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This item is filmed at the reduction ratio checked below/ Ce document est film6 au taux de reduction indiqu« ci-dessous. ^°'^ ^X^" ON THE f: 7/1^ CHEMICAL AND MINERALOGICAL IIELA.TION8 or METAMORPHIC ROCKS, BY T. sr^ HRY Hrar, m.a., f.r.s. Reprinted from the Dublin Quarterly Journal of Science/or July, 186S. I Montreal : PRINTED BY JOHN LOVBLL, ST. NICHOLAS STREET. 1863. ON THE CHEMISTRY OF METAMORPHIC ROCKS, BY T. STERRT HUNT, M.A., F.R.S. 3 At a time not very remote in the history of geology, when all crystalline stratified rocks were included under the' common designation of primitive, and were supposed to belong to a period anterior to the fossiliferoi's formations, the lithologist confined his studies to descriptions of the various species of rocks, without reference to their stratigraphical or geological distribution. But with the progress of geological science, a new problem is present- ed to his investigation. While palaeontology has shown that the fossils of each formation furnish a guide to its age and stratigraph- ical position, it has been found that sedimentary strata of all ages up to the tertiary inclusive, may undergo such changes as to obliterate the direct evidences of organic life; and to give to the sediments the mineralogical characters once assigned to primitive rocks. The question here arises, whether in the absence of organic remains, or of stratigraphical evidence, there exists any means of determining, even approximately, the geological age of a given series of crystalline stratified rocks;— in other words, whether the chemical conditions which have presided over the formation of sedimentary rocks, have so far varied in the course of ages as to impress upon these rocks marked chemical and mineralogical ditlerences. In the case of unaltered sediments it would be difficult to arrive at any solution of this question withou t greatly multiplied analvses; but in the same rocks, when altered, the crystalline minerals which are formed, being definite in their composition and varying with the chemical constitution of the sedi lents, may perhaps to a certain extent, become to the geologist what organic remains are in the unaltered rocks, a guide to the geological acre and succession. '^ It was while engaged in the investigation of metamorphic rocks of various ages in North America, that this problem suggested itself; and I have endeavoured from chemical considerations, con- joined with multiplied observations, to attempt its solution. In the American Journal of Science for 1858, and in the Quarterly Journal of the Geological Society of London tor 1869 (p. 488) will be found the germs of the ideas on this subject, which I shall e ndeavour to explain in the present p aper. It cannot be doubted • Read before the Dublin Geological Society April 10, and reprintTd from advance sheets of ths Dublin Qaarterly Jouraal for July, 1863. 4 ON THE CHEMISTRY OF METAMORPHIC ROCKS. that in the earlier periods of the world's history, chemical forces of certain kinds were much more fictive than at tlie present day. Thus the decomposition of earthy and alkaline silicates under tho combincid influences of water and carbonic acid, would be greater when this acid was more abundant in the atmosphere, and when the temperature was probably higher. The larger amounts cf alkaline and earthy carbonates then carried to the sea from the decomposition of these silicates, would furnish a greater amount of calcareous matter to the sediments ; and the chemical eflfects of vegetation, both on the soil and on the other atmosphere, must have been greater during the Carboniferous period, for example, than at present. In the spontaneous decomposition of feldspars, which may be described as silicates of alumina combined with silicates of potash, soda and lime, these latter bases are removed, together with a portion of silica ; and there remains as the final result of the process, a hydrous silicate of alumina, which consti- tutes kaolin or clay. This change is favoured by mechanical division ; and Daubree has shown that by the prolonged attrition of fragments of granite under water, the softer and readily cleav- able feldspar is in great part reduced to an impalpable powder, while the uncleavable grains of quartz are only rounded, and form a readily subsiding sand ; the water at the same tmie dissolving from the feldspar a certain portion of silica, and of alkali. It has been repeatedly observed, where • potash and soda-feldspars are associated, that the latter is much the more readily decomposed, becoming friable, and finally being reduced to clay, while the or- thoclase is unaltered. The result of combined chemical and mechanical agencies acting upon rocks which contain quartz, with orthoclase, and a soda-feldspar such as albite or oligoclase, would thus be a sand, made up chiefly of quartz and potash-feld- spar, and a finely divided and suspended clay, consisting for tlie most part of kaolin, and of partially decomposed soda-feldspar, mingled with some of the smaller particles of orthoclase and of quartz. With this sediment will also bo included the oxide of iron, and Uie earthy carbonates set free by the sub-aerial decomposition of silicates like pyroxene and the anorthic feldspars, or formed by the action of the carbonate of soda derived from the latter upon the lime salts and magnesia salts of sea-water. The debris of horn- blende and pyroxene will also be found in this finer sediment. This process is evidently the one which must go on in the wearing away of rocks by aqueous agency, and explains the fact that 'xl^ ON THE CHEMISTRY OP METAMORPHIC ROCKS. 6 ■while quartz, or an excess of combined silica, is for the most part wanting in rocks which contain a large proportion of alumina, it is generally abundant in those rocks in which potash-feldspar predominates. So long as this decomposition of alkali ferous silicates is sub-aerial, the silica and alkali are both removed in a soluble form. The process is often however submarine, or subterranean, taking place in buried sediments, which are mingled with carbonates of lime and magnesia. In such cases the silicate of 8(;da set free, re-acts either with these earthy carbonates, or with the corresponding chlorids of sea-water, and forms in either event a soluble soda-salt, and insoluble silicates of lime and magnesia, which take the place of the removed silicate of soda. The evidence of such a continued reaction between alkaliferous silicates and earthy carbonates is seen in the large amounts of carbonate of soda, with but little silica, which infiltrating waters constantly remove from argillaceous strata; thus giving rise to alkaline springs, and to natron lakes. In these waters it will be found that soda greatly predominates, sometimes almost to the exclusion of potash. This is due not only to the fact, that soda-feldspars are more readily decom- posed than orthoclase, but to the well-known power ot argillaceous sediments to abstract from water the potash salts which it already holds in solution. Thus when a solution of silicate, carbonate, sulphate, or chlorid of potassium is filtered through common earth, the potash is taken up, and replaced by lime, magnesia, or soda, by a double decomposition between the soluble potash salt and the insoluble silicates or carbonates of the latter bases. Soils in like manner remove from infiltrating waters, ammonia, and phos- phoric and silicic acids, the bases which were in combination with these being converted into carbonates. The drainage-water of soils, like thatof most mineral springs, contains only carbonates, chlorids, and sulphates of lime, magnesia, and soda; the ammonia, potash, phosphuric and silicic acids being retained by the soil. The elements which the earth retains or extracts from waters are precisely those which are removed from it by growing plants. These, by their decomposition under ordinary conditions, yield their mineral matters again to the soil ; but when decay takes place in water, these elements become dissolved, and hence the waters from peat bogs and marshes contain large amounts of potash and silica in solution, which are carried to the sea, there to be separated—the silica by protophy tes, and the potash by alore, 6 ON THE CUEMISTRY OP METAMORPHIC ROCKS. which ktter, decayin}? on the shore, or in tho ooze at the bottom, restore the alkali 'to the e^rth. The conaiticiis under which the vegetation of the eoal formation grew, and was preserved, being similar to those of peat, the soils became exhausted of potash, and are seen in the fire-clays of that period. Another eff>'('t of vegetation on swlimpnts is due to the reducing or de-oxidizitig aicncy of the organic matters from its decay. These, as is well known, reduce the peroxide of iron to a soluble protoxide, and remove it from the soil, to be afterwards deposited in the forms of iron ochre and iron ores, which by subsequent alteration become hard, crystalline and insoluble. Thus, through the agency of vegetation, is the iron oxide of the sediments with- drawn ♦'rora the terrestrial circulation ; and it is evident that the proportion of this element diffused in the more recent sediments must be much lens than in those of ancient times. The reducing power of organic matter is farther shown in the formation of metallic sulphu rets; the reduction of sulphates having precipitated in this insoluble form the heavy m-'tals, copper, lead, and zinc ; which, with iron, appear to have been in solution in the waters of early times, but are now by this means also abstracted from the circulation, and accunuihited in beds and fahlbands, or by a sub- sequent process have been redissolved and deposited in veins. All analogies lead us to the conclusion that tho piimeval condition of the metal?, and of sulphur, was, like that of carbon, one of oxidation, and that vegetable life has been the sole medium of their reduction. The source of the carbonates of lime and magnesia in sediment- ary strata is twofold :— first, the decomposition of silicates con- taining these bases, such as anorihi.> feldspars and pyroxene ; and second, the action of the alkaline carbonates formed by the decom- position of feldspars, upon the chlorids of calcium and magnesium, originally present iu sea-water; which have thus, in the course of ages, been in great part replaced l>y chlond of sodium. The clay, or aluminous silicate which has been deprived of its alkali, is thus a measure of the carbonic acid removed from the air, of the carbonates of lime and magnesia precipitated, and of the amount of chlorid of sodium added to the waters of the primeval ocean. The coarser sediments, in which quartz and orthoclase prevail, are readily permeable to infiltrating waters, which gradually remove from them the soda, lime, and magnesia, which they con- tain ; and if organic matters intervene, the oxide of iron ; leaving ON THB OHBMISTRY OF METAMORPHIO ROOKS. jottom, licli tho 1, being ^Hh, and aducing decay, solnble 3positcd isequent through ts with- that the idiraents ■educing ation of ;i»)itated nd zinc ; vaters of from the ly a sub- n veins. ;undition ), one of edium of ediraent- ates con- sne ; and e decoin- .gnesium, course of The clay, li, is thus ir, of the e amount ral ocean. e prevail, gradually they con- I ; leaving f at last little more than silica, alumina, and potash — the elemenU of granite, trachyte, gneiss, and mica-schist. On the other hand, the finer marls and clays, resisting the penetration of water, will retain all their soda, lime, magnesia, and oxide of iron ; and con- taining an excess of alumina, with a small amount of silica, will by their metamorphism, give rise to basic limo and soda-feldspars, and to pyroxene and hornblende — the elements of diorites and doleritcH. In this way, the operation of the chemical and mechan- ical causes which we have traced, naturally divides all the crystalline silico-aluminous rocks of the earth's crust into two types. These correspond to the two classes of igneous rocks, distin- guished first by Professor Phillips, and subsequently by Durocher, and by Buusen, as derived from two distinct niagmaj ; which these geologists imagine to exist beneath the solid crust, and which the latter denominates the trachytic and pyroxenic types. I have however elsewhere endeavoured to show that all intrusive or exotic rocks are probably nothing more than altered and displaced sediments, and have thus thtir source within the lower portions of the stratified crust, and not beneath it. It may be well in this place to make a few observations on the chemical conditions of rock-metamorphism. I accept in its widest 8onse the view of Hutton and Boue, that all the crystalline strntified rocks have been produced by the alteration of mechanical and chemical sediments. The conversion of these into definit*" mineral species lias been eSected in two ways : first by molecular changes ; that is to say, by crystallization, and a re-arrangement of particles ; and, secondly by chemical reactions between the elements of the sediments. Pseudomorphism, which is the change of one mineral species into another, by the introduction, or the elimination of some element or elements, presupposes metamorphism ; since only definite mineral species can be the subjects of this process. To confound metamorphism with pseudomorphism, as Bischofi*, and others after him, have done, is therefore an error. It may be farther remarked, that although certain pseudomorphic changes may take in some mineral species, in veins, and near to the surface, the alteration of great masses of silioated rocks by such a process is as yet an unproved hypothesis. The cases of local metamorphism in proximity to intrusive rocks go far to show, in opposition to the views of certain geologists, (hat heat has been one of the necessary conditions of the change. The source of this has been generally supposed to be from below ; 8 ON THE CHEMISTRY OP METAMORPHIO ROCKS. but to the hypothesis of alteration by ancomling heat, Naumann has obj(!cted that the inferior strata in some cases escape change, and that in doBcending, a certain plane limits the niotanior()hi»m, Bcparating the altered strata above, from the unaltered ones beneath; there beinrr no apparent transition between tiio two. This, taknn in connexion with the well-known fact that in many cases the ititrnsion of ignectns rocks cames no apparent change in the atljacent nnaltere(i sediments, shows that heat and moisture are not the only conditions of metamorphism. In 1 867, I showed bv experiments, that in addition to these conditions, certain chem- ical reagents might be necessary ; and that water impregnated with alkaline carbonates and silicates, would, at a temperature not above that of 212° F., produce I'lemical reactions among the elements of many sedimentary rocks, dissolving silica, and gene- rating various silicates (1). Some months subsequently, Daubr^e found that in the presence of solutions of alkaline solutions, at tem- peratures above 100^ F., various silicious minerals, such as quartz, feldspar, and pyroxene, could be made to assume a crystalline form; and that alkaline silicates in solution at this temperature weald combine with clay to form feldspar and mica (2). These observations were the complement of my own, and both together showel the agency of heated alkaline waters to be sufficient to effect the metamorphism of sedimetita by the two modes already mentioned, — namely, by molecular changes, and by chemical reac- tions. Following upon this, Daubree observed that the thermal alkaline spring of Plombieres, with a temperature of 10()« F., had in the course of centuries, given rise to the formation of zeolites, and other crystalline silicated minerals, among the bricks and cement of the old Roman baths. From this he was led to sup- pose that the metamorphism of great regions might have been effected by hot springs; which, rising along certain lines of dislo- cation, and thence spreading laterally, might produce alteration in strata near to the surface, while those beneath would in some cases escape change (3). This ingenious hypothesis may serve in 1. Proc. Royal See of London, May 7, 1857 ; and Philos. Mag. (4) XV., 68 ; also Amer. Jour. Science (2), xxii., and xxv., 435. 2. Comptes Rendus de I'Acad., Nov. 16, 1857 ; also Bull. Soc. Qeol. de France (2), xv., 103. 3. It should be remembered that normal or regional metamorphism is in no way dependent upon the proximity of unstratitied or igneous rocks, which are rarely present in metamorphic districts. The ophiolites, ON TOE CHEMISTRT OF METAMORPHIO ROCKa. 9 lomc cjisea to meet the difficulty pointed out by Naumnnn ; but while it is uiid