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'J ': i:i:iyr «>•: VI , ', '•i ,:• ■V ,' f ■r •t.v I r'->;i/.' ■^' \ ■ mk Mtm' i - i kicMie^ ft eiy— -.Wa-»* • "A^jj^ifct-fc.i^inin lairfhiw oir SOME POINTS IN CHEMICAL GEOLOGY,* BY T. STERRY HUNT, M.A., P.R.S. I. In a paper read before the American Association at Montreal in August 1857, as also in some previous communications to the Royal Society, and in the Report of the Geological Survey of I Canada for 1856, I have endeavoured to explain the theory of the transformation of sedimentary deposits into crystalline rocks. In considering this process we must commence by distinguishing between the local metamorphism which sometimes appears in the vicinity of traps and granites, and that normal metamorphism which extends over wide areas, and is apparently unconnected with the presence of intrusive rocks. In the former case, however, we find that the metamorphosing influence of intrusive rocks is by no means constant, showing that their heat is not the sole agent in alteration, while in the latt«r case dificrcnt strata are often found affected in very diflFerent degrees ; so that fossiliferous beds but little altered are sometimes found beneath crystalline schists, or even interca- lated with them. We cannot admit that the alteration of the sedimentary rocks has been efiected by a great elevation of temperature, approaching, as many have imagined, to that of igneous fusion ; for we find un- oxidized carbon, in the form of graphite, both in crystalline lime- 1 'NoTB. — This paper,written and sent to the Geological Society of London in August 1868, was not read before that body until the 5th of January 1859. An abstract of it appears in the proceedings of the Society in the Philosophical Magazine for February, and it is published at length in the Quarterly Journal of the Geological Society for November 1859, pp. i8-496, from which it is now reprinted, with the addition, by the author, > a few notes, which are distinguished from those before published by I being enclosed in brackets. "I.: >■ ,!' :M,r' .,. (■H^ stone and in beds of mngnctic iron-ore ; and it ift well known that thcHC substances, and even the vapour of water, oxidize graphite at a red heat, with formation of carbonic acid or carbonic oxide. I have however sliown that HolutionH of alkaline carbonates in presence of eilica and earthy carbonates slowly f^ive rise to silicates, with disen- gajj;einent of carbonic acid, even at a temperature of 212° Fahr.,— the alkali being concerted into a silicate, which is then decomposed by the earthy carbonate, regenerating the alkaline salt, which serves as an int' 'idium between the silica and the earthy base. I have thus ti. iOiivoured to explain tlic production of the various 8illcat«s of lime, magnesia and oxide of iron so abundant in crystalline rocks, and with the intervention of tho argillaceous clement, the formation of chlorite, garnet and cpidote.f I called attention to tho constant presence of small portions of alkalies in insoluble com- bination in these silicates, both natural and artificial — a fact which had already led Kuhlmann to conclude that alkaline silicates have played an important part in the formation of many minerals ; and I suggested^ that, by combining with alkalies, clays might yield feld- spars and micas, which are constantly associated in nature with the I silicates above mentioned. This suggestion has since been verified by Daubruc,§ who has succeeded in producing feldspar by heating together for some weeks to 400° C. mixtures of kaolin and alkaline I silicates in the presence of water. The problem of the generation from tho sands, clays and earthy I carbonates of sedimentary deposits, of the various silicious minerals I which make up the crystalline rocks, may now be regarded as solved, I and we find the agent of the process in waters holding in solution alkaline carbonates and silicates, acting upon the heated strata. [ These alkaline salts are constantly produced by the slow decomposi- tion of feldspathic sediments, and are met with alike in the waters of I the unaltered Silurian schistp of Canada, and of the secondatyt strata of the basins of London and Paris. In the purer limestoneel however, the feldspathic or alkaliferous elements are wanting; a iV ■.: .(■• *i,.:- • fPrcceedlngs of the Royal Society, May T, 1857. JReport Geol. Surv. Canada, 1856, p. 479. §Bull. Soc. Q6o\. de France (2) vol. xv. p. 103. '..•>■. ft well known that ;idizc ji;ruplato at a nic oxide. I have iitca in presence of licatcH, with iliscn- oof212°Fuhr.,- 8 then decomposed le salt, which Hcrvcs irthybuse. I have the various silicates dant in crystalline iccous clement, the [ called attention to 1C8 in insoluble com- ificial — a fact which ikalinc silicates have any minerals ; and I ays might yield feld- jd in nature with the IB since been verified 1 f feldspar by heating )f kaolin and alkaline [ids, clays and earthy ous silicious minerals be regarded as solved, rs holding in solution | )n the heated strata, ly the slow dccomposi-j alike in the waters of I md of the secondary I 1 the purer limestonaj nts are wanting; 7, 1867. I, p. 103. ». theso strata often contain soluble suits of Hmo or magnosia. Thcso would noutrulizo the alkaline salts, wliich infiltrating iVoni adjacent strata, might otherwise effect tho transformation of the li)reign mat- ters present in tho liniestonos into crystalline silicates. liy a similar process theso calcareous or magncHian silts, penetrating tho ad- joining strata, would retard or prevent tho alteration of the latter. These considerations will servo to explain tho anomalies presented by tho onmparativoly unaltered condition of some portions of tho strata in mctamorphio regions. || II. As tho history of the crystalline rocks becomes better known, wo find that many which were formerly regarded as exclusively of plutonio origin arc also represented among altered sedimentary strata. Crystalline aggregates of quartz and feldspar with mica offer tran,sitions from mica-schist, through gneiss, to stratified granites, while tho pyroxenio and hornblendic rocks of tho altered Silurian strata of Canada pa.ss, by admixtures of anorthic feldspars, into I stratified diorites and greenstones. In like manner tho interstratificd serpentines of theso regions aro undoubtedly indigenous rocks, re- sulting from tho alteration of silico-magncsian sediments, although I tho attitude of tho serpentines in many countries has caused them I to be ranked with granites and traps, as intrusive rocks. Even the crystalline limestones of tho Laurentian scries, holding graphite I and pyroxene, arc occasionally found enveloping broken beds of II DcSenarmont' in his researches on the artilicial formation of the min- erals of metalliferous veins by the moist way, has shown that by aid of heated solutions of alkaline bicarbonatcs and sulphurcts, under pressure at temperatures of 200" or 300" C, we may obtain in a crystalline form many native metals, sulphurets, and sulpharscniatcs, besides quartz, fluor-spar and sulphate of bary tes. Daubrdo'' has since shown that a solution of a basic alkaline silicate deposits a large portion of its silica in the form of crystalline quartz [when heated to 400" 0. We have here, beyond a doubt, a key to tho [true theory of metalliferous veins. The heated alkaline solutions, which are at the same time the agents of metamorphism, dissolve fVom tk sediments the metallic elements which these contain disseminated, and subsequently deposit them with quartz and the various spars in the 1 fissures of the rock. *Ann. de Ohim. et de Phys. (3), vol. xxxii, p. 129, 'Bull. See. G^ol. de France (2), vol. xv, p. 99. ■(■( •v m !• ^ it ,1 „ ♦ i-.'ii ;•:.«■■' I? ir •' ■i'i' fer: i:' v" quartzitn, or injected amon^ tho flHHurcA in adjacent Hilicious strata. Frniu Hiniilur iiictH, obHcrvorH in other ro^ionrt huve been led to OHMlKn 11 pliitonic origin to certain cryHtulline linieHtoncs. We aro thuH broii^lit to tho concluHJon thut nietiunorphio rockH, Hueli an grunito, diorite, doleritc, Her|)cntinc, iind linieHtone, may under eertuin conditionH, upiMjiir an intruHivo rocicM. The pusty or Hcnii-fiuid Htuto wliieli tboMO rock.s must huve uHHunied iit the time of tlieir displaciv Dient iH illuHtrutcd by tlie obHorviitionH of Duubr(5e u|Mjn tho Hwelling up of f,'la.sM and obsidian, and the development of cryHtaln in their uuiHS under tho action of heated water, indicating n considcnibio degree of m(»bility among tho particlen. Tho theory of igneo- aqueous fusion applied to granites by Poulett Serope and Scheerer, and supported by Elio do Beaumont and by tho late microscopic observations of Sorby, should evidently be extended to other intru- sive rocks ; for wo regard the latter us being in all cases altered and displaced sediments. III. Tho silico-aluminous rocks of plutonio and volcanic origin aro naturally divided into two great groups. Tho one is represented by tho granites, trachytes and obsidians, and is distinguished by containing an excess of silica, a predominance of potash, and only small portions of soda, lime, magnesia and oxide of iron. In tho other group silica is loss abundant, and silicates of lime, magnesia and iron predominate, together with anorthic feldspars, containing soda and but little potash. To account for tho existence of these two types of plutonio rocks, Prof J. Phillips supposes the fluid mass beneath the earth's crust to have spontaneously separated into a lighter, silicious, and less fusible layer, overlying a stratum of denser basic silicates. In this way ho explains the origin of the i supposed granitic substratum, of the existence of which however, the study of tho oldest rocks aflFords no evidence. From these two layers, occasionally modified by admixtures, and by partial separa- tion by crystallization and eliquation. Prof. Phillips suggests that wo may derive the different igneous rocks. Bunsen and Durochcr have adopted, with some modifications, this view ; and the former has even endeavoured to calculate the composition of the normal trachytic and pyroxenic magmas (as ho designates tho two supposed zones of fluid matter underlying the earth's crust), and then seeks, from tho proportion of siliea in any inUsrmediatfl spooios of rook, to doJuco tho (luantitioa of alkalioH, limo, magnesia and iron which ihii) should contain. iMo lon^ as tho traohytio rooks are composed essentially of ortho- close and quarts, and tho pyroxonio rooks of pyroxene and labradorito, uT a foldHpar approaching it in composition, it is evident that the calculaiiouH of Bunsen will to a certain extent hold good ; but in the anulv.HOH, by Dr. Strong, of tho vokanio ii»jks of Hungary and Anncnitt, we often find tliat the actual proportions of alkalies, lime, and mognoHia vary considerably from those deduced from calculation. This will necosflorily follow when foldspors like albito or anorthite replace tho labradorito in pyroxonic rocks. Tho phonolites are moreover highly basic rooks, which contain but very small amounts of limo, magnesia, or iron, being esHontially mixtures of orthoclaso with hydrous silicates of alumina and alkalies. IV. In a recent enquiry into tho probable chemical conditions of a cooling globe like our earth, I have endeavoured to show that in the primitive crust all tho alkalies, lime and magnesia must have existed in combination with silica and alumina, forming a mixture which perhaps resembled dolerite, while tho very dense atmosphere would contain in the form of acid gases, all tho carbon, chlorine and sul- phur, with an excess of oxygon, nitrogen and wat«ry vapour. The first action of a hot acid rain, falling upon tho yet uncoolcd crust, would give rise to chlorlds and sulphates, with separation of silica ; and tho accumulation of tho atmospheric waters would form a sea charged with salts of soda, lime, and magnesia. The subsequent decomposition of the exposed portions of the crust, under the influ- ence of water and carbonic acid, would transform the felspathie portions into a silicate of alumina (clay) on the one hand, and alkaline bicarbonates on the other ; these, decomposing the lime-salts of the sea, would givo rise to alkaline chlorids and bicarbonate of lime — the latter to be separated by precipitation, or by organic agency, as limestone. In this way we may form an idea of the generation from a primitive homogeneous mass, of the siliceous, calcareous and argillaceous elements which make up the earth's crust, while the source of the vast amount of carbonate of lime in nature is olsc explained.* * Am. Jour. Sci. (2) xxr. 102, and Ganadian Journal for May 1868. I'M :..'■'■' 'i 4 ' !• . -I ^ 1 ,■1 „ ^frWi> ■-' 1.', ■'<. !■! I:!:. ■H ■■ ''*■■■ .k: I .v: :«'" ft' : • T Wben we examine tlie waters charged with saline matters which impregnate the great mass of calcareous strata constituting in Canada the base of the Silurian system, we find that only rbont one-half of the chlorine is combined with sodium ; the remainder exists as chlor- ides of calcium and magnesium, the former predominating, — while sulphates are present only in small amount. If now we compare this composition, which may be regarded as representing that of the palaeozoic sea, with that of the modern ocean, we find that the ohlorid of calcium has been in great part replaced by common salt, — a process involving the intervention of carbonate of soda, and the formation of carbonnt/^ of lime. The amount of magnesia in the sea, although diminished by the formation of dolomites and magnesite, is now many times greater than that of the lime ; for so \oag as chlorid of calcium remains in the water, the magnesian salts are not pre- cipitated by bicarbonate of soda.* When we consider that the vast amount of argillaceous sediment- ary matter in the earth's strata has doubtlessly been formed by the sama process which is now going on, viz. the decomposition of feld- spathic minerals, it is evident that we can scarcely exaggerate the importance of the part which the alkaline carbonates, formed in this process, must have played in the chemistry of the seas. We have only to recall waters like Lake Van, the natron lakes of Egypt, Hungary and many other regions, the great amounts of carbonate of soda furnished by springs like those of Carlsbad and Vichy, or contained in the waters of the Loire, the Ottawa, and probably many other rivers that flow from regions of crystalline rocks, to be remind- ed that the same process of decomposition of alkaliferous silicates is still going on. V. A striking and important fact in the history of the sea, and of all alkaline and saline waters, is the small proportion of potash-salts which they contain. Soda is pre-eminently the soluble alkali ; while the potash in the earth's crust is locked up in the form of insoluble orthoclase, the soda-feldspars readily undergo decomposition. Hence we find in the analyses of clays and argillites, that of the alkalies which these rocks still retain, the potash almost always predominates * See Report Geol. Surv. Canada, 1867, pp. 212-214, and Am. Joar. Science (2) zzriii. pp. 170 and SOS. greatly over the soda. At the same time these sediments contain silica in excess, and but small portions of lime and magnesia. These conditions are readily explained when we consider the nature of the soluble matters found in the mineral waters which issue from these argillaceous rocks. I have elsewhere shown that, setting aside the waters charged with soluble lime and magnesia salts, issuing from limestones, and from gypsiferous and saliferous formations, the springs from argillaceous strata are marked by the predominance of bicarbonate of soda, often with portions of silicate and borat«, he- sides bicarbonates of lime and magnesia, and occasionally of iron. The atmospheric waters filtering through such strata remove soda, lime and magnesia, leaving behind the silica, alumina and potash — the elements of granitic and trachytic rocks. T'le more [ sandy clays and argillites being most permeable, the actios of the infiltrating waters will be more or less complete ; while finer and more compact clays and marls, resist'ng the penetration of this liquid, will retain their soda, lime and magnesia, and by subsequent altera- tion, will give rise to basic feldspars containing lime and soda, and if lime and magnesia predominate, to hornblende or pyroxene. The presence or absence of iron in sediments demands especial consideration, since its elimination requires the interposition of or- ganic matters, which by reducing the peroxide to the condition of protoxide, render it soluble in water, either as a bicarbonate or combined with some organic acid. This action of waters holding organic matter upon sediments containing iron-oxide has been de- scribed by Bischof and many other writers, particularly by Dr. J. W. Dawsonf in a paper on the colouring matters of some sedimentary rocks, and is applicable to all cases where iron has been removed from certain strata and accumulated in others. This is seen in the fire-clays and iron-stones of the coal-measures, and in the white clays associated with great beds of green-sand (essentially a silicate of iron,) in the cretaceous series of New Jersey. Similar alterna- tions of white feldspathic beds with others of iron ore occur in the altered Silurian rocks of Canada, and on a still more remarkable Male in those of the Laurentian series. We may probably look -214, and Am. Jo^. t Quart. Journ. Geol. See, vol. v, p. 25. i^'r .■I. J:' • • .'■4 ■ m^ It,- v.. ,f^ .tit' )'■'■■■ ■i.l :} I'M ■ . •5,'H ■ ' ^>.V• ,..• I'll]!:*- i^B: '• W'^' ■■<':•.< '» ■ i/'^-'- •j. .H:ii' J]-'::i 1.;! ; . 10 upon the formation of beds of iron-ore as in all cases due to the intervention of organic matters, so that its presence, not less than that of graphite, affords evidence of the existence of organic life at the time of the deposition of these old crystalline rocks. The agency of sulphuric and muriatic acids, from volcanic and other sources, is not however to be excluded in the solution of oxide of iron and other metallic oxides. The oxidation of pyrites, moreover, gives rise to solutions of iron and alumina salts, the subsequent decomposition of which by alkaline or earthy carbonates will yield oxide of iron and alumina ; the absence of the latter element serves to characterize the iron-ores of organic origin.J In this way the deposits of emery, which is a mixture of crystallized alumina with oxide of iron, have doubtless been formed. Waters deficient in organic matters may remove soda, lime and magnesia from sediments, and leave the granitic elements inter- mingled with oxide of iron; while on the other hand, by the admixture of organic materials, the whole of the iron may be re- moved from strata which will still retain the lime and soda necessary for the formation of basic feldspars. The fact that bicarbonate of magnesia is much more soluble than bicarbonate of lime, is also to be taken into account in considering these reactions. The study of the chemistry of mineral waters, in connexion with that of sedimentary rocks, shows us that the result of processes continually going on in nature is to divide the silico-argillaceous rocks into two great classes, — the one characterized by an excess of silica, by the predominance of potash, and by the small amounts of lime, magnesia and soda, and represented by the granites and trachytes, while in the other class silica and potash are less abun- dant, and soda, lime and magnesia prevail, giving rise to pyroxenes and triclinic feldspars. The metamorphism and displacement of sediments may thus enable us to explain the origin of the different vari :.Ies of plutonic rocks without calling to our aid the ejections of the central fire. [I Hjdrated alumina in the form of gibbsite is however met with incrusting limOnite, and the existence of compounds like pigotite, in which alumina is united with an organic substance allied to crenic acid, seems to show that this base maj, under certain conditions, be tal^ea into solution bj organic acids.] often 1 we have of sedi ruins greate our pala soda of in the The lir bonate, basic, silica, a less ex mica, cL kyanitCj are rare The 11 sc, not less than VI. The most ancient sediments, like those of modern times, were doubtlessly composed of sands, clays, and limestones, although from the principles already defined in IV. and V., it is evident that the chemical composition of these sediments in different geologic periods must have been gradually changing. It is from a too hasty generalization that an eminent geologist has concluded that lime- stones were rare in earlier times, for in Canada the Laurentian system — an immense series of stratified crystalline rocks which underlie unconformably both the Silurian and the old Cambrian or Huronian systems — contains a limestone formation (interstratified with dolomites), the thickness of which Sir W. E. Logan has esti- mated at not less than 1000 feet. Associated with this, besides great volumes of quartzite and gneiss, there is a formation of vast but unknown thickness, the predominant element of which is a triclinic feldspar, varying in composition between anorthite and andesine, and containing lime and much soda, with but a small pro- portion of potash. These feldspars are often mixed with hypersthene or pyroxene ; but great masses of the rock are sometimes nearly pure feldspar. These feldspathic rocks, as well as the limestones, are associated with beds of hematitic and magnetic iron-ores, the latter often mixed with graphite. Ancient as are these Laurentian rocks we have no reason to suppose that they mark the commencement of sedimentary deposits ; they were doubtlessly derived from the ruins of other rocks in which the proportion of soda was still greater ; and the detritus of these Laurentian felspars, making up our palaeozoic strata, is now the source of alkaline waters by which the soda of the silicates, rendered soluble, is carried down to the sea in the form of carbonate to be transformed into chlorid of sodium. The lime of the feldspars being at the same time removed as car- bonate, these sedimentary strata in the course of ages become less basic, poorer in soda and lime, and comparatively richer in alumina, silica, and potash. Hence in more recent crystalline rocks we find a less extensive development of soda-feldspars, while orthoclase and mica, chlorite and epidote, and silicates of alumina, like chiastolite, kyanite, and staurotide, which contain but little or no alkali, and are rare in the older rocks, become abundant. The decomposition of the rocks is more slow now than formerly. ij-v •1<1 I r. i ■i ■■.■''■'■ I '<'.*•■■■ !»•; .;, '. f yi%. ■■■, ■ ' , ,1' i ► >• f \ >;; ■.: > t Ibll. Tol i, p. 669. [tThe notion that volcanic phenomena hard their statin the seidinrattt- hrj formations of tlie eartli's cruBt, and are dependant upon the cottbuB- Ition of organic matters, is as Humboldt ttmskAa, one which belongs'to V'^, »',. P ■'■}'.':?■, r^ '•'.:■■• 14 VIII. The volca&io phenomena of the present day appear, so far as I am aware, to be confined to regions covered by the more recent secondary and tertiary deposits, which we may suppose the central heat to be still penetrating (as shown by Mr. Babbage), a process which has long since ceased in the palaeozoic regions. Both normal metamorphism and volcanic action are generally connected with elevations and foldings of the earth's crust, all of which phe- enomena we conceive to have a common cause, and to depend upon the accumulation of sediments and the subsidence consequent there- on, as maintained by Mr. James Hall in his theory of mountains. The mechanical deposits of great thickness are made up of coarse and heavy sediments, and by their alteration yield hard and resist- ing rocks ; so that subsequent elevation and denudation will expose these contorted and altered strata in the form of mountain-chains. Thus the Appalachians of North America mark the direction and extent of the great accumulation of sediments by the oceanic cur- the infancy of geognosy {Cotmos, vol. v, p. 443. Otte's translation). la 1834 Christian Keferstein published his Naturgesckichte des £rdk6rpert, in which he maintains that all crystalline non-stratified roclcs, from gran- ite to lava, are products of the transformation of sedimentary strata, in part very recent, and that there is no well-defined line to be drawn between neptunian and volcanic rocks, since they pass into each other. Yolcanic phenomena according to him have their origin, not in an igneous fluid centre, nor an ozydizing metallic nucleus, but in known sedimen- tary formations, where they are the result of a peculiar process of fermentation, which crystallizes and arranges in new forms the elements of the sedimentary strata, with evolution of heat as an accompaniment of the chemical process. {Naturgesckichte, vol. 1 p. 109, also Bull. Soc.Giol. de France (1) vol. vii. p. 191.) These remarkable conclusions were unknown to me at the time of writing this paper, and seem indeed to have been entirely overlooked by geological writers ; they are, as will be seen, in many respects an anticipation of the views of Herschel and my own ; although in reject- ing the influence of an incandescent nucleus as a source of heat, he has, as I conceive, excluded the exciting cause of that chemical change, which he has not inaptly described as a process of fermentation, and which is the source of all volcanic and plutonic phenomena. See in this connection my paper On the Theory oflgneout Rocki and Volcanoei, in the Canadian Journal for May, 1858.J ■r.., 16 day appear, 8o i by the more lay suppose the [r. Babbage), a regions. Both itally connected 11 of which phe- to depend upon onsequent there- y of mountains, ide up of coarse hard and resist- ation will expose mountain-chains, the direction and J the oceanic cur- e's translation). In hte des ErdMrpeu, pd rocks, from gran- jedimentary strata, line to be drawn IBS into each other, in, not in an igneous in known sedimen- peculiar process of forms the elements n accompaniment of t,\3oBull.Soc.Giol. me at the time of lirely overlooked by many respects an although in reject- urceofheat, hehas, , chemical change, »f fermentation, and nomena. See in this ;fcf and Vokanoet, in rents during the whole palnozoic period ; and the upper portions of these having been remoyed by subsequent denudation, we find the inferior members of the series transformed into crystalline stratified rocks.§ 1 1 [§ The theory that volcanic mountains have been formed by a sudden local elevation or tumefaction of previously horizontal deposits of lava and other volcanic rocks, in opposition to the view of the older geolo- gists who supposed them to have been built up by the accumulation of successive eruptions, although supported by Humboldt, Von Buch, and Elie de Beaumont, has been from the first opposed by Cordier, Constant Prevost, Scrope and Lyell. (See Scrope, Geul. Journal, vol. xii, p. 326, and vol. xv. p. 600 ; also Lyell, Philot. Trans, part 2, vol. cxlviii, p. 703, for 1858.) In these will we think be found a thorough refutation of the elevation hypothesis and a vindication of the ancient theory. This notion of paroxysmal upheaval once admitted for volcanoes was next applied to mountains which, like the Alps and Pyrenees, are com- posed of neptunian strata. Against this view, however, we find De Montlosier in 1632 maintaining that such mountains are to be regarded as the remnants of former continents which have been cut away by de- nudation, and that the inversions and disturbances often met with in the structure of mountains are to be regarded only as local accidents. {Bui' Soc. Geol., (1) vol. ii, p. 438, vol. iii, p. 216.) Similar views were developed by Prof. James Hall in his address be- fore the American Association for the Advancement of Science, at Mon- treal in August 1857. This address has not been published, but Ihey are reproduced in the first volume of his Report on the Geology of Iowa, p. 41. He there insists upon the conditions which in the ancient seas gave rise to great accumulations of sediment along certain lines, and asserts that to this gpreat thickness of strata, whether horizontal or in- clined, we are to ascribe the mountainous features of North Eastern America as compared with the Mississippi valley. Mountain heights are due to original depositions and subsequent continental elevation, and not to local upOeaval or foldings, which on the contrary, give rise to lines of weakness, and favor erosion, so that the lower rocks become exposed in anticlinal valleys, while the intermediate mountains are found to be capped with newer strata. In like manner J. P. Lesley asserts that " mountains are but fragments of the upper layers of the earth's crust," lying in synclinals and preserved from the general denudation and translation, (/ron Manufacturer't Guide, 1869, p. 63.]