and there snlidifyinu' to a mass of Holid, sometimes crystalline rock ; and wo n)ay also visit a stamp mill where valuable metallic particles arc beini^ extracted from poor vein stones, and find, in the slimcpits of the estahli>.hinent, banded layers of half solidified strata, re(|uirin_u, but a little time to ('fleet their perfect eonsniidation. These two means employed by nature in jiroJueini; rocks have boon steadily reco;;nized by the majr)rity of geoloL'ists, and the two classes which result have been indicated by a suporabuiub'.ncc of names. Tustratified and stratified; i<:neous and af|ueous ; eruptive and sedimentary ; exotic and indiL'enous ; prinuiry and secondary; (protoL'ene and deuterouene;) crystalliiu' and^lastie • Q massive and frairment iry ; oriL'in:d aiii'. derivafe, arc all terms which have been used for distinguishing those two j^reat classes, and the least ohjcctionabh' anioni,' them would appear to be the two last mentioned. The lirst of these, ori-^inal (rrspriinulichc.) was first adoptiMl by Zirkel!= for dcnotin'^' ii^ncous or eruptive rocks, while the term derivate was iirst su^ruested by L>avid Forbcsf as equivalent to secondary or sedimentary rocks. The latter term wo have ventured to mndifv, and in the I'ollowing pnyes we shall usi; the names criirinal and derived for indie iting the two groat classes. These names would seem to deserve the preference, for tho following reasons. It is admitted by geolo- gists, on all hands, that the material which constitutes the various sedimentary formaticms, consisting of limestone, hardened clay, or consolidated sand, although it may have been immediately derived from pre-existing rocks of a detrital nature, originally came from the decomposition and disintegiation of crystalline rocks, of such as are known to constitute tho oldest formations of tho earth's crust or to have broken through and deposited themselves on the • Pclrogrnphie I., p 173. t Tlic Microscope in Oeoh^gy. p 0. 6 outside of it. It is further an accepted theorem, universally acknowledged by scientific men, that our globe was originally in a state of iuncous fusion, and tha^ all the material which consti- tutes the rocks of our day existed in the form of a melted zone encircling the central part of the globe. It is evident that, before the conditions for the formation of sedimentary rocks could exist, the liquid globe must have become, to some extent, solid ; a crust at least, must have been formed upon it, from the disintegration of which the material ot such sedimentary rocks could have been derived, and upon which that matu'ial could have been deposited. This crust, and the rocks wl.ich from time to time after its solidification penetrated or were erupted through it, must conse- quently, liavebeen the first rocks, and they must have yielded the material for all those subsequently formed by aqueous agencies. It would, therefore, .ippear legitimate to name the former class original and the latter, derived rocks. AN'horc, as in the ca^e of the volcanic and sedimentary rocks which are being formed at the present day, we can observe the process of their formation, no doubt can arise as to their origin. Those rocks, however, fjrm but a very minute fraction of those which build up the earth's crust, and it becomes necessary, in order properly to discriminate among the latter, to point out the distinguishing characters of original and derived rocks. The further we go back in geological time, and the older the rocks are which we arc called on to classify, the greater is the difficulty of doing so, and the more divergent the opinions of geologists become as to their origin. The stratigraphical relations of rocks are most clleotive in determining this, but it will be necessary at present to confine ourselves to considerations of a more purely pctrological nature. This is the more easily done, since the lithological characters afford abundant means of recoiinizinj; oriijiaal and derived rocks, and distinguishing them from each other. Original rocks arc made up of crystalline particles of one or more minerals, principally silicates. These are seldom perfect in crystalline form, arc i'rcquently more or less irregular or distorted, and are intimately bound together to a compact whole, without the intervention of any foreign substance as a cementing material. They are thus mutually interlocked to a crystalline mass, which, however, posfcsses at the same time an average mincralogical and Oil ical chemical composition. This wouM soom to indicate that the mass must have been originally liquid, and, to some extent, in the same condition during crystallization, otherwise it would have been impossible for the various chemical constituents to move toward the points where the minerals wore being ibrnied into whose composition they enter. On the other hand, this lifjuidity must have been somewhat limited in degree, fur the miiienils stem to have pressed against each other, so as to have mutually interfered Vkith their crystalline development, and .so as also to have fitted perfectly Into each other on complete soliditieation. The size of the crystalline particles varies from a foot or more in diameter down to that of mi:jroscopical minuteness. It is even the case that they become so minute as to occasion a pcrtVctlv vitreous structure which even the micrtiscope is incapable of resolving into distinct minerals. In all such eases, although the rock can scarcely be termed crystalline, it remains, what its mode of occurrence plainly shows, an original rock. Derived rocks are made up of the disintegrated fragments (ir particles, and the chemical constituents of previously existing / rocks, abra/ed or dissolved away by water or other agents. Those fragments or particles are sometimes angular, somctiujcs rounded ofT, and always bound together by means of an intervei!- ing cement, which is independent of, and may be altofjether diffe- rent in nature from, the enclosed fragments. Tln-y vary in their dimensions even more widely than the constituents of oriuinal rocks. There arc sometimes found in them blocks of several cubic feet contents ; and, on the other hand, they arc frer|uently composed of the finest particles of dust. The cement which unites these panicles is subject to great differences, both as re- guards its quantity and its nature. t»oiiietimes it con-ists of the material of a newly erupted original rock M'hich has happonod to envelope and bind together fragments of a ;"i re-existing crystalline or sedimentary rock. Sometitiies it consists of the finely divided detritus of the rock of which the larger fragments are composed. Somtimes the finely comminuted cement is from a ditferent rock than the fragments. Sometimes it is of an infiltrated crystalline nature. In some cases the fragments, and in others the cement predominates. Apart from the finely divided sandstone or clay which sometimes fills the interstices between the fragments, carbonate of lime, silica and iron oxide arc the substances which, more frequently than any others, form the cementing material in these fragmentary rock-;. Recent investigations rc_narding the cliemical ccmpositiou of rocks have rciulercd the distinction between the original and derived classes still more marked, and made it possible to point out another essential point of dilTfrence between them. Original rocks possess a chemical fomposition in which a dclinite relation exists between the quantity of silica and that of the various bases which they contain. In derived rocks this dclinite relation is not to be observed. This peculiarity of chemical composition possess- ed by original rocks was first pointed out by Bunsen, and has been quite recently insisted upon as a feature distinguishing them f Irom derived rocks by Von Eichthoycn in liis " Communications i from the West Coast of North America."' -^• Those two great divisions do not, however, exhaust all the classes into which rocks have been divided. It has long been supposed, and more recently the belief has gained ground, that many of the rocks belonging to the divisions above indicated have experienced, since their soliditicatiun or dejiosition, certain changes in their chemical and niincraloj. ieal composition, and in their jihysieal characters, whereby they have been rendered quite unlike their originals, and this without their liaving been disin- tegrated or displaced. The influences to which these elianges have been ascribed are various. Heat, water holding dillerent substances in solution, gases, atmospheric agencies acting sepa- rately or combined, have all played an important part in ellecting these changes. The rocks thus modified have been called meta- morphic, altered or hypogenous rocks, without very marked refe- rence to the classes from which they have resulted. In the following pages the name altered will be applied only to those original rocks, and the term meUimor[)hie only to those derived rocks which have cxpciienced, in situ, such changes as those here indicated. Ii is not, however, proposed in the present paper to discuss the relations of derived and metatnorphic rocks, but, in endeavouring to classify those of the original class, the altered rocks sometimes resulting from them will be noticed. * Zoitschrift der Dcutschen Gcologifclion Gesellscliaft, vols, xix and xx. CCS which, material in jiosiliou of igiiial and blc to point Original itc relation irious bases ation is not ion possess- nil has been sliing them munieations ;iust all the s lonir been rround, that lieated have on, certain tion, and in idered quite been disin- ese changes :ig dilfcrent ctiiig sepa- in ellectiiig :illed nieta- Kiiked rele- 1. In the ]y to those lose derived those hero lit paper to ks, but, in the altered xixaud XX. ( 9 III. — TEXTURE OP OUKilNAL ROCKS. In adverting to the origin of rocks, those which have been called original were described us analogous in nature lo furnace scoriae. This may seem a forced comparison, and it may bo supposed that crystalline rocks are not likely to be influenced by lieat ; but the truth is that nearly every one of them have been shewn, experimentally, by Hall, Bisctiof, Dele-ise, and Sorby, to be fusible, and to bo reduced by a high temperature to the same condition as furnace scoriae. But while the latter general'y exhibit, on cooling, a homogeneous mass, original or compound crysta.line rocks are mist frequently seen to bo composed of Various and different minerals. While the furnace slags, in r!>()id coolinir, h id no time during which their ch 'inicul con. Btitutcnts coulil arran:;e theuHeives into difforent compounds, the greater number of original rocks, having solidified in enormous masses, and, doubtless, during long periods of time, their con- stituents had opportunity for arranging themselves in such a manner as their chemical affinities suggested. The minerals, which were the result of this re-arrangement of the chemical elements, are not, however, always readily recognized in rocks. The latter have in some rare cases solidified so hurriedly that they present merely the appearance of n itural glass. Others have hid time to lay aside the vitreous ch;iract(T and assume a stony appearance, but they appear so homogeneous and fine- grained that tht;ir compound nature would s arcely be suspected. This is, for instance, the case with basalt, which, on this account, Was, at one time, regarded as a simple mineral. ( in grin^tory of rocks, but which really possess a deeper nieaning than we are inclined at first to iin.igine. Although neither the furn:ice nor the volcano can give us any concep 'on of the magnitude of the scale upon which the earlier o iginal, or, as thoy have been n;inicd, the plutouic neks, wore erupted, still, tho}' furnish us with hints wlaL-h we cannot . afford to neglect. To the metallurgist, it is an every day occur- rence to observe that the same scoriio yields eithora vitreous slag or a stony mass, accordingly as it has been (juickly or slowly cooled. Slag cakes, a few inches in diameter, arc Ibund to be impdpablo or glassy on the outside, while on breaking them, the interior is found to be porcelain-like or crystalline. Bischof made some interesting experiments on this matter at the iron-works of Miigdcsprung in the Ilartz. He allowed common iron furnace slag to run into cold water, where it disengaged sulphuretted hydrogen, and yielded a white, easily friable pumice stone. lie next allowed the slag to solidify upon cold, somewhat moist, sand. This ^iw'c a harder pumice, still retaining some of the original color of the slag. In the next experiment the slas was allowed to cool on a completely dry bottom of sand, and the result was a brownish-grccn transparent glass. Under a protecting cover of dry sand, the solidified slag was found to contain cry>talline quadratic prisms in considerable numbers, and between them lay spheric. d concretions, consisting of regular radiating fibres, ex- tending from the middle point in every direction. In the hist experiment the slag was exposed to slow cooling in a basin lined with a warm mixture of charcoal powder and clay. When broken, after cooling, it did not exhibit a trace of vitreous substance nor any (juadratic prisms, but a fine radiated texture had spread itself equally throughout the whole mass. The ex. perimcnts of Sir James Hall have often been mentioned in con- nection with this subject. Nearly seventy years ago he applied experiment, for the first time, to the elucidation of geological phenomena. It occurred to him to melt a small piece of basalt, and the result was a dark vitreous substance. But on fusing a much larger quantity, and allowing it to cool slowly, he obtained a crystalline mas.<». Since that time geologists gradually became accustomed to look upon the original rocks of a glassy appearance, which occur in nature, as the products of rapid, and those of a i n for cliaracteriz- to be rcgnrdcd •ks, but wliieli nod at iir.4 to ulcano c;in give upon which the plutoiiic rt cks, lich we cannot .'ery d;iy occur- a vitreous shig ckly or slowly re ibund to be king then), the Bischof made le iron-works of n iron furnace d sul[)liurcttcd ice stone. He lat moist, sand, of the original as was allowed he result was a ctiiig cover of in cry.-t:illine ween them lay ig fibres, ex- In the liist a basin lined clay. When of vitrcoui^ iatcd texture iss. The ex. ioncd in con- go he applied of geological iece of basalt, t on fusing a y, he obtained dually became ■iy appearance, ud those of a granular texture as the products of slow cooling. Nor are there wanting instances to show that other physical causes have in- flueiiCL'd the strutture of such artificial silicates as slags. At the Eilinton iron-wi rks in Scotland, and those of Bethlehem, Penn- sylvania, the writ'r observed that there is freijuently dovelnped in the slags, as they flow from the furnace, strciki-d bands of different colors, not at all unlike those developed in m;iny slate rocks. "J'hen ag.iin, wiicn the workmen, at the establishment first n imid, tip off the iron and cool the small amount of .scoriic W lich follows after it with a plentiful supply of water, the sl.ig froths up :in 1 S(didifios to a porous cellular sulistanee, the exact parillel of which is to bj found in the [mmice stone of volcanoes. In observing tin slags of c ippLT furnaje-i, nothing is more com- mon than to see tiiose which are all nved to fl )W over dan;p ground rise up into porous s(!ori;v;, while those which run over wet portiims of the smoltiug-house fl )or, b)il up into loose pieces, or throw themselves about in the form of little volcanic bombs and lapilii. Similar phenomena are observed in the lava streams of active and extinct volcinoes. Those of Alta Vista, in Teneriffo, consi>t, on the surfa^ o, of glittering, transparent bottle-glass-like obsidian, which, towards the interior, changes into a less glittering pitchstone-like mass, which is so filled with crystals as to resemble a crystalline rock. These instances have been given in order to shew that, in studying the varying textures of original rocks, it is well to bear in mind that such textures arc, in all likelihood, the result of the influence of the physical conditions under which their respective rocks solidified, and of the temperature and plasticity of the mass frouj which they were produced. The Ibllowing modificati(Mis in the texture of original rocks may here be distinguished ; — ■ 1st. The constituent minerals arc of a comparatively large size, ranging from several inches to nne eighth of an inch in diameter, generally large ennuiih to be easily tested as to hardness, cleavage, and other physical characters. The mode of their arrange ment is altogether irregular, and, although the individual mineral may sometimes have a greater length than thickness, no parallelism ot their larL'cr axes can be noticed. Granite, syenite, and diorite are examples of this order of texture, which may be called the coarse and smaU grained. 2nd. Tiie constituent minerals arc? of a size varying from the smallest individuals to thoHC of an inch io diameter. One or 12 more of them have their longest axes arranged in the same direction and parallel with cacii other, there being thus developed a fibrous or laminated texture. This may be called the schistose order, to which gneiss and hornblende schist belong. ord. The constituent minerals are finer grained than in the prcci'iling order, and more ditlicult of determination. A similar parallel structure, however, is vi>ible, which occusions an easier fracture of the rock along a particular plane, or what is called a slaty cleavage. Coinmon roofing slate may be regarded as the type of this slafi/ order of texture. 4th. The next order of texture to bo distingui.slied is the pnrphyrltlc. Large individuals, or crystals of one or several minerals, are enclosed in a fine-grained or impalpable matrix. Augitic, sycnitic and felsitie pori)hyry are examples of this order of texture, the rocks of which are distinguished from each other i.s well by variitiona in the nature of their matrices, as 'u the compositions of the crystals devel(>ped in these. 5th. The next order may be called the viirloliti'c, and regarded as incipient poipliyritic texture. In a fine-grained matrix, snjall rounded concretions are developed, without, however, being sharply separated from it. These concretions sometiinej possess a fibrous structure in the interior, the fibres radiating from the centre, and their existence is frequently betrayed ou the weather ing of the rock. Gth. The minerals are here of a much smaller size than in the coarse-grained order, so as to be in most cases difficult of deter- mination. This texture is the same as that often posse-sed by the matrices of poi{)liyries, and, being destitute of parallel structure, bears the same relation to the coarsely granular which the slaty docs to the schistope texture. Trap and lelsite belong to this order, which may be called the finvgritimd. 7th. This order may be denominated the trachytlc, and. although its rocks have frequently a pnrphyritic development, they are distinguished from those of that class, in having a rough, porous, sometimes even cellular, matrix, and felspar crystals developed in it of a vitreous appearance and full of small fissures. The same rough uneven surface and fracture is developed in those trachytic rocks which contain no largely developed crystals^ and even in many of a much more basic composition than what are usually termed trachytes. Ilhyolite, andesitc and dolerite are examples of this order. 13 ill tlic suinc hu3 dovolopeJ J the schistose (1 than ill tlie )ii. A similar jions an easier hat is called a i;ardod as tlie ;uishod is the )iie or several [)ablf> matrix. 3 of til is order >u\ each other cos, as ni the , and regarded matrix, small owever, being etiinoj possess ting i'rom the the weather zc than in the icult of doter- posse.>-sed by e of parallel sely granular p and lelsite ii-griiimd. ac/iytlc, and, development, ving a rough, spar crystals small fissures, developed in oped crystals^ on than what d dolerite are 8th. In this order of texture the porous appoaranec above referred to is developed to such a degree that a >C(iriaceous or cavernous structure results. This structure is peculiar to volcanic roeks, which also afford examples of purely vitreous texture, iu which no "grain " nor any mineralogical constituents are observ- able, but an impalpable glassy appearance predominates. This order may be called the volcanic textnrc^'AnH lava, pumice-stone, and obsidian, mentioned as examples of it. It is not to be supposed that these varieties of texture are at a'l sharply S'^parated from ca 'h other. On the contrary, roeks the most varied in their structure are found to be connected with e sell other l»y insensible gr.id itions. Thu'^, Titreous roeks are gra-luilly found to assume an impalpable and then stony character. Then again, they frc(juontly become porous and cellular, and graduate into scoriaceous lavas. Rocks of tiie latter order have very often well-defined lainerals developed in them, and when also the cellular texture becomes more subdued, trachyiic rocks result. These, when they gradually become more Compact or their felspars gradually lose t!ie r vitreous and fissured ap[ie ir inco, become indistinguishable from fei-iites and porphyries. Farther, when the matrices of the last mentioned rocks gradually become coarser 'r lined and their cry>tals reduced in size, they piss into thoroughly granular rocks. When, on the contrary, the well-developed crystals of porphyries gradually disappear, fine-irrained roeks are the product. Nothing is more common than to find the latter gradually assuming a slat/ structure or gradually becoming coarser in the grain, and so giving rise to schistose or granular rocks. And nothing is more common than to fi id the constituents of granul ir rocks, little by little, arranging themselves in a given direction, and so producing coarsely schistose structure. But with all the fre(]ucncy of gradation between original rocks of various textures, it is to bo rem irked that those which diflfer widely from each other in structure, do not exhibit sudden transi- tions the one into the other. Cavernous and coarsely granular rocks are never found to constitute part of one and the same mass, or to pass into each other, without gradually assuming the characters of intermediate impalpable and fine-grained rocks. Nor is it ever the case that coarsely schistose rocks become trachytes all at 0Hce< A certain consistency or method is recognisable in ."ill thfse transitions, and it is only those orders which arc more 14 n^irly related to each other as rop;ar(l-( tixfure, or are more iiitiiii.itoly associated, j^oolot^ioally, tlmt jiraliiato into c;ich otlier in the inimiior above dc^oribcd. In the do-oiiption of the various species of texture given above, those have been phiced ncnrost to oaeh oilier wliicli nro most prone to pass into each other by niodifieations of texture. To account satisfactorily for these vnrliitions of texture among oiiiiiiial rocks is no easy matter ; but if the facts already ^iven, J!S ros ; that tho coarsely j^ranular rocks solidified very slowly, but in oomparativo rest ; that porphyritio and small-<:;rained rocks cooled more (piiekly than coarse granites, although crystallisation evidently took place while tliey were in a plastic condition ; that fine-grained schistose rocks solidified while in motion, but are ihe products of compara- tively rapid cooling; that porous trachytes cooled rapidly, but in comparative rest; that very cavernous rocks came into contact with water during cooling, and wo may suppose that, where tliat clement was y)resent in great quantity, many original rocks underwent disintegration whi e their solidification was in process, giving ri.-e to the tufaceous series of derived rock-:. Many of those generalisations are supported by observations ricent'y made on tho microsi opic structuro of rocks to which, however, it is impossible here to refer. IV. — CHEMICAL COMPOSITION. Crystalline or original rocks have been hitherto regarded and described as aggregates of minerals. No doubt the larger number of them may be correctly enough thus ciiaracteri-ed, but it is d)'ibtful whether the descri|ttion applies to all the original rocks. For instance, obsidian has alwajs been classed among thiSJ, and, on all hands, it is admitted that no miiicnls are disccrnable in it, that it is perfectly vitreou^;, as n;ueh so as bottle or window glas<. A similar vitreous substance, unresolvable by the mici-oscope. forms, according to Vogelgesang, part of the matrix of all true porphyries. Then we have mr.r.y instances of rocks, almost impalpible in texture, belonging to various families, iu which the microscope certainly reveals the presence .J^ 15 or nro more o idi other (f tht! various ed ne:irc8t to icli other by xture among ih'f.'idy uiven, iVL' ;iiiy value n-e that tho the lajiso of h'ly cxtoiided 'd, but it is the oriujiiial asscd among miner lis are 1 >o as bottle I'osolvahle by part of the riy instances ; to various tho presence of Foparnto minerals, but, frequently, leaves their nature nnd, nlway-i, tlicir coinpmition undotcriiiiiud. liesides the uncertainty which thus very fre<|ueiitly surrounds our knowledge of tho niinoraionioil constitution of fine-jrrained recks, tlicro are otiicr consi leratioiiH which tend to show that the composition of a rock, is not ascortaincd even aftiT its constituent minerals liave been detenniiied. In tho first place, the relative quantitits of theso present cannot bo ascertained, and, secondly, even when this is done approximatively, the uncertain composition of the mineral Bpecics renders the chemical coinpo>ition of the rock almost as doubtful as before. Tt would therefore appear simpler and tend to a justcr view of the nature of original rocks, to regard them not so much as aggregates of minerals, as mixtures of their choniieal components, alkaline and earthy !«ilicatcs. Vihich, during cry>tallisition, arranged themselves into compounds of n.ore deflnite atomic composition, namely, into minerals. As has boon already remarked, the primary source of all original rocks must have bjcn the original fluid glob.', and also that pirtof it, which, until the present day, has remained in a state of igneous fluidity. The elements which originally com- posed the fluid globe must have be'cn the same as those wiiich enter into tho comp isitiou of the earth at the present d ly. If, liowevcr, we leave out of consideration those volatile and gaseous elements which, from their nature, must have gone to form tho primitive atmosphere, and aL-o the greater bulk of the metals, which, from their gravity, must li ivo accumnlated at the centre of th,' earth, we have the following list of substances, which in all likelihood, constituted the uppiT zone of the original fluid- globe: — Silicic, boracic, phosphoric, stannic, titanic, ninbic, tung- stic, and tantalic acids: among b ises, potash, sod.i, lithia, lime, magnesia, alumina, ferric oxide, zirconia, manganic oxide, man- ganous oxide, ferrous oxide, glucina, ceria, yitria, oxides of zinc, lanthanum and uranium. All of these substances make their appearance in oriuiiud rocks, many of them however in compara- tively minute quantity and enteiing only into the composition of their .so-called accessorial constituents. If we, for the sake of clearness, lea\c these rarer substances aside for the present, we have the following, which may be regarded as the es.seotial chemical constituents of original rocks: Silicic Acid Aluiuira, rrotoxiile of Iron, Magucbia Lime Soda, I'ota.'ih. I 19 Tlii'80 subsfances, wo luny suppose, wcro, in the orli^iiml fluid m;ii;uius I'rniii wliu-li t)ri^iii!il mcks frystalli>cJ, prcsoiit in the Bauio uiituiier in which wo seo tlieiu t'oiiil)im'(l to^ctluT in I'urn ico slai^s or j^las.s. Kach tif tlicst! i-onstitui'iits, tiio aikalics cxcoptod, is of u niost rclrat'ior)' nature by il.si'lf, but. wiien soveral of the cartlis unite witli tho silica, eou»|Mtun is result of varinus de:;;ret'S of fusibility. In liiis there is merely u repetition of the well-known plu'iioiiiena ol' clieinical eonibiiiatiitn, wliere elemoiits the most aniajionistic eombiiic to tbrni a substance innocent of any of the pn'pcities of its const tueiits. The silica oi cpiartz infusible and clKinically inditlVrent as it may api c ir under oi dmary ein-um- Htaiiocs, acts in this ease as an acid, and, wit . t « aid of heat, coiiibincs wiih the equally rii'rictory basc;^. foiiniiu readily fu>ible compounds. The simple t>ilicatcs, Ibrmed by the uni' n of silica or silicic acid with one base, are not always fusible. Tho-o of tlie alkalies and iion oxides are, but the silicates td" alumina (clay), inajj;Meda (serpentine), and lime (woll istoidte),are almost or com- pletely inlu>ibie. Nevertheless, the three latter comLiiied form the scoriae ible to elas>ify original rocks in a similar manner. When the student of chemistry has gradually added an acid to an alkali, or other base, until the mixture neither reddens litmus nor browns turmeric paper, he has formed a neutral salt consisfinijj of one atom of base to one of acid, such as sulphate of iron iFcO S.O.,) and nitr ate of potash (KO N.O j). The salts of the peroxides, altl)ouj.;h frequently possessing acid properties, are, nevertheless, also regarded as neutral or normal, and contain, for every atom of base, three of acid, such as per- sulphate of iron (Fe.^ O3 3 SO3) or tcrsulphatc of alumina (A, Os 3 SO,;. Similarly in mineralogy those silicates are regarded as I 'I ^ . 17 filial fluid I'Ut in thu ill (urn ICO excepted, .Till of the di'i;ret's of I'ell-kiiowQ the most my of the usjble and ry eirrum- ti of ilOUt, ily ru-iblo •i' silica or >e of the la (chiy), it or com- ncd form ely, those a present puddling termed Such B accoui. pliysical us shigs, 3r. occur in just as oriijuke, icks in a radually mixture formtd id, such N.OJ. ng acid normal, as per- (A,0, 'ded as I neutral whloli contain one atom of monoxide combined with one of isiiiea ucid or silica, or one atom of Hf^iuioxide comhined with three of wilioa. Thus tiie mineral leuciti-, which consists of one atom of potiish, one of alumina, and four of silieio acid, may be regarded as the typo of a neutral mineral. Its formula is KO. Al, • 'a 4 Si. 0, and it will bo observed that its bases contain four while its acid contains eight equivalents of f)xygen. Neutral or monosilicates, therefore, are thoso in whioh the proportion of oxygen in the bases, to tliat in the acid, is as I is to 2. If we search among crystalline rocks for those in which ♦his ox_\gea ratio exists, we shall find them to }»« wclj-delint'd rock sptciis which are not usually considered from a cluuiic il point of view at all. These rock species are syenite, mel.ii)hyre and andesito, which rcspictively represent the neutral development 0.83 5H.-J8 50-— 56-22 .'■)918 50-73 18 IT. Anjritio nndosito from T.Kwon. liiirjf, ill Sii'lionjri'liirifo.— KjiTulf 1 TH ;fl Tm OS II. I|i'riil«lciii!ii' Aiiriil)l(Miiliii AiKli'oito fniin St;iiT Swi.'lliiii.— TM'horiimk. 1 2'flOI fiH'W IV. linnilili'iMlic AiKlffiti*, fVtiiii tiluiiatill»i'i>', ill Sicix'iipiliirgc, -liiiiiuuoi>iicrK' I y-J-i MMi Avoriigo I yutit; W-J It would Boom tlioroforo from tliONc fi^'urc'', tliut (lio»c rocks wliicli, ill (•niiip()>itinii, arc noiitral or iiiouosilicalc."', contain an uiixiuiit of HilitM !ivera;j;in^ 57.02 por cent. As ill clicmi'-trv we have acid salts, in which one atom of base is coiiibino I witlj nioro than ono atom of acid, po in litliolo^y wo liave rooks in which the silica is present in nmeli lar;;,citic porphyry an 1 rhyolitc. Proceediiiji; in the same niaiiner as with the neutral rocks we (ind the followin,!,' ainon;^ this scries to approach most closely in composition to bi-siii'iates: O IIATIJ. (J'la-itltv nr Mhnt III liKI Bases. Slllou. p.uts loik. I. Granite from nciilclherfr,— Strong 1 3 bDI} 72 II JI. (jiuiiite IViiiii Doiicliiirv liritlgo, Di.iiega!,— lldiigbtdii ' 1 3.7t)0 7'J.'J4 III. (inmito of Fox Uock, near Duiilin, — iliiiijrlitoii 1 4 077 73 IV. OiaiiittMirijtiU'giai iicarSilfsia, — Sii.iifT 1 4 ;u5l 73.13 V. Giauilc (if I?lacl OS •GO r.'-(>:i 57 -o iiitain iin tij (jf base li<>l<);iy wo 'imntity I scries of cess as to ntoiiis of for mcry ccn bases ritic jiml pi evented the saiiio iiotii^ this tcs: ntllv nf (II III |IKt Its luck. '2 II ) 24 :? 4G Tt (ipponr?, tliorcCorp, that tho oxyKcn rfi*'" 1 ^^^ ^ corrosponds to nil av('riifj<( ^iii("l pcrct'iit iiio of 7-.(il, aiiil to >iu'h bi-siliouto rtK'ks the n ime silicic iiii;;ht hi; applioil. Uut bi'siilrs this siliitii! Kories of rooks thiiro is foiui«l aiintlifr Bcrios of very ditfiTotit chemical constitution, and in wliicli tlio base.*!, and not the hilica, prcjionderatc. It is only, however, in rare instances union;^ those rocks that tho silica disappears to hucIi nn extent as to form a disilicato. i.c.u eomponnd of one eipiivalent ol' silica with two of hise, or in which th*; (piaiitities of oxyiien contained in acid and base arc e(nial. A very well marked series of basic ntcks may, however, be pointed out in which two e(|iiiva- Icnts of silica arc combined with throe of base, and in whicii tho oxygon ratio is as 1^ t(» I. Tho rocks which represent this ba»ic development of the porphyritic and tracliytic textures, are, respectively, uuijiiic porpliyry and ne|ilu'linitc. Tho followiii<^ are instance (if these rocks in which the oxygen ratio most closely appioaelns 1 ;{;>;)=1 : — Aug" tic inrpliyry from Fassatluil ill Tyrol N»'i)lii'Iiiiiti' fniiii Wickcnsteiii in liiiwi-r Silt!-';c rocks. Between the basic and neutral rocks, on the one hand, and the latter atid the silicic rocks on the other, there exist many other rocks of interincdiate composition and forming gradual traiisitioiis between each of tho series, which iiavc been more niinutily refoired to in the foregoing. It thus becomes possible to point out a series of rocks passing gradually from the basic extreme to that of acidity in compositioi), not otdy for each of the granular porphyritic and tr.iohytic order of rocks, but also for every variety of texture specified in the preceding chapter. The following 1'abic gives an arrangement of these va'-ious scries of rocks and au czhibiiioQ of the diatiuctive characters as to texture and liO chemical compojiitinn possessed by each. In constructing tliis table, it has been found that by limiting the variations in silica con touts of each family to 7 per cent, very correct lines of separation may be drawn betwixt them : — Table I, Showing the General Chcniical Composition of the Families of Original Hocks. Onlcr of Texture. HiiHio IliickB. ■ '"Mltlllllili^' I' 8' tl.aii 49 pir fpiit. silica. BiisouR Rock8 I'ontiiiniiiK I rem »it t(t -6 Ill-cunt, .-ilica .Niutrnl Hnckt.. (iiKin.^ilicatcri o<>iitninii]).r from .'j6 to tilt IHTccni .silica. Sllicoous U.ickB, cont;iitiinp Ifoiii 6:1 t'.7ii P'-rcont. .sl.ica -ilicic R.ipkB, ciintairiiiitr mere than 70 jiiT cent, .-"ilica I 1 oil '• ami n iuli- grai cl II Sell St. su III Slatv A''Orth"sltc. B ,sic bchsl. (}'•(> nst"n'>. llo nlilfiule C l8'. ijrei'i I.Stone si to. li il'll t TIP P'l'plnrv. V ir . lite. r a p. D.ileritP. 1 loin It lava "iy-nite. Sy,'til-e sell St. CI o .-.lute. Melai>hyre. V ir.b"3ftltite Granititc. Quflss. ."Uicpons si ,to. Poiphyrlie. 0-a"itp. G lissite. S' 111 Ic slate. IV Porphyrltlc V V ir'n it \c .\'ic:itic por- I'liry, Fi'isiti.' por- liln ry. p:.ilri.llte, F i-itc. Rhy-. lie. tiliskllaii. VI Fiiii'-K Hi eil V!I. T NOilVllo VJII VoKaillc ii.sal . N plu'lliiitc. Nt'|ilii'ljiiit>' lava. Basal tl e. A .1. sit... A (Ipsite i IV . Knr'tp. Triciiyte. 1 railiytlc liiva. Before proceeding to explain the foregoing table, it may be iiioiitioncJ that no new names have been used in its construction; that names to which definite ideas as to mineralogical con>tituti(m ■ire attached, have been, as much as po.ibility, and gradual transitions in each of these respects are found to exist from rock to rock along each horizontal series. The basic rocks are generally darker coloured, less hard, and more readily fusible than the rocks which correspond to them in texture but differ i'rom them in containing a larger percentage of silica. On the other hand 22 tl-.e more siliceous a rock is, the lighter it will generally be found to be in colour, the liardcr and more difficult to penetrate or excavate, and the more refractory on exposure to high tempera- tures . There is yet another physical property belonging to those original rocks, in which they show a similar correspondence with their chemical composition. Still speaking generally, the more siliceous a rock the lighter it is, not only in colour, but in weigiit; the more bisic the rock, the heavier it become*. Thus it is the case that, in each order of tf^xture on passing from the siliceous to the basic rocks, a gradual increase of density takes p'ace, and on the otlicr hand, the transition from the basic rocks to the more siliceous exhibits a gradual diminution of specific gravity. So constant is this relation that it may be taken advantage of in determining the general compo^^ition of a rock. To take as an nstance the coarsely granuhr series of rock families the general range of their specific gravities may be said to be as follows : — Granite - - - - - 2.G5 and under. Granititc - - - - 2.6.') to 2.8. Syenite 2.8 to 2.875. Greenstone- - - - 2.875 to 3, — This part of the subject is one of very great interest, but it would be premature at present to discuss it minutely. V. — MINERALOGICAL CONSTITUTION. Having, in the foregoing, adverted to the texture and chemical composition of original rocks, it now becomes necessary to refer more particularly to their mineralogicil constitution; In order to continue the analogy which has boon shown to exist between furnace slags and original rocks, it will be well here to refer to those instances which have been observed of the formation of well developed crystals in the cooling of artificial silicates. TliC rapid manner in which furnacG slags are commonly allowed to cool is of course detrimental to the formation of any mineral-liko airgregations, but it is sometimes possible to obsei- -> in copper furnace slags, that when they have been allowed to polidity in large blocks or cakes, they show an acfynolitic structure in their mass, often closely resemble hornblende rock, and very commonly contain cavities lined with the most beautiful crystals. The formation of pyroxene in slags from iron furnaces has been frequently obsei'vcd and well authonticuted. Noggcrath described 2d )G found ;tratc or (Miipora- those ice witli le more weL^lit; it is the bilioeoiis ice, and to the ;ravity. ai,^e of in lie a.s an 2 general ws : — r. st, but it chemical to refer In order between refer to lation of :^. The owed to leral-liko 1 copper lidiiy in in tiieir >mmonIy 3. The as been escribed augitc crytals from the slags of the iron furnace of Olsborg, near Biggc, in Westphalia; Montefiori Levi analysed augites taken fiOin the t^l.igs of the iron furnace at Ougiee, near Liege. Richtcr described and examined similar crystals from the iron woiks of Kufskberg in the Banat ; Vou Leonhard mentions acieul.ir augite crystals in the iron furnace slags of Skis-hytta in Swede). F. SanJberger describes similar oc3urrences, and uuiuerous others might here be mentioned. Mitscherlich and Jierthier obtained by melting silica, lime, and magnesia together, in a charcoal cruijible placed in a porcelain furnace, a mass possessing cleavage corresponding to the faces of augi' ., and the hollow cavities in which were crowded with the most beautiful crystals of that mineral. These are also of very common occurrence in the lava streams, not only of extinct but of active volcanoes, and well developed augite crystals have not unfroqupiit'y been ejected from their craters. Olivine has been observt'd in the slags of iron furnaces quite as freciuently at augitc, and it, as well as magnetite is one of the conimoncst minerals in streams of basaltic lava. So ia Icucite, although it lias not yet been produced artificially. xAIit?cherlich ob>erved transparent six-sided tabular crystals of mica, and laminae of it seveial inches broad, in the c;ivities of old copper furnace slags near Garpcnbcrg, in Dalccarlia. Gurit also mentions artificially formed mica, and it appears frequently in ancient and modern lava streams. With regard to felspar, ILiusmann makes mention as early as 1810, of felspar crystals which had been formed in one of the Mansfield furnaces. In 1813-1, Heine found similar crystals in the copper furnace of Sangershausen after it had been blown outj and in the iron furnace of Josephshiitte, in the Ilartz they wei'e also detected. In 1810 the formation of fel>par crystals in glass works was first observed, and in 1848 Prechtl gave an account of their occurring in a mass of glass weiizhitig 133^ lbs. which had been incited in the plate glass factory at Neuhaus. They were of various sizes, some an inch in length, with perfe.'tly sharp edges. The formation of sanidine and other varieties of felspar, in lavas of recent age, is a matter of common occurrence. No instance is known of the production of quartz from artificial .silicates, nor do those lavas of the present d;iy which are highly siliceous, develope it in cooling. These solidify as vitreous uncrystalline masses, but many lavas of extinc- volcanocs in the Andes and the Siebengebirge contain it in well M 24 formed crystals, showing that it must have crystallised out from the m;iss of the rock. The number of minerals which enter into the constitution of rocks in very small compared with the number of the mineral ppccies which are found described in the various treatises on njiiieralo^'y. Of the latter there are upwards of six hundred, but the great majority of these are rare minerals, occurring in veins or cavities and not entering into the constitution of the rocks themselves. The number of minerals which are found in original nicks is still n)ore limited, and if from it we deduct the / ^ sparingly occurring, or so-3alk'd accessorial constituents, the 'J-i. o t -K number is reduced to two tt ty minerals which may be c-illed the essential constituents of original rocks. The following table givts their names and the silica contents of the extreme acid and baSiC varieties : — ■ Miner ul Ber cnitage of Silica. Qmrtz ItHi 00 - OO'OO Orilioc'lase G'J 00 — G::^ 75 OiifjDcliise {,4'26 — L9^8 Liiln'iidniite 55 h;} — 50 ;U i\n(ntliite 47'63 — 4"^'ul L 3 essential ithologists rizing the s we have, chemical aluiuina, snda and tuents of 3S, and no the table. bitrary as 3urnialine essential constituent. Tourmaline, however, contains, besides some of thfl subtances just mentioned, boracic aeid and fluorine, and, in its mode of oecurrence, resembles such accessorial or accidcntiil min- erals as zircon, apatite, titanitc and others. Garnet, corundum, opidotc, cordierite and scapulite arc rock minerals, containing no other chemical constituents than those above mentioned, but they have been excluded from our list because they resemble the acces- sorial constituents in the manner of their occurrence. Witli regard to these essential minerals it is first to be remarked that the analyses which have been made of them are not, in every c;ise, of such specimens as have actually formed part and portion of some rock species. To obtain pure specimens of the minerals of rocks is often a matter of great dilliculty, and well- develojjcd crystals frou) veins or geodes have been preferred for analysis to the generally amorphous particles of the same species which enter into the constitution of rocks. Tlie composition of these minerals cannot, like tiiat of well-crystallised artificial che- mical compounds, be unequivocally expressed by chemical formulae. Attempts, the most painstaking and persevering, have been made in tliis direction by mineralogists, and the result has only been to shew that, in the mnjority of cases, each analysis of the same species demands a different formula for expressing its composition in ehcmieal equivalents. The composition of micas, augites and hornblendes is especially variable, and even with regard to the felspars it has been maintained that those of our list are not dis- tinct or independent species but aremixturesof one witl: the other or with other supposed speeies,such as krablite,albite or adularii. It has thercrore been considered best here to neglect their various assumed chemical formula and to regard principally their average chemical composition. Certain differences in the composition of these minerals cause their subdivision into two different classes. The minerals of the first class arc mostly silicates of alumina, lime, potash and soda, and it may he called the fclspathic class. It includes, however, leucite and nephclinc, which can scarcely be called felspars, and quartz, which, although of very different composition, nevertheless possesses lithological affinities connecting it closely with the acid felspars. The minerals of the second class also contain lime, but alumina and the alkalies are less frequent or absent altogether, being replaced by magnesia and protoxide of iron. They are generally of a more basic nature than the felspathic class, and the M ,>6 purely b:isic mincrnl magnetite may be placed, as lithologically ri'liifcil, :ilong with them. Tlic minerals of this class may there- fore be cilleJ the b.isic essential constituents of rocks. We have thus tiie Ibllowing classilieation of these essential rock minerals. Class 1st. — Felspathic — Quartz, Orthoclase, Oligoclasc, Labra- dorite, Anorthite, Leucite, Ncpheline. Class 2nd. — Basic — Potash mica, Magnesia mica. Hornblende, Pyroxene, Diallage, Eustatite, Hypersthcne, Olivine, IMag- nctitr. The extent to which these minerals enter into the constitution of origin;il rocks will bo best seen by repeating here the general view given of tiic families of rocks, placing at the head of each column the names of the principal constituents. Table II, General View of the Mineralogical Constitution of the families of Oriiiinal Rocks. m linaic li'icks. Bnnoui Rncl-g NeutrnlRncks SiUceoua IVhs Silxric Rockf. (iuartz Qnartz. il Orthnelafc. Orthoclase- Orthoclase. Folspathie Miu'ls i Oligoclase. Oligoclase. Oligoclase. 1 Labradoritc. Anorthite.... Andrtliile. Nepl'.eliiie . . • Neplu'lino. Mug: mica. .Mag; mien. Mag: mica. Pot: mica. Basic Minerals . Hm iiblcndo. Hoinblende. Pyroxene Pyru.venc. 1 Olivine Olivine. I. Coarse and Mienetito ... 1 small-grained Anorthnpito.- ttrcon?tonc. Syenite. fJranitito. (Jrnnite. 11. ;5ohisto«i — lJa?io schist.- Orcenstono schist. Syenitio schi.-t. Gneiss. Gncissito. III. Slaty Green>tono slate. Clay slato. Siliecous slate. Silicic Slato. IV. Porphyritic. Aucitic por- Greenstone Mclaphyro. Porphyrite. Porphyry. ,.,..,.. I''""y- (lorphry. \ . \ ■inolitv,'" Sphcrnjyto. Fclsitc. VI. Finu Brain jd .Anhydrous Trap. Basaltito. Eurito. „ 1 bisilt. ^ II. Tn-hytiC! Xeplielinito. Dolcrito. Andesito. Trachyte. Rhyclite. Mil. Volcanic Xeiiholinitc Doloritic lava Andesitic. Trachytic Obsidian. lava. lava. t&BJm 1 lithologically ass niiiy thcrc- ks. We have rock minerals. joclasc, Labra- I, llorntlonde, Olivine, Mag- e constitution ■e the general head of each the families (8 li'kg SiliricBocka. nso. Cjnnrtz. Urtliotluse. ise. lioa. Pot: mica. 0. Grnnite. Gncissito. ! Silicic Slato. tc. Porphyry. Sphcriilyto. Fclsitc. 1 Rhyolito. Obsidian. tB i n It will be observed from this table that a certain degree of consistency is observed by the essential minerals in entering into the constitution of original rocks. Such acid minerals as quartz and orthocl ise never occur in the b:isic rocks; nor, on the other hand, do wc find augitc or labradoritc entering into the composi- tion of siliccousgr.inites or trachytes. Towards the b:i,^ic extreme of chemical composition in rocks, the tiliccous minerals diminish or disappear, and, towards the acid extreme, basio minerals act in the same way. This behaviour alone is sufficient to thew that the mineralngioil constitution of a rock is not the result of accident, but mainly the consequence of the chemical nature of tlic plastic magna from which it resulted, an inference which is borne out by the varying composition of the mincriils themselves. It will be seen that at the heads of the columns the minerals have been arranged according to the classification already pivcn. Now it would appear, with regard to the members of each of the classes which wc have distinguished, that not only do they laseinble each other in chemical composition but they seem to replace each other when they enter into the composition of origi- nal rocks. That is to say, the increase, in quantity, of one of them in a rock is generally accompanied by a decrease on tlut part of another member of the class, and generally of that member which most closely approachei the first in chemical composition. This appears to be well borne out by the table, and numerous examples of «ueh substitutions might be cited. Thus hornblende replaces mic;i in granitite forming syenite ; oligoclasc replaces orthoelase in the p\3Sigc^from syenite to diorito ; anddiillago replaces pyrox- ene in that species of greenstone called gabbro. There arc thus formed gradual transitions fronr one rock species to another in mineralogical constitution as well as chemical composition. In the subjoined table ( III) the nature and manner of these; tran- sitions arc exhibited. It will be seen that the distinctions already made as to the orders and families of rocks are kept steadily in view while at the same time an attempt isiiiadc to give a systematic arrangement of the dififerent species of original rocks ,and their mutual relations. » TIT. — T.iblo showin^• the Constitution of the various Species ol' Jh'xcn'/iliiiii iij T'jiiire (III 'I mniirx (if' Kxxcilliill JSdsir C'dllKlilllClllK. Basic N(iil-fil!iilhic Noels. lidStC lldclcx (^Sii/iKi/icdlcs.) /lllStlllS li UlitliT l!) ['. c. 4;i to ")(; Silica. Silici SailSsllfitr, Aiioithitf. Xfpliclinc. ( iliumliisr. Liilii'iiiloiit' I. Coarser 1111(1 siu.'ill-nijiiiird : with iioliiisic < oiistiliunts (I u II II II II II II (1 II II I. II II .1 Willi .Mi( a '• lloiiililciiilr i Anipliiliolytc. " I'yrnxilli' I'vidXrliytc. " J>iulla,u'(' or Siiiaiiiuditc. (tiiiphazyti' " IlyprrstlRnf " JlristMlitc " Olivine Diiiiytc. " Magnetite [ .Magnctyti Llicr/.olvte. Corsytc. Kukryte. JOiipiidtide, I'rotoliastvtc If. Sfliist().'iileritic hi' 4\ f tlio various Species of Ori,i-in;il Rocks, by TrroM.vs MArFARLAXE. /■-■((• Uncles Xi'utnil Hocks. i {Miinoxilinites.) I >''"•'""••' /.'"'■/,-.s'. I'T 411 jl. V. : 4i) to .")(; |(. f. Silicii. I Silica. "'•' t'> ^''^> ]>• <■. ' (i;; t(i 70 |i. r. Silirfi. I Silica. Silii'ic liorkn. (/>i.ii/ic llrn.) <»vci' To |i. (_•. Silica. Enseiilial Fc/.yi,it/itc (.'onslilucrita. smite •tiiitc. u'linc (tlinoclasc. Luluiuloritc, < l| tliocliisc. Oli.mxlasc. yiiaitz. Ortluiclasi;. Oli.ui'clasc. j Quartz. Urtiioclaisc, Si/ii'ir Xon-/r/.iji,i/hi(; Hnrh s (''niliii/iiii'/ (JiKirl:. ytc. mtidc iliastvtc. XdlTtc. Micaccdus diorili'. Didi'itc. Dialiasf. (laiilu'o. ilyiHiytc. Miascytc Svcuitc. (iniiiitytc. Horiili. (iiairitvt( (iranitcllc. (JiHuitc. Kidiitc schist. Itialiasc schist. 1 slat( lute jjorph, ic poi|)li. ytc. cnytc. esvtc. (iiiciss. Sycnilic schist. Svciiitic unciss. Aphanytc slate. [ Artrillvt 'i>iiart/ytc CJi'cisi 11. ,'t(« ;\Iclaphyrc. Jfiiictte. Hoiiili. .Mclaphyre. i\crsaiit(iMc. Mioritc porphyry. Diahasc poijiiiyry. i An.ijritic iMda'phyrc, Varidlvtc. liintc. Aphaiiyti'. Dioritic aphaiiyti Trap. Siliceous slate. Porphyrite. Mica porphyrite. Hornh. jiorphyrite. liasaltvte. Ito|cr\te. ic lava. Doleiitic lava. And.'syte. AiiLcitic Andesvte, Enrvte. Andcsitic pinnice. Aiulesitic lava. 'J'rachyte. Au"itie trachvtc Obsidian. Puiiiict'. (i rami lite (iiK'i.s.syte. Felsitic slate. Felsitic porphyry Mica jiorjihyry. S])hcruiyte. Felsvte. Tilivolyt. Daiyti'. Silicii' obsidian. Quartz schist. Mica schist. In preparing,' tabic III, the samo caro lias boon taken as with those alrc-uly ^'ivcn to introduce no new terms, and to use tho "arious names of the speeies only in the sense whieh at present is generally attaehed to them by petroloj,'ists. In a lew instances, where siieh names have hitherto borne a too general or a more or less indefinite meaning;, an attempt has been made to conline their application to one speeies. The name rhyolite is ibr instance used in a somewhat more restricted sense than that j,'iven it by its originator, and the very vague, generally condemned, but still much used or misused, name, luelaphyrc, is, as applied to a parti- cular species, limited to those porpliyriteroeks whieh are neutral in chemical composition and in which crystals of tricliiiie felspars only are developed. In some other cases, where the same species possessed several synonyms, a slightly dilferent signiiicatioii has been given to one, and generally the least used of them, in order to make it of use in our system. For instance, eurite and felsito have hitherto been synonymous. In our table the latter term is made to indicate the more silicic speeies of fine grained rocks. Such names of rocks as have been derived from tlio.se of minerals have their terminations, in accordance with Dana's suggestion, altered from ite to ijte. It will be observed that, in table III, the minerals of the fels- pathic class only are placed at the head of the vertical columns, while the other essential minerals have been itlaced under each variety of texture on the left hand side. The cause of this arrangement may here be stated. The felspars, being of very constant occurrence in original rocks, and being fre((ueiitly diffi- cult to determine, have not been much made use of in distin- guishing species until quite recently, For instance, oligocluse very often can only be distingui.shed from ortliocla.se by an experienced mineralogist, and only an experienced chemist after a minute analysis, can distinguish between oligoclase, labradorite and anorthite in a compound rock. On the other hand the minerals of the other class possess very well marked physical characters, and the presence of one or other of them was readily detected by the earlier petrologists and made use of by them for characterising different rocks. Thus, mica, hornblende and olivine are very widely apart both as regards form, colour, hardness and fusibility. The only tv.'o minerals of the second and third classes which are difficult to distinguish from each other arc hornblende and augitc, and this is only the H ^J o case in fine prnincd compound rocks, By pivinpr proniinPnce to each of these non-fclsputhic rnincnils Jind placing; their nanics on tho horizontal lines of our table, it becomes pos.Mble to shew at a planco the rocks which they form with the fclcpathic minerals named nt the heads of tho vortical columns, and the manner in which, by (gradually replacinji each other, they form the different species of original rocks. Thus it will be observed that among tho schistose rocks the most bahic is diabase schist ; that the latter becomes diorito schist when liornbleiide replaces pyroxene ; that the diorite schist, as its oli^oelasc is replaced by orthoeiase. becomes syenite schist, and, as (juartz makes its appearcncc and increases, syenitic gneiss is produced. At the next step in a silicic direction, mioi replaces the hornblende, producing common t^nciss, then when tho mica disappears, granulitc results. If, instead of the mic;i, the orthoeiase dis:ippears, mica Bchist is developed, and when from the latter rock the mica in ;,'reater part is withdrawn, it becomes quartz schist. The other varieties nf texture, such as the pori)liyritic and traehytic, each exiiibit a similar scries of transitions, the most fully developed bcin;^ the crranular order. In tho latter it becomes possible, by means of the peculiar arrange- ment of our table, to shew the mineralogical nature of each of the species of the complicated family of the greenstones. Diorite, g;ibbro, hyperyte, diaba.'JC and protobastyte rock arc shewn to be respectively characterised by hornblende, diallagc, hypersthene, pyroxene and enstatite in combination with various felspars. The great majority ol' original rocks contain some variety of felspar, but there are a few species in which that mineral is absent and which are called non-felspathic rocks. In order as I'ar as possible to shew these also iu our table, two columns have been added to it, one at each side. The right hand one shews the silicic, and the left hand the basic rocks void of felspar. VI. — ACCESSOIIIAL CONSTITUENTS. Uesidcs tho minerals mentioned in the foregoing chapter as the essential constituents of crystalline rocks, there are others of less frequent and only accidental occurrence, which have been called by German lithologists the accessorial constituents. Among these such minerals are not included as arc only found in the veins, cavities, or even joints enclosed in rocks. Only those which are found in intimate mechanical union with the essential constituents io the body of the rock itself are regarded as accessorial const!- ^^/ tucnts. They arc somctitnca made up of the patno common chemical components as the essential rock constituents, but much more frequently other and rarer elements enter into their eomjio- sition. It is indeed almost exclusively from these accessorial minerals that many of the rare simple elements have been derived with which chemists alone have any intimate accjuaintance. Thus glucinum, cerium, yttrium, lanthanium, colunibium, t;int:ilum, tungsten and zirconium are only found as eomponentsof accessorial rock constituents, while other elements, such as sulphur, phospho- rus, boron, fluorine, chlorine, tin, eopper, lead, ehroniiuin and titanium are frequently found in them, which but rarely occur in essential rock constituents. The followinjr is a cat;dogue oftlio accessorial constituents of rocks, arranged according; to Dana's system, which at the same time indicates briefly their chemical nature. I. Native elementa. Gold. Silver. Mercury. Iron. . Diamond. (frajihitL'. II. Sttlpliidcs, . Mengite. Woll'iamito. VII. Phofphaten. Apatite. Monazite. Tryphllite. From this list it will be seen that the accidentally-occurring minerals in crystalline rocks are five times as numerous as the essential minerals. It is scarcely possible to take a general view of the list without noting not only the number of rare elements which arc found among their components, but also the preponde- rance of bases in their composition. The number of subsilicates and unisilicates largely exceeds that of the bisilicates. The rare tantalates, columbatcs, Sec, are exceedingly basic, while no less than ten consist exclusively of anhydrous oxides. Another pe- culiarity in the composition of the silicates among them is the presence of scsqui-oxides in large quantity. Epidote, lievritc and others are silicates of alumina and peroxide of iron, while anda- lusite, cyanitc, topaz and many others contain the former base in great abundance. With regard to their distribution among original rocks, it is to be remarked that by far the greater number are native to the coarsegrained and schistose series, and occur in largest quantity in their neutral or siliceous families. Granites and syenites are especially rich in them, a remarkable instance being the zircon syenite of Fredericksvaern in Norway, in which no less than fifty difi'erent minerals are found, among whose components there are nine rare elements. These accessorial minerals become less fre- quent in the porphyritic and trachytic rocksj until among modern lavas very few of them are to be found. ^^^'J The following statement shews the distribution of the accessor- ial minerals among the various orders of original rocks: In coarse and small-drained roc/cs. Ae.uirite. Acscliinito. Acniitc. AUanito. Anakimo. Andiilusiti! Apiitite. Apnpliylliti'. hcvyll. Bleiule. Calcspar. Catapluiito. Colniubitc. Copper pyritt'S. Cordit'riti'. Coruiidiiin. Crocidolite. Clirys(il)(.Tyll. Cyauite. Diamond. Kpidotc. KudiKipliite. Kukdliti.'. FliKircritc. Fliidiiti'. (■adnliiiitc. Galiniti.'. Galena. Gold. Graphite. Hematite. Hypo.stilbite. llmeuite. Iron pyrites. Lepidolite. Leucopliane. Mafinetie pyrites. Mentiite. Mereury. Molylidenite. Monaziii'. Mosamlrite Plieiiakite. Finite. I'olycrase. i'olymi.Lcnite. I'reJiiiite. Fyroehlore. Piutile. Saponite. iSaussiirite. Sca])()lite. Silver. iSodaiite. Spudumi'iie. Tantalite. Thorite. Tinstone. Titaiute. Tourmaline. Tripiiylite. 'J'ritonite. Vesuvianite. Wolframite. Wohlerite. Yttrotantalitc. Zireon. x^^^ In Schistose rocks. Andahisile. Apatite. Beryll. Calcspar. Coidierito. Coiuudum. Cyanite. Dolomite. Fluorite. Graphite. Hematite. Iron pyrites. Lepidolitc. Molybdenite. Eutile. Spinello. Staurolite. Titanite, Tonrmaline. Zircon. In Slaty rocks. Cliiastolite. Cidoritoid. Damoiuite. Dipyre. Fald unite. Ottrelite. Paraconite. Sericite. Staurolite. In Porphyritic rocks. Apatite. Calcspar. Crocidolite. Delessite. Ei)idote. Fluorite. Gicseckite. Hal loy side. Iron pyrites. Liebenerite. Titanite. Tourmaline. In Impalpable rocks. Hauynite. Ilmenite. Iron. Iron pyrites. Maguetic pyrites. Nepheline. Noscan. Sapphire. Titanite. Zircon. In Trachylic Rocks. Apatite. Faujasite. Hauynitu. Hematite. Iron pyrites. Leucite. Melilite. Nepheline. Nosean. Sapphire. Titanite. Zircon. With regard to the origin of these accessorial minerals it may be maintained that by far the greater number of those just men- tioned have been developed during the solidification of the rocks containing them, and somewhat in advance of the essential con- stituents among which they are found. The evidence of this statement will, however, be given in the following chapter. VII. — ON THE ORDER IN WHICH THE CONSTITUENTS OF ORIGINAL ROCKS WERE DEVELOPED. [t cannot be assumed that, in the slow crystallisation of a rock from igneous fusion, its minerals were all developed at one and the same instant. On the contrary, mnny of them are found under circumstances which prove that, even after their formation, the mother magna still possessed some degree of plasticity, and many of the constituents of rocks are so associated and surrounded as fairly to lead to the conclusion that a certain order was main- tained in their gradual developement. The well-known phenomena of fractured crystals in original rocks first deserves mention in this connection. Felspar crystals are frequently found in granites, broken in two pieces, these frag- ments being displaced, and the space between them filled up with granitic substance. This is the case with the orthodasc crystals of the porphyry of Elba and of the quartz porphyry of Ihnenan ; with the sanidiuc in the trachyte of Drachonfels, and with the tourmaline of the j^Tanite of Winkelsdorf in Moravia. These phenomena serve to prove that the solidification of original rocks took place very gradually, and that their crystallisation was iu progress long before they became completely consolidated. Very many of the f tcts recorded regarding the occurrence of accessorial minerals in rocks go to prove that they were the first to separate from the fluid magna and assume their characteristic forms. Blum has observed that the long tourmaline crystals which occur in the chloritic schists and granites of Aschoffenburg and of Winkelsdorf in Moravia, and which are frequently found fractured, have their separated fragments frequently bent out of their proper direction and cemented together by mica. The proof here seems plain as to the formation of the tourmaline prior to that of the mica.-''^ In the lar<>e irrained <>'ranite of Bert'stietic, near lluhla in Thuringia, Scnft has observed that the (juartz partly surrounds the tourmaline and wholly surrounds the mica plates, and regards this occurrence as proving that the formation both of the tourmaline and of the mica preceded that of the quartz, f Very many instances have been observed which go to prove the formation of tourmaline prior to quirtz, and not a few from which it may reasonably be inferred that it crystallised before both mica and felspar. In connection with the ore deposits of Scmidinavia, mention is made of the occurrence of iron pyrites completely enclosed i;' a crystal of tourmaline. A similar relation has been obser\cd in the case of garnet, which very frequently encloses in its crystals a kernel of magnetite. Garnet is, however, noted for enclosing many other minerals, quartz, mica, iron glance, vesuvian, epidote, copper pyrites, iron pyrites, galena, blende, and especially hornblende varieties, having been found iu the interior of its crystals. According to Blum the orthoclase crystals of the porphyrite of the Baranco cos las Angustias, on the Island of • Ziikel, i'etrograpliie 1, 63. t Die Krystalliniseho Felsgementhcilc, p. 512. n ^J/ of lauy ote, illy of the of i P.ilma, contain racliatinj:; particles of cpidoto which gradually merge into the mass of tlie orthoclasc. This and similar instances can scarcely be explained otherwise than on the supposition that the formation of the epidote preceded that of the orthnclase. Other facts concerning the occurrence of epidote in syenitic rocks would scorn to indicate that the formation of the hornblende pre- ceded or took place contemporaneously with that of tiic epidote. Senft has observed, near Brotterode, staurolite crystals enclosed in tr;insp irent plates of mica, and G. Rose describes both stau- rolite and cyanitc columns as occurring in a similar m inner. According to Senft, tourmaline, garnet, staurolite and cyanite are very constant companions of potash mica in crystalline rocks^ and most frequently occur bedded in it as well developed crystals, and when separated from the surrounding mass of mica, leave in it an accurately bounded, smooth sided and sharp angkd im- pression of their several forms.* The order of the formation of the minerals of granite has been a matter of frequent discussion, and the impression prevails that the mica preceded the Ibrmation of at least the (ju irtz in that roek. Senft thus gives the result of his observations on this matter : " Potash mica shews itself most frequently associated ** with amorphous quartz and with orthoclasc; with the fu>t *' usually so that it lies imbedded in its mass, which would in- " dicate a later formation for the quartz; with the orthoclase, on " the contrary, I'reciuetitly so that it appears to sit upon it, so " that one must regard the mica as tlie newest mineral. Ilow- " ever, there are not wanting cxamjdes of the occurrence of mica *' sitting upon the quartz, nor of others in which it .ippears so " evenly intermixed with fresh orthoclase that one must ascribe " to them a contemporaneous origin." f Senft has also the following remark on the mutual relations of oligoclase and hornblende : " Where oligoclase occurs in very "distinct intermixture with crystals of hornblende, it, for the " most part, surrounds them, and, indeed, often completely encloses " them in its mass. This relation })lainly indicates that although *' both minerals were produced in one and the same original " magma, nevertheless, the hornblende vas the lirst born, and the '• oligoclase was obliged to produce itself out of that part of the " ma"ma remainin"; after the formation of the hornblende." l"\'lsf,a'lnL'ngtlioile, p. 707. f Fclsgtmcngtluili', p. 7<)7. -^ ^'^^ Jl '. r The study of the manner and order of the formation of cry- stalline minerals in coarse-grained, compound crystalline rocks, has not, on the whole, had that attention which it deserves. On the other hand many of the results obtained in the microscopical exiimination of fine-grained original rocks have an important bearing upon this subject. Vogolsang^'' has described with the mos^t piinstakinix accuracy his observations on the mutual rela- tions of :;i. i ' orals of many pitchstones, trachytes and por- phyries. Meniion must first be made of a very interesting phe- nomenon which he has detected in the microscopical structure of many trachytic and porphyritie rocks. This is called Fluidal- structure, and secni'^ to have been discovered somewhat earlier and indcpci, . . .,) ..; iii, W'eiss.f This term is to be uiuler- stood to denote r^nu'; i ition of the constituents of a rock re- latively 10 each other, :.s tn illo>v of the inference being drawn that a Movement 'f tlie mu.v- '.n" as a whole or in its smallest parts, had taken phi'. w''.i:i; "^ ! process of crystallisation or solidification was going on. ]]igl. (.iilieient illustrations of this phenomenon arc given in the beautifully coloured plates accom- panvinir Voti;elsanht be drawn re^ardin"' the silica contents of these rocks from their ascertained specific gravities. Although the same relation has been observed to exist amonLr the granitic and porphyritic rocks, and doubtless runs through all the orders, it has not been found that a certain specific gravity invariably corresponds to a certain degree of silieifieation or that, for instance, because a syenite containing 59.83 p.c. of silica has a specific gravity of 2,730, a trachyte having the same silica contents will have the same specific gravity. On the contrary we find decided differences as to specific gravity in rocks of similar composition, but belonging to diff'erent orders of texture. The following table shews the average specific gravity of the various families of granular, porphyritic and trachytic rocks : It will bo obsorvccl from this tiiblo that the spocitic j^nivity of priinular rocks is jrencrally greater than that of the trachytio rocks which correspoml with them in tli'grec of acidity; granites arc heavier than rliyolites, anil greenstones than doleritcs. (The rule docs not hold good when applied to the basic rocks, but this ni:iy be owing to the ficility with wjiich tiiey become d composed and absorb water, which causes a material diminution of gravity.) The porphyritic rocks seem to occupy a position between the other two scries, being neither so dense, relatively, as the granular nor so light as the trachytic rocks. This would seem to indicate that tlio coarsely granular rocks crystallised more slowly and perfectly than the porphyries and the latter more than tlic tra- chytes. This ditferencc in density between rocks having the same percentage of silica is even more observable between trachytic and vitreous rocks. Obsidian has invariably a much less specific gravity than a quartzosc trachyte which possesses the same percentage of silica. Thus we have the specific gravity of Khyolite from Palniarola witli 74 54 p. o. Si. Oo = 2 529 Obsidiiin frcm Lipari witli 74 05 " = 2.L170 Quartz tracliytu fruni Bcpolulal, Asia Minor, with 7G.5G " = 2.G5G ObsidianfromLittle Ararat with 77.27 " =2.394 The cause of the difference seems merely to be that while the rhyolites cooled slowly and shrank together to a denser mass, the obsidians are quickly cooled unannealed natural glasses. It is well known that garnet, vesuvianite, orthoelase, labradorite, augite, and olivine have their densities much decreased by being fused and quickly cooled, and the same thing has been remarked with regard to rocks. St. Claire Devillc, and Delessc experi- mented on several rocks, and found that their specific gravities were diminished after fusion. St. Claire Deville's results were as follows : f^pcc'llMi Sioi'ilio (iiavitifs Grnvitios lietbre i'lition. al'ti;rl'iision. Vitreous lava from the Peak of Tcncriffe 2.570 2 4G4 Traciiytc from Cliaiiorra 2.727 2.(1 1 7 Basaltic lava from tiiu I'lak of los Majorqiiinus 2.945 2.8M iJasalt from I'if do Foga, Cape of Good Hope.. 2 971 2.879 (Jraniti; from Andoux 2.GG2 2.3G0 Delessc found the loss to be lc.«s with fine-grained and semi- vitreous rocks than with those of a distinctly crystalline character. According to his results, if the rocks experimented on be arranged according to thcdegreeof diminution which their specific gravities undergo in fusion, beginning with those which experience greatest ^^J loss, those rocks will be found at the head of tlio list which arc cotnniotily considered to bo tiic oldest in ajje. Delessc found the folldwiniz; per ccnt.iiics of diminution, the specific irnivity of the various roeks before fusion being regarded as .- 100. (Iranitc, jrraniilitc and (jiuu'tz porpliyry 9—11 p. c. Syi'uitic graniti', and syciiite 8 — U " Torpliyry witli oiIIhu lust' uml oli;j;oela.se, witli ami withiiiil (|iiai't/, 8 — 10 DioriU' and dioi itc porphyry (J — 8 I^Iclaiiliyrc 5 7 IJasalt, tracliyte, and old volcanic rocks 3 — 5 Lavas and voh anic rocks o — 4 (I i: As early as 1S41, Gustav Bischof made observations on the comparative volumes of Basalt, Trachyte and Granite in their crystalline, molted, and vitreous conditions, with the following results : Vuliunc in vitreous condition. in crj'stnllinc. Basalt 1 0.9208 Trut liyte 1 0.9214 Granite I 8420 Volumo in a fluid state. Basalt 1 Tmcliyte 1 Granite 1 in crystalline. ... 0.89(50 . .. 0.8187 . .. 0.7431 Nothing can be more obvious from these data and experiments than that original roeks in cooling, solidifying and crystallising, underwent contraction, increasing thereby their density, and that the amount of contraction was the greater the more thoroughly and coarsely crystalline the rock, and the earlier the dates of its eruption in the geological history of the earth. It is not custo- mary in treating of eruptive rocks usually to entertain any very definite ideas as to their age, but it ought not to be forgotten that the geologic d experience of Europe has shewn that they nride their appearance on the earth's surface somewhat in the same order as they occupy in Table III. It would therefore seem that tIio.se rocks which liave experienced most perfect crys- tallisation and the greatest amount of contraction or increase of densi'y during that process arc the oldest in geological age, that those which have crystallised imperfectly and experienced but a moderate amount of contraction, belong to the middle age of geologic d history, and that those which have solidified quickly to a semi-vitreous condition, and have experienced in so doing scarcely any contraction, are exactly those which are the most recent, and have been denominated volcanic rocks. Such results ought not to surprise us, but ought rather to be anticipated if x^/ the llionry of the orl^'inal Ivrnoous fluidify of the ulnbo bo well fDiiinlrd. Thi- ciioiiinius dcirroo of heat, which (nily cniihl have occiisioiiod such a condition, etmhi not liave disappcarod su(hh'Mly. A;.'radud (h'croaso of temper it uro must have t.ikcn place from tlu^ time wiion the solidilic ition of the earth he;j,in down to recent oln the jirwcNsnl' tlic solidifuMtidii (if miuinal rocks aiitl probtildy nftcr their CMMistitiU'iit iiiiiionils liiid .is.mhihmI h1i!I[k; iiinl position, a certain fiii'tlicr coiisnlidjitioii scciiis to have taki'ii pl.ic'c' acconipanii'il hy ;i contract ion or (liniinMtioM in the volume of the rock, durint;' which the space oriuin.dly occupied ty its mass remaine(l the same, Th(! amount of contraction ill the most of cases was very small liut nevertheless sutlicient to dimiui.-h the cohesion of the rock aloiin' certiiin planes or curved suil'aces. No actual fissure oi' open space would seem to have heen developed at the time this contr.iction took pl.ice, but merely weak jilaces, plani'S ol' least resistance, which hy the action of atniosphi'ric forces were substMjuently revealed as joints of .scj)ar,itioii (AbsondcruiiLis kliiften. ) The word .structure would seem to he ii eonvenieiit term wherchy to designate the ineipiiiit joiutiui: caused hy the contraction of oiiL:inal rocks. Tin' joints thus 1'orme(| occur ;it varyiiij: intervals ,ind run in various direc- tions and hy such vari.-itions cause the scpai'atioii of the .ilircted rock into blocks of very different shajies and sizes. Accordini;' to the form of the resulting' fragments the followini; different .sorts of sej)ai-ation or structure have been distinguished. Ciililnil stniftiiri is caused by the occuri'ence ot iiirce set> of joints which ea(di occur at s(Uni'what rei^ular distances and inter- sect each other almost at riuht anjilcs, so that the rock becomes separated into lartic masses of a more or less cubical shape. (Jranitc shews this structure in the most marked decree, and wlien, after dcsintciiration. the blocks have their corners and edji'cs weathered off", they somewhat resemble cotton bales built \\\) into hune piles. This phenomenon is ob.servable around the Brocken in the Hart/., at Loui.senburi;' in the Fichtelgebirue, near Jjiskeard in Cornwall and on the north shore oi' Lake Supe- rior between Michipicoten and 3Iichii)icoten Island. A modifi- cation of cubical structure is occasioned by one of the three scries of joints (tho.se lying horizontally) occurring at shorter intervals and causing the formation of blocks having their length and breadtli almost equal but a lesser tliickness. Granites and syenites sometimes shew this form of .separation. Polijh'drlc structure results when the sets of joints intersect the rock in several irregular directions, and divide it into angular blocks of the most irregular shapes. This form of structure is D ■i •id coimnoii ;iinoi)t;- t^raiiitcs, porpliyrios ami uroenstonc. It is also lVt'([uent aiiiniiti- t;niees Intin each other and the roek heeonies split up iiitii thin hlneks or flans. Phdnolite yields the best examples of tlii.s iorni III' >trueture. At .Mont D'oi' in the Auver^ne thin ttaii'S of i)luimiliti' are used lor riMilinu'. Porphyries, trachytes and basalts often shew the same sort of si'peralion. Cnrriil y/rmiurr is caused by the surfaces ol' the flag's heinti; bent and ajipeariiiu hollowed out in a saneer-like form. Of this there are Ninie ;:ooil examples anmni: the melaphyri's ol' the south shore of Miehipieoteii I>land. (ihiliul'tr iir Sjilii niiiliil ,toiies oi' Nassau and the Fich- telui'birm'. basalts : tlu' trachyte of Chiajo di Luna on the Island of Ponza ; the trachyte of Alten;iu in the JMi'el. Kveii porphyry and uranite occasionally exhibit thi> structure. Portions of the diorites ami corsites whicdi occur in dykes between Piucon Ixiver and ]*oint Por))liyry. liade Superior, are i'reipiently broken up into heaps of little roULih balls fnun half an inch to an inch in diameter. ('olidnnur .-•'nicfurc is, of all the varieties, the one which has attracted most attention. It cau.ses the seperation of the rock into pillars having from three to nine but generally six sides. The length of these jjillars is variable but remains tolerably equable at each particular locality. Those of Ailsa Craig in the Firth of Clyde are 400 feet, and those of the Palisades in the Hudson 200 feet high. Just as variable is the thickness. The elegant trachyte pillars of Baula in Iceland sometimes sink to • Zirkel — Petrographie I. 99. ,. 47 hey tlu' tliit'kiiess of ii finger, tlios^e oi' tlic Meiidobcri; on tlie Uliim- arc ot'tiMi only 4 inelu'S thick, while soiiic ;iiv kimwii to jior-x'ss ;i diameter of i'roni 1(1 to 15 feet. 'J'he position of tliesii jiillars varies from beiiii;' pesfeetly vertical to jieri'ectly Jiorizontal ; some- times tlu'y lie parallel with each other, sometimes they diveriic in a radiating' form. At one jtlaee tluy are sti'aii;lit, at others l)ent and sometimes even S-shaped. Tlu' lonui'r pillars are rre((uentlv intersected at varions intervals throuiihout their Knulh hy cross joints ^^cncrally runnini;' at riu'ht angles to the direction of the column. The ends of the mendters thus formed have mostly even surfaces, but they are often concave and converse the latter shape tittini;' into the former. A moditicotion (d' columnar struc- ture is occasionally met with in which c\ lindi'ical columns, con- sistinu' of concentric y.iuu's. an' found soiiMtimes alone in>ide (d' atiii'ular pillars. These are found in the ,;iidesite of the llm/.el- berij,' in Si('beni:ebir!.i(' ami in the tr;.chue .-it Kelheri: in the Eil'el. In whatever positions the columns staml. or are ^roupcil. they ai'i' invariably placi'd at rit:ht auLiles to the .-url'ace at which the coollnu- bei;an. If the- roek his solidilied in the i'onn of a liori/.ontal layer lU' coverini:-. the pillais stand vertical ; if it has tilled init vertical fissures the cidmnns lie hori/,ont;d, like cord wood. ( iraiutrs. (Ireeiistoiu's. por])hyi'ies and traehytes all oc- cassionally t'xhibit columnar structuix'. but l)a-alt and ;iiiame>ite shew it ;ilmo>t inv.ariably. and in the most maiiinticent examples ;is for inst:ince at (!i,int"s Causeway. St.atVa. the Varoi' Islands, and in Iceland. 'Idu' d-'ufee oi' distinctness with whiidi tlie>e ditfrrent struc- tures ari' developed >eeni^ in general to depend upon tin' decree of rapidity with which they cooled. The ((uicker tlu' coolini:- the m(U'i' decided the jointinL;'. and this would derive conlirmation from the fact that those eruptive rocks which .are vi'ry much broken up l>y joiutiiiu- .are less distinctly cryst.dii/ed. It wouUl appear however that the more remdar of these forms of structure have not in all cases been occasioned by contraction, but that the influences which Lioverncd tlu' miiieralogical grouj)- iug were also active in producinu' it. The followini;circum.staiice3 secii to indicate this, (ji lobular structure is frequently accom- panied by a globular arrangement of the rock constituents, and by their forming concentric rings around a middle point. In some rocks^ crystals of their constituents are luuch larger, more frequent and more perfectly crystallized near the joints than in fc 48 the centre of the blocks. Tlie peculiar fracture of certain rocks seems also to stand in connection with their structure. Many thoroughly <;ranular rocks are more easily split in one direction than another and this direction is frequently parallel with the horizontal joints occurring in the rock. In the north of England this property is called the grain by the (juarry-men there, and filo mastro (tilime maestro) by the workmen of Baveno. X. -THE ALTERATION OF OKKilNAL MOCKS. The greatt'r })art of our globe is surrounded by a li((uid en- velope, rlio ocean, and this, as well as all the dry land, is com- pletely enclo.'^ed by a gaseous envelope, the atmosplicre. Both of these envelopes by means of their own properties, or those of cer- tain ingredients which they contain, exert, and have, in byi'-past geological ages exerted, a powerful influence upon the original rocks to which our attention has been directed in the foregoing- pages. These rocks after consolidation occupied very different positions both as regards their geological relations and their situation with respect to the sea level. -According to their posi- tion they became expo.sed to the disturbing influences of the atmosphere aiul its contents, or to the attacks of tlie ocean and the substances it held in solution. These disturbing influences ^Yere in both cases of a twofold nature: they were both clu'mical and mechanical, both decomposing and disintegrating, but, in general, chemical decomposition preceded and aided meclianical disintegration ; the superficial jiarts. at least, of rocks became weatliered and altered before the delu"intanci's resulting from their alteration, forming with them a series of hydrated minerals which enter extensively into the constitution of the weatliered ami altered rock-masses of the earths erusi. Next to water in importance comes carbonic acid which, although but slightly soluble in water at ordinary pressures, is sufficiently so to obtain access to the interior of roek minerals, and begin the attack upon them by combining with their bases. After it has combined with a small (juantity of lime or alkali it forms a solution of a carbonate, wliich, being capable of absorbing a further quantity of carbonic acid, giving rise to a bicarboiuite, becomes a much more powerful agent in decomposing silicates than the carbonic * Nauniann ; (^eounosie I. 442. 50 aci d 111 OIUV n <>!ird s oxvgc'ii its Motion ill (It'conijiosiiiu' sili cat(.'s is iiKtstly coutiiiod to briiiuing to a lii^lior state oi' oxidation the jirotnxides of tlie heavy iiietals which tliey contain and esjie- cially tlie })rotoxide of iron The chemical components ui' dviuiiial mcks are principally t^ilicati's of alumina, protoxide of iron, lime, luauiicsia, potasli and soda. The manner in whieh these ari' chemically acted on by the atmospheric ii-eiicies in the juncess of Aveatherinu' is as follows. The carlionie acid coiitaiiu'd in the humidity which almost continually moisti'iis rocks, lirst decomjioses a small quantity oi' the silicate of ju'otoxide oi' iron forming,- a carbonate of the latter base, which however is almost instantaneously con- verted into peroxide of iron l»v oxyii'en. the carlionie acid beinu' set i'ree to attack fresh portions of silicates. The silicates of lime and m:imie>ia are next decomposed by it. carbonates of lime and maLiue-ia iH-iiip' ]iroduee(l. dissolved in the water as bi-carboiiates and either uradually removed, or. owinu' to the evaporation of carbonic acid, redepo.-ited. The s;ime tliinu' takes place with the silicates of ]iotash and soda. l)ut part of these alkalis dissolvi' away part oi' the silica of the .-ilicate of alumina anil remove it in the ioi'm oi' silicati' of ]iot .sh or >oil;i. ()f all the bases alumina alone remains undissolved liy the carbonic acid. and. it. combineil with aceitain auiount oi' silica a, id water and soiiietiiHo wit.i piirt oi' the other bases, is tiu' principal pro- duct of the weatlieriiiL: process, and in the i'orni of ;in impure clav forms tiie weatiiered cru>t of the rock. Hut original reeks uith weathered surl'aces are' not by any means .altered rocks. The latter result i'rom a much more thoioULili ciianue in the i'omier. The proci'ss of alteration diifers fnun that ol' wcatluiaii'^ in beinu' much more deeji .seated and in liaviiiL;' taken pi u-e out oi' contact with.iir or atmospheric oxyticii. The latter seems to have been retained in the superficial portions of the rock and only the carbonic icid and water appear to have penetrated to urea terdcpths. 'J' hoc have had the s:i me decomposing eliect upon silicates as in the upper regions, but the resulting bi-carbonates together with the alkaline silicates and perhap.s aluminates were retained i'or .sometime in these greater depths, none of them decomposed by atmo.splieric oxygen and many of them causing further alteration among the rock components, or forming and depositing new products in the interior of tlie rock. Here the steadiness ot their action must have been uninflucuced 51 by changes of temperature and uninterrupted by the evaporation of the carbonic acid, the action of whicli seems to have been en- tirely ditferent under tlie new circumstances. Probably the alkaline silicates were the first to be decomposed, then the silicate of lime seems to have bei'u attacked, afterwards the silicate of protoxide of iron and lastly the silicate of magnesia. The order of the solution and removal of tlie alumina and silica was doubt- less dependant upon the quantities of the alkalies present. Where the texture of the rock or the occurrence of veins and cavities in it afforded the necessary space, tlie dissolved products re-arranged and deposited themselves as new minerals generally as carbonates or liydrous silicates, while, in the spaces occupied by the original minerals are found their lixiviated residues consisting of similar minerals and frequently possessing the forms of their originals. Only those rocks in wliich this more thorough alteration lias taken place to very L;reat deptlis are to be understood as included in our class (,)f altered rocks. In some cases tiie results oi' such altering processes are so thorough and I'xtend to such considerable depths beneath the surface that the atmospheric agencies above mentioned would appear ti) be altogether incapable of producing them. It must not however be forgotten that, in earlier geological ages, the atmo.sphere must have been differently constituted from that of (.lur day. and must have coutaini'd not only an immensely lar^'^er amount of carbonic acid but also large (juantities of sulphurous, •sulpliuric and hydrochloric acids, the action of which upon the original rocks then existing must have been incomparably more energetic than that of the agencies now at work. These rocks must at that time have been deluged with showers of acid raiu or submerged beneath a strongly acid and saline sea. Besides being subject to weathering and alteration, some original rocks simply absorb water one or more of their consti- tuents becoming converted into zeolites or other hydrated mine- rals. By this process of liydration phonolytes and hydrous basalts become formed from fine grained, and perlyte and pitch- stone from impalpable rocks. Although these processes of weathering alteration and hydra- tion have not unfrequently affected original rocks it is neverthe- less to be remembered that the latter are not invariably or even generally subject to them. Those which allow themselves to be acted on by such decomposing processes seem to do so more or less 52 rcliK'tiiiitly and tlio ilcj^reo of the dcconipositioii effectecl differs with tlieir varyiiiir coinpositioii. Oriniiial rocks, as lias been already shewn, are for the most part made up of silicates and, havinii' been formed under circumstances which admitted of a very ])erfect union of their various elements into crystalline minerals, their decomposition is no easy matter. Even when exposed in the state of fine powder to the action of concentaated acids very few of tliem are com|)letely deconijiosed. The same cheniicMl rule applies to rocks as to artificial and natural silicates, namely, tlx^ more acid they are, that is to say, the hiulur the percentage! of silica which they contain, the more efiectually do they resist tiie acti(Ui of acids. In a similar manner we find, gent'rally speaking, tliat the more siliceous a rock is. or its minerals are, the less liable it is ty of action with concentrated mineral acids but their want of power is compensated for by their cjuantity and tlii' unlimited time at their dispos il. Their deconijiosinji' effect is however as in the case of tiie mineral acids the greater, the laruer the (|uantity of bases contained in the mek upon wliicli they act. It is on thi.s account that we IVe(|iiently tind silicic granite to have remained luiatttcked while basic diorites. !j,abbros and diabases have ex- perienci'd more or less alteration ; liranulite and uneiss, jiorjiiiy- rytes and silicic porphyry, trachyte and rhyolyte remain compar- atively unchanged while the correspondinu' basic rocks, diabase schist, auuitic porphyry and basalt are fre((uently found more or li'.ss deeomjiosed. 'J'he capacity of an oriiiinal rock for alteration depends also upon its texture and the state of auiireuation of its minerals. Many of the coarsely uraniilar granites and syenites beloniiin<;' to a primitive izeolouical a<;e have to this day remained imaltered by atmosjdieric infiuences because of theirdense, hiii'hly crystalline state. It is only some of the more recent granites and porphyries that are found in such a state of decomposition as to be the source of much of the kaolin of commerce. Tho.se of the uiines of Scandinavia and Saxony which occur in ])rimitive rocks never require elaborated timbering the latter being very little given to weathering, while those on the Comstock Lode in Nevada expend enormous sums for timbering on account of the breaking up or "slaking," which the nmch more modern enclosing rocks (propylite and doleritic trachyte) undergo after the vein is opeued up. A fine grained or almost vitreous texture seems to 5:} be no security !i,-:;ainst the iiccrss to the iiitiTior of m icck of do- couipoisiiiu' solutions, for ni;iny rocks witli tliis tcxtun' sudi ;is basalt, piionolite, and pitclistonc have experienced hydratinn and partial decomposition throughout their mass. A slaty texture seoms especially adapted for assisting' atmospheric infltuMices to penetrate rocks and induce their decomposition. Indeed thr greatest nundjer of altered rocks will, in our .system. l)e Inuiid amonii' those of a schistose or slaty texture and comprise the most of the semi-crystalline schists and slates wliicli have been by many geologists termed metamorphic rocks. It has already been suffi- ciently wi'll exj)lained why a schisto.se or even slaty structure is not to be considered as indicatim:' a sedimentary origin, and gneis.s and granulite have therefore Ijeen cl.isst'd unhesitatingly with other eiystalline original rocks. The ureat m;iss ol' schists and slates which occupy a ueoloLiical position betwixt such uiieis>oid rocks and those of an evidently detrital n.iture woidd appear to have tlu! same original origin as the former but to have ac(|uired their semi-crystalline habitus iVom the acti(jn ol' processes oi' alteration subse(|uent to their sojidilication. As is well-known the formation of these rocks, mie i schist, argilhict'ous mica schist, talc and chlorite schists, has heretofore been explained in an op- posite manner. It is im])Ossible to de.-^cribe here the various agencies which are usually supposed to have caused the alteration, or the manner in which they have o])erated, so diver.se are the many metamorphic theories. They all however agree in assum- ing that the .H-histose rocks in (juestion were originally sedimen- tary ; stratified clays and sandstones. In this paper, on the other hancL they have been regarded as products of igneous fusion. Tliere are many reasons why .schistose and slaty rocks solidified from igneous fusion should have been more accessible than other original rocks to altering influences. Their fine slaty urain is peculiarly fitted for the introduction of altering solutions into the minutest pores and around the minutest particles. Then they have almost invariably a highly inclined position and consequently pre- sent their edges to the atmosphere in .such a maimer as to cause the formation of little reservoirs of water all over their outcrops, from which the grain of the rock leads the moisture directly into the interior. It is also highly probable that these slaty strata occu- pied this position at the time when the acid atmosphere and ocean existed on the earth and were therefore much more exposed to alteration than those original rocks which only made their appear- ance at later periods. B 54 XI. — TEXTURE OK ALTERED ROCKS. As ii goueral rule altorod rucks retain tho texture of their ori<;iiials. This is very phiin in tiie case of tlie granuhir rocks whicli have suffered ciiatij^e. However much their minerals may have been affected tlieir texture remains intact. Scliilleryte, cliloritic syenite and protoj-ine remain . ) absorbt'd by it tbe result being Kaoline (AloO;; 2 Si ()o + 2 II ().) On account of its invariably con- taining lime, oligoclase weathers more easily than ortboclase, and tbe more lime it contains the more rapidly it decomposes. Water saturated witii carbonic acid dissolvesout the lime as bi-carbonate and also the alkalies in combination witb silica, so that tbere remains, as in the case ol' ortboclase, kuoline, generally, how- ever, more imjnu'c and containing earthly cirbonates. .Vccord- ing to Senft (Felsgemengtbeile \). i)\^'2,} it is only tbe layers first formed of the weathering crust which are frt'c from carbonate of lime. After decomposition lias })enetrated to the mass of tbe crystal tbe crust very fre(|Uently consists of a meebanical mix- ture of kaoline witb silica aiul carbonate of lime, and sometimes even carbonate of magnesia. Senft accounts for this plienomenon by supposing tbat tbe later f(»rmed kaoline crust retains tlie water containing the carbonate of lime and silica Ironi tbe under- lying decomiiosing oligoclase so long as to cause the loss of the carbonic acid Avhich held these substances in solution, where upon, they become deposited and mechanically combined with the kaoline. Labradorite bein<>; more basic and richer in lime * tSenft ; Folsgcmcn^fthcilo p. 577. 67 than oliji'oclaso is cvoii more prone to (k-ednipositioii. Its woath- (■rin<; begins a.s usual with a liydratioii of tho mass nl'tlM' liabni- ilori to, then progresses vith carbonatinj;' Jiml IcaebiiiL; out first the lime and a jtart of the siliea, afterwards of tlir iii;i;j,iiesia and soda, aixl ends with the I'ormation of a white ci.iycv siibstanee, or, if iron oxide is present, of oeiirc yt'llovv (d.iy. Tiic poreelain earth of J'a>san is the jtroduet of the alteration ol' a mass of so- ealled passauite or porei'huii spar which, according; tn Senft (Feloii'emenutlu'ilf p. (iOO). rosfiidilis, in cnmpdsitidn, l.ibradorite in which the ]iroecss of altcratiijo li.is already bcL:uii. Amorpiious siliea. in the form n[' opal and chalcedony, are alxi |irn(liicts of its alteration. With regard to anorthite its history as a rock, eonstitneiit is very modern and very tew ob.servat ions have as yet been maih- renaidiiiL:' the manner of its weatherim:. Senl't men- tions an annrthitic rnek from Thurinn'ia the small crystals of wliieh become did!, white and ell'ervesee with acid> in weatlier- inu. The lariic crystals of the anorthile pdipiiyry nf 'I'hnnder Cape, Jjake Superinr. becume (ipa(pie and bleached on weatlii'r- iag, but the thin weathered surfaces do not etlervesce with acids_ In L;eneral therefore it may be stated that the products of tiie alteration ol' the feldspars are kaolinlte sometimes contaminated with silica, iron oxide or carbonate of lime ; soluble silicates of the alkalies and carbonate of lime in urcatt'r part reincved: and hydrated siliea. The I'ormation of the l;itt<'r ,-eems esjn>cially jtroiK! to occur with I'eldsjiars deficient in the alkalies wliicii are necessary for its solution and removal. Amonu; the basic essential rock minerals, ]iota>ii mica resists decomposition most stidjbornly. TuiUhmI were it not il)r its foli- ated structure it mi^ht lie regarded as indesti'uctible. It.s nornnd formula is three atcnns mono silicate of alumina and one tcr-silieate of potash { ?> Alo 0;i Si Uo + K () ;! Si O^. ) but it very seldom occurs so pure and simi)le in composition as this. Small (|uantitiesof iron oxide, mannesia, soda and lime fre(|uently occur in it and, like the feldspars, the nu)re it contains of these substances the more prove it is to decomposition. If its position is such that water can readily penetrate between its leaves, a loosening of its mass and an opening of its leaves may be observed ■which, when frost cooperates, gradually results in their complete disintegration and the formatiou of a loose mass of small scales. And this is all the alteration that atmospheric agents can effect when the mica contains very little or uo protoxide of iron or 58 lii!ih hiown in colour, until an ocliro yellow iihii is developed which deci'euses the transparency but not the lustre of the plates which it covers and lemls theni (il'len a lu'onzed or L:ildid appearance. II' this skin is di.sxilved away by muriatic acid, the pure tran-parent mica reappears. In the ordinary wcathcrinu however this skin becomes thicker and thicki-r and at la.««t the mica loses its transparency, lustre and coherence, ll' water eontaininu' ciidtonic acid now has access to it. it <:radually dissolves tuit t'roni it its alkalies ami alkaline I'artlis \intil then' remains an impure clay coloured yellow by iron oxide and lillcd with iiinunn r.dile x'alcs id" undeeomposed mica ol'ten iincroseopically small. .Ma;^ncsia mica differs from jiota^h ndca in coiitaininr li'ss silica, tluMjuantity of which s Idoui exci'e'ds 40 per cenl. and in containinu' less potash, usually about 5 and seldom I'l'acliin;;' 10 percent. It contains further i'rom 15 to 30 percent inaLiuesia. and sometimes as miudi as liT) per ci'Ut of protdxiile and peroxide of iron. 1 Is percent lue ol'.dumina vai'ies from 111 to liO and besides tlioe principal con.-titiients there are present O',') to 4 jier cent tluoriiie, O',") to ',', per cent water. O-,") to 5 per cent soda and sometinu's about 'J, per cent lime. I n a sinular manm-r to potash mica, liut with less difficulty, it under- lidcs alt 'ration ti a ri'ddish brown earthy mass consi>tin,i:', to a lar,i:e extent, of undeeomposed scales of ndca, with traces of car- bomites of lime and m iirnesia in a ferruginous clay (Senl't. Fels;/emenL:tlieile p. 71-1. j It llius a)i|iiiirs that the weatiieiini;- of ndca does not result in the Inrnnlion of a new hydrated mineral ri'taiiiiuu' the place of tlie oriiiiiial but eouti'ibutes ehieHy to its disintt'Liration and piepares it for subse(pient removal by lucehanic d means. Sonu'times when oxygen is e.xcluded mica apjiears to be subject to .dtcration to chlorite and talc but the proofs of this are not veiy distinct. Hornblende has an extremely variable chemical composition, which can scarcely be reduced to any general formula applicable to all its varieties. Its components are principally silica, alumina, protoxide of iron, magnesia and lime. The light coloured varieties such as tremolite, actynolite and anthophyllite contain little or no alumina ; but they seldom occur as rock constituents. 59 The Mack, <,'mMii.«
  • " MiiKnchiu '.'1.12 to 2.(10 " •'ime l'j.(;,-it(> 8.00 « f^'"!" 'J'JMo 0.00 " I'otash 'J.IH to 0.00 i< Tli(! weatJieriM^' of horiililcnilc icscMible.s in rationali' that of the feld.'^pars. The; silicate of protdxiih! of imn is, in an early stage, (lecompo.xed by carbonic acid, the resulting carbonate being at once decomposed by atmospheric oxygen, and the ]»eroxides of iron depositeil ii,^ ochre yellow hydrateon the surface of the horn- blende. Sometimes a shining violet coating of ferroso ferric oxide is lirst produced, which however gradually changi's to jieroxide. This process then progresses aeeompaiiied by a de(;omposition of the silicates of lime and magnesia also, the lime and then the magnesia being removed as carbonates. Tlu^ alkalies in condtin- ation with silica and carbonic acid also disappear having at last a leatlier yellow ferruginous clay, (.^uite a ditferi'iit product results however when atmospheric oxygen is excluded and water, containing carbonic acid, alont; has access to the hornblende, which is the case when it is contained in deeper lying portions of the rock. In this case tiie whole ol' the lime is removed as car- bonate, together with the alkalies and part of the silica and chlorite is jiroduced. 8onu' of the magnesia -and }irotoxidc of iron may likewise be ri'inoved by the carbonic acid, cidorite still being the result. In the decomposition of very ferruginous horn- blende, containing little magnesia, the product is oft<'n ferruginous .ihu'ite or delessite. When howevi-r the greater part ol' the ba»es disappear, a sort of Fullers earth (smectite) is produced, while, at the same time, (|uart/,. iron spar, dolomite, brown spar, and calcspar become deposited in the minute cracks of the rock. (Senft Felsgemengtheile p. G81.) This decomposition of horn- blende with exclusion of oxygen is, as before explained, altera- tion, wh the common decomposition in contact with air is underst* by the term weathering. Py K' much resembles hornblende in chemical composition. The CO ->onent8 are the same and their quantities equally as much si ject to alteration. Alumina is frequently present but (Ill 111 sniMllcr (|u:iiitity, tlio silicntcs of liiiic, mnpiosiii nnd ju'dtoxule orii'im |ir(']M)ii(l('r;itt\ tlu' ;ilk:ilies arc entirely :il)seiit. as are also fluorine ami titanic acid wliieli are neeasionally I'ountl in the Innaililendes. Alnniinous and ndii-alnniinous ityroxenes are usu- ally distinuuislu'd. as in the east' of hornblende, and those whieh enter into the eoniiiosition of I'oeks are mostly aluminous, jiossess- inti' (lark iireen. hrown ami hlaek colours. Coninion auuite AVi'athers somewhat more (|uickl_\ than hornblende but, at the same time, more slowly than the t'eldsiiathic minerals usually as- sociated with it. When weath(!rim: once begins. [>t'euliarly shin- ing;' vinlet eoloniH'd jioints and s]iots may be observed on its sur- face, which uradnally become yellowish Lireen and ochre yellow, a |iidol' that the prolo.xide of iion l)ecoines sepei'ated out as iiy- drated [leroxide. As the weatheriiiu prouresses it is remarked that the yellow spots ejl'ervesci' with a<-i(ls. and even nnder the peroxide a white coating may be detected which t'll'ervesces with acids. The spets have now an earthy appearance. lia\i' an ai'Liil- laceons smell and allow theniselvt's ♦(> be wa.du'd away by rain, exposing small depressions nn the surl'aces of the crystal. These spots, dissohcd in muriatic acid, yield a sdlution (•ontainin^- peroxide of iidn. silic.!. alumina and carbonate of lime with traces of mauni'sia. In this niainiei' the aniiitc i^rad ua 11 y loses its iron, limi' and mauni'sia ami is converted into a ferru<;im)U8 clay eontainint; t-arbonate of lime and often silicate of mamiesia. (Senl't. Felsm'inenLitheile p. l!;");!.) iitimmelsberi:' and \'oii Ilauer have analy>ed weathered crystals of auiiite. from Hilin in liohemia, and their results abundantly shew the correctness of the above explanation of theii' manner ol' decomposition, An-iuiiiii-: til It.'iinniolslier^'. Ai'i-ordiiii;- t(i \'(iii Ilniior, Silica (lo.(;:{ ."i4.24 Alnniina 'Jli.e.s lia.ii^ l'ci-(i\i(lr of irmi . . . t.'Jl .").•_'•_' I-iuic l.'JT 0.87 Ma^nrsia ().;i| (i-.'.d Water ;> ]L> I4.;j7 '.t',) 22 10(1.28 The yellowish brown weathered crust found on augite crystals I'rom Ceruosin had the foUowiiii"' composition. Silica ;!,-). Ahnniiia i .^. „ Peroxide of iron j '"' Jjime (!.") Magnesia 4 1 Water 18,0 101.8 S ii C( C.l WluMi il('('(Mii|tositi()ii lakes place iindtT cin-iiiii^tanccs wliicli oxcliule the ]ir('S(Mi('e ol'air, aiiuitc scciiis to iitiiicrLro an altcratioii similar ill iiatiin' td that ('Xiicricin'cd hv luiriiltlciiilc ; walcr is iibsorhtMl and niic or ninn' ol' the iiKMidxidcs are reinoved wliile liydrous aimite. ]iyrall(ilite, eldorite lU' delosile result. Seiit't liu'iitioiis its alteration ti> Lireeii earth in the aiiLiitie |>i'r|ih_vry ol" Fassatlial. He aeemints inr the |ireseiiee of tiie alkalies, wliieli siH'iii to ha\(' replaeeil the niaunesia. as lia\ ini;- been derived IVoiii the deeonipositioii ol' the oli^^delase which oeeiirs in the inatiixol' the ro(d< : Kjei'nU' has ol)served similar alterations in the anuite porphyiy of llolnu'strand. Norway. 'I'liis rock Lii'adiiates into an aniyiidaloid. ealcs))ar. surrounded hv Lirt'eii eartli taking the place of the spaces previously nccnjiied l)v auuite cry.^tals. 'IMu' calcspar aniyudnles I'nMpiently indicate hy their I'orin that (d'tiie oriuinal au^ite. All ihestaucsol' tliis alti'ration may he oli^-erved in the rocks of llolme>trand IVom tiie uiidecoinpo>cd aiiLzite to the hall' decomposed with eales])ar in the interior, .md ultimately to those wiiich have heen i ntirelv altered into calespar and '^ri'eii earlh.-'- The writer has h.id o]iportnnitv of making' similar oh- sorvations amoiiu the amyudaloids of Lake Superioi', the results of the alteration heinu. however, p'nerallv calespar and didessite. According to IJiiim a iveathered doleritic amyudaloid of liiitzcl- ])vv'j: contain> lULiites cliaiiui'd in a similar inamier into steatite (Senft l''el>L:-ement:lheile p. (ilill.) Diallaut'. a silicate of ]iroloxi r^ iZ F-! « c « to > cS 1—! 'r. •Sb J 4S .r » «S « .d H .1) _ ^ iJ o a* ■§. CO O a 3 fl X 3 '5 j= « -~r ■yj •J2 -3 o a; ~ ; o ■ ^ ^ '-' C3 O O w ■s &4 og o 13 9!^ a .£„ oj CM «) c4 4) o « O 13 «2 a a a tn O m ]a o -a a eS o aj rt a ;s ^ 3 o a u, o •- -s-^ 13 .a 3J3 a a & o P, 3 O c S 3 -3 c "3 T35 a _2 2 V a '3 ID M ^ 64 The foreiroing tablo bciiiu' construetotl on the same principle as No. Ill re'(|uiri's but little cxplanatinn. It has been our object to includ(! iiu'l arraniit^ in it ;ill tlie altered rocks and even some of somewhat doubtrul charactei. As in i'ormer tables no new names have been invented, and only in the e-isi' of ariiillaceous niiea slate and phyllite have synonyms lieen so ai)plied as to in- dicate two difl'i'rent rocks. Tlu' former ol these terms is apjilied to the siliceous and the latter ti> the silieii- rdek^ of the series of slates to which they belonu'. The ]irincip;il accessorial constitnt'nts of oriiiinal I'oeks appear to be but vi'ry slightly subject either to weatlii'rini;- or alteration. 'J'opaz, tounuiliue. crysobcryll. brryll. eorimdum, chromite, ilmenite, /ircoii. diamond. ^iduHnite. per(d'skite. uahnite. spinello tinstone, rutile and many others are. elienncally, almost iudes- tructiblr nunerals. Many of them ]io-sess a hardness e(pial to and sometimes exceedinu' ([Uartz, resist tdn'mieal decomposition as well as it, and are fmuid accompanyinu' it amom:' the ruins of rocks in the sands of rivers and sea shores. .Most ol this indif- ference is attributable to their extremely dt'use and crvstalliiu! (WY-nature. and [icrhays also to tlu'ir peculiar ehemieal composition, ' for, althouiih many of them are l)asie. their components ai\' such as art! but littU' influenced by atnu>spheric aucucii'S. Many of these acct'ssorial miiu'vals whicli seldom exhil);t decomposed sur- faces, such as garnet, epidote and tourmaline, an- nevertheless said to uive rise to the formation ol' other miiu'rals by tluir altera- tion. This is owini:' to the oecurn'nce of the latter in forms belonLiiuLi' to the former minerals, but in some eases it may be doubted whethei' the prooi' of alteration is sufficient. The uratlua! change of the ::arnef or tourmaline into the supposed new mineral is not always traceable, and tie; latter beiuLi' freijuently very crystalline and anhydrous, bears very little resemblance to an altered mineral. These remarks •d\)\)\y to the alleged cham;e of bcryll to mica, tourmaline to chlorite, leucite to saiudine, garnet to specular iron ore, e|»idote to [jotash ;ica ami otbei's recorded by Seuft. On the other hand allanitc, kacite, liaiiynite, sodalite and others, have been found on analysis to contain water and to have undergone hydration and other changes. Cordierite and scapolite are the principal occasional minerals of original rocks which have been very thoroughly altered. The number of new minerals to which tliey are said to have given ri.«e is very remark- able. Senft mentions praseolite, csmarkite, aspasiolite, bonsdor- ffite, fahlunite, wei.ssite, gigantolite, pinito and potash mica as pro- 66 ducts of tlio Mltenitioii of cnrdierito, tlie first four by its absorbing- water ami losing silica, tlie next four by absorbiiig water and losing magnesia, and tlic Uist by again losing its water and aj)pro- priating potasb. ( Felsgcniengtheil(> p. o')! ). Tlieso alterations witli the exception ol' tin; one said to yield mica are well autlien- ticated. It is however worthy nf remark that SiMil't failed to discover cordierite in any granite containing pinite. [t is un- necessary here to specify the numerous minerals regarded as altered scapolite as it is very seldom tliat tluy are found in altered rocks. None of the auxiliary miner-ilsof original rocks yield so readily as tlie sulphurets to the decomposing influences of the atmospliere, in the process of weathering, but they seem to have entirely escaped the more subterraneous process of alteration. Iron pyrites, markasite, magnetic pyrites, copper pyrites and galena which are frequently found iu tine particles impregnating rocks readily decompose in contact with the atmosphere and weather in a very noticeable maniu'r. At greater dejiths in similar I'ocks they appear entirely unchanged alth(uigh in close contact witli thoroughly altered minerals. This is owing to the exclusion of atniosplieric oxygen in the process ol' alteration. That flementis very potent in the weathering process and readily oxidises these sulphuret^. but the water containing carbonic acid wliich alone picnetrates to greater depths is altogether without action on them. XIII. -coNri.rsiox. We have endeavoured in the foregoinii' pages to set forth some of tl le mor e interestimi relations which exist amoiiLi' tlie vari ous families of original and altered rocks, and have also attempte(l to make use of these for their better classification. Whether or not the system advanced be regarded as satisfacts be admitted, at any rate, that Petrology as at present developed is not the very confiLsed department of (leology which some have represente(l. This coidd be rendere(l even more evident by de- scribnu qn ,^iy eacn roe k -pecu and showing more minutely its various relationshii)s. This would ln)wever carry us much be- yond our jiroposed limits, aiul go far towards making our {tapciv a regular treatise on petrology. It is lioped, however, that the preceding pages will be found to contribute a little towards a better uiulerstanding of the subject, in wliicli case our object in writing them will have been accomplished. AcTo.NVALE, VM, May, 187: