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Un des symboles suivants apparaitra sur la dernidre image de cheque microfiche, selon le cas: le symbols ^^ signifie "A SUIVRE", le symbols V signifie "FIN". Maps, plates, charts, etc., may be filmed at different reduction ratios. Those too large to be entirely included in one exposure are filmed beginning in the upper left hand corner, left to right and top to bottom, as many frames as required. The following diagrams illustrate the method: Les cartes, planches, tableaux, etc., peuvent dtre filmds d des taux de reduction diffdrents. Lorsque le document est trop grand pour §tre reproduit en un seul clich6, il est filmd d partir de Tangle sup^rieur gauche, de gauche d droite, et de haut en bas, en prenant le nombre d'images ndcessaire. Les diagrammes suivants illustrent la mithode. 1 2 3 1 2 3 4 5 6 ON THE CHEMISTRY OF THE EARTH. By T. Sterky Hunt, LL. D., F. R. S. In tbo following pages 1 have endeavored, at the request of Professor Henry, to give a brief snmmary of certain views in chemical geology which have been put forward by me in various scientific journals dur- ing the past twelve years, the germ of them having appeared in a com- munication to the Americmi Journal of Science in January, 18.58. In addition to the foot-notes, I have ai)pended a list of my principal pub- lications on the subject, where those who desire to follow up the various questions here suggested will find them treated more at length. The last three of these papers are in part reprinted in the present abstract. I take this occasion to say that the views here embodied will be devel- oped in a work on the chemistry of the globe, the preparation of which is now well advanced. T. STEEliY H.^T. Montreal, January 6, 1870. List of ihe principal impers, by Mr. T. Sterry Hunt, relating to Chemiml Geology. Ou tho Choinistry of the Primeval Earth. Amcricaa Journal of Science, January, 1858. On the part which the SilicatoH of the Alkalies may play in tho Metaraorphism of Rocks. Proc. Royal Society of London, May 7, 1857, and American Journal of Science, March, 18.'>8. Ou tho orisrhi of Feld.spnre, and on some points of Chemical Liihology. American Journal of Scieiico, May, 18r)h. On the Theory of Igneous Rocks and Volcanoes. Canadian Journal of Toronto, March, 1^58. Ou Home points in Chemical Geology. Quarterly Journal of tho Geological Society, Noviiuber, 1851). Re\ itiw of the last-named paper. American Journal of Science, July, 1800. On Gyi>sum8 and Maguosian Rocks. American Journal of Science, September, No- vember, 1859. Ou tho History of Lime and Magnesia Salts. American Journal of Science, July, 1861). (.'onlribntiouH to liithology. Amerieati .Tonrual of Science, March, May, .f uly, 1804. Choiuistry of Natural Waters. American Journal of Scieuce, March, July, Septem- ber, 1>^5. (Munnistry ami Mineralogy of Metamor]:>hio Rocks, Dublin Quarterly Journal, Jul.\, I8(i:i. C>i igiu of Noino MaguoKian and Aluminoua Bocks. American Journal of Scionco, September, 1801. La Ohimii^ de la Terre. Comptes-Rendus do rAcad(^mip, June 9, 1802. Tlie Eavlh'fi (Jliuuite in Paleozoic Times. American Journal of Scicnci', 1>*03. On some poiufs in American Gnologv. Anu>rieau Journal of 8nien(5e, M.a.\, ISfil. On tho Chemi( al (leologv of Mr. David Forbes, (ieological Magazine, Fobruarv, 1«08. On the Chemical Geology of Mr. David Forbea. Chemical News, February, 1868. ■ ThM Chomistrv of tho Primeval Earth. Lecture before the Royal Institution, May, Volcauoia aud Earthciuakes. Lecture boforo the American Geograplucal Society. April, 18(59. Ou the Probable Seat of Volcanic Action. Geological Magazine, Juno, 1869. I C35) CHEMISTRY OF THE EARTH. CONTENTS OF SECTIONS. 1-3, classification of tlio sciences ; 4, objects of cboinical geology ; 5, uebnlar hyi)o- thesis; fi, lUissociatiou ; 7, 8, tho sun; 9, the cooling earth; 10-l:i, its solitliticatiou ; 14, relations of solution to pressure ; 15, on the earth's crust ; 1(5, 17, probable composi- tion and relatrion of earth's crust and atmosphere; li^, fixation of carbonic acid, its efl'ect on climate ; 19, chemical influence of vegetation as a reducing agi rt ; origin of carbon and sidphurets ; 20, origin of limestones, dolomites, and gypsums ; .'1, silicated ^Yatc^8; 22, relations of potasli and soda ; 23-2f), disintegration of silicatta rocks ; 27, their division into two groups; 28, chemically-formed aluininons and non-alnminonu silicated rocks; 2r, changes in composition of sediments; U(t, 31, metamorpliism of rocks ; 32, cheniical alteration ; 33, molecular alteration ; 34, porosity of sediments, their aondensation ; 35, uUimato result of chemical motamorphism ; 3l), th(>ory of vol- canic action ; 37, views of i' yferstein and Herschel ; 38, the prinntiv:? crust ; 30, internal heat ; 40, 41, infiu(>nco of pressure ; 42, 43, types of igneous rocks ; 44, origin of granites ; 45, indigenous and exotic rocks ; endogenous rocks or vein-stones ; 4(5, filling of mineral veins ; 47, source of metals, and theory of metalliferous depos-ts ; 48, cirrhiiuakos ; 49, volcanoes ; 50, 51, their distribution ; .52, causiM of subsidence and accumulation of sedi- ments ; theory of mountains; 53, origin of lavas; 54, relations of sedimentary deposi- tion to volcanic phenomena ; ancient volcanoes ; 55, modern volcanoes. § 1. In ai^proacliiiig the study of the chemistry of the earth, or what may bo designated chemical geology, it becomes necessary to dedue the natural objects of that complex study to which is given the general name of geology, and also to consider its connection witli the various sciences. To this end, some notions as to the order and the relation of these sciences may not be out of place. Following the classitication established by Comte, we distinguish between the abstract sciences, which deal with laws, and the concrete sciences, which have to do with things. In their order the abstract sciences foiui an ascending series, aci;ording to the degree of complexity of their phenomena, " so that ciich science depends on the truths of all those which precede it, with the addition of peculiar truths of its own." — (J. S. Mill.) At the base of this series are thus i)laced — 1st, Matli^nnatlcs, with its successive divisions of number, geometry and mechanics; 2i\, Abstract Astt'onomt/, wliiah considers, in addition to the.se, gravitation, taking coguizjuice of luuuoer, extension, equilibrium, and motion ; 3d, Physios, coniprehending the laws of weight, cohesion, sound, light, electricity, and magnetism ; -Ith, Chemistri/, which treats of the relations to one another of the diiferent forms of mineral matter, and their transformations under the physical agencies of light, heat, and electricity ; 5th, Jiiology, or Physiology, to which belongs tho study of the laws of organized growth and developjuent; (Jth, Psychology, whicii considers the laws of mental phenomena j and, 7th, Sociology, or tho laws of human society. § 2. Parallel with these abstract sciences is a series of concrete or his- torical sciences, dealing not with laws and general princii)les, but with objects and fa(!ts. Of these concrete sciences tLj lirst is Descriptive Astronomy, which is the natural history of tho planetary and stellar worlds, treating of their movements, dimensions, andcosinical relations. Coming, in tho next place, to the history of our own planet, the study of tho accidents of its surface and its interior gives ri.se to Physical (Icography and to Strttotural aiul Dynamical Ocognf>sy ; while the boilies which it presents to us are naturally divided into two groat classes, the inorganic or mineral kingdom, and tho organic, including tho vegetable and ani- mal kingdom. The study of these two chisses gives rise to two great branches of natural history, Mineralogy and Organography, the latter including Botany and Zoology. The concrete scieiu;e of mineralogy has for its subject the natural history of all the forms of unorganized mat- CHEMISTRY OF THE EARTH. tor ; that is to say, those substances which are exempt from biological laws, but come within the domain of physics and chemistry. Chemical change implies disorganization, and all so-called chemical species are inorganic, that is to say, unorganized, and belong to tho mineral king- dom, whose natural history is thus physical and chemical, while that of the vegetable and animal kingdoms is biological.* § 3. It might, at first sight, seem foreign to our present subject to speak in this connection of the moral, social, and political history of mankind, dependent upon the laws of psychology and sociology. Jt is, however, to be remarked, that while in the abstract order each science is independent of that which follows it in the series, it is far different in tbe concrete sciences. This is seen in the familiar example of the de- pendence upon each other of the animal, vegetable, and mineral king- doms, and it is evident that man puts in movement agencies which are constantly at work modifying alike physical geography and the rela- tions of the miner.al, vegetable, and animal world to such an extent that human history must not be disregarded in the studj^ of the lower reigns of nature. § 4. From what has gone before it will be evident that under the com- mon term of geology are generally confounded two distinct bran(5hes of study; the first or abstract division being that of the physical, chemical, and biological laws which have j) resided over the develoi)ment of the globe, and the second or concrete division, the natural history of the earth as displayed in its j)hysical structure, stratigraphy, mineralogy, paleontology. The name of geoguosy, emi)loycd by some .authors, may very approi)riately be retained for the latter, while that of geologj- is restricted to the former division. It is proposed in the following jmges to consider brietly some of the more im[)ortant points in the chemical history of the globe ; in doing which it will b(^ necessary to notice also its astronomical and physical history, and the relations of organic life, in so far as they ari^ concerned in the chemical history of the earth in its various stages of develoj)ment. The scheme thus embraced is so grea^ that in tlui limits of the present essay nothing more can be attempted than a sketch which shall embrace some of the most striking facts in the history of the forming globe considered as a condensing nebulous mass, in the chemistry of the air, sea, and earth injiast ages, and in the relations of the central heat to the superficial portions of the earth, by which we shall endeavor to explain certain facts in dynamical geology, such as the great movements of the earth's crust and the phenomena of earthquakes and volcanoes. § 5. The nebular hypothesis, as it is called, which supposes that our solar system and all the worlds of space have come from the condensa- tion of diffused vapors, has received strong contirmativin from the dis- coveries made by the spectroscope. We now know that there exist in the heavens ncbuhe consisting of luminous gas; that is to say, vaporous matter shining by its own liglit, wliich we may, with great probability, regard as the primal matter out of which, as the ehler Ilerschel sug- gested, suns and planets h;iveb(>en formed Ity a process of condensation. By the aid of the telescope and tlu' spectroscope we find in the heavens, l>lanets — bodies like our earth, shining only by reflecte*! light; suns. — self-luminous, radiating light from solid matter ; and, moreover, true nebuhc, or nmss<»s of luminous \apor. These three forms represent three distinct phases in the condensation of the primeval matter, from which our own and other i)lauetary systems have been formed. * T. S. Hunt, on the Objects ami Method of Mlnorulogy. Amoricau Joiurual of Sci-» encc, ['i,! xliii ; !i03. CHEMISTRY OF THE EARTH. This nelmlouM matter is concoived to be so intensely heated as tol)e in the state ot true gas or vapor, and, tor this reason, feebly Inminons when eoni] tared witli the sun. It would here be out of place to diseuss tlie detailed results of spectroseopie investif^ation, or the beautiful and injj^enitnus methods by whidi nu)deru science has shown the existence in the sun, and in nmiiy other luminous bodies in space, of the same chem- ical elements that are met with in oiu' earth. § 0. Oahulations based on the amount of light and heat radiated from the sun show that the temjierature which reigns at its surface is so great lliiit we Ciin hai'dly form an ade(pmte idea of it. Of the chemical rela- tions of such intensely heated matter modern chemistry has nuuU' known to us some curious facts. \vhi(!h hell) to throw light on the constitution and liuninosity of the sun. Jleat, under ordinary conditions, is favor- able to chemical combination, but a liigher temperature reverses all aHinities, Thus, the so-called noltle nu'tals, gold, silver, mercury, «S:e., unite witli oxygen and other elements ; but these compounds are decoin- jtosed by heat, ami the i)urenu!tals are regenerated. A sinular reaction was many years since shown by Mr. (jlrove with regard to wgether in the contlition of what we t-all cliemical elemmits, whose fur- ther dissociation in stellar or nebulous masses may even give us evi- dence of matter still nmre elemental than that revealed by th<' ex)>eri- ments of the laboratory, where we can only conjecture the compound nature of many of the so-called elemiMitary substances. § 7. The sun, then, is to be conceived of as an innneiisc mass of intensely heated gaseous and dissociated matter, so condensed, however, that not- Avithstanding itsexeessive temperature, it has a specitic gravity not much below that of water; prol)ably otfering a condition analogous to that wliicii Cagniard de la l\>ur observed for volatih* bodies when submitted to great pressure at temperatitivs much above their boiling pitint. The radiat ion of heat going on from the surface of such an intensely heated mass of uneombined gases will produce a superlicial cooling, wliieh will peiinit the eombinatiini of certain elements, and the production of solid or liipiid particles ; these, suspended in the still dissociated vapors, become inttmsely lumiiH)ns. and form the solar photospiiere. The con- densed particles, carried down into the intensely heated mass, again meet with a heat of dissociati*)n, so that the process of condjination at th«' surface is incessantly lenewed, while the heat of the sun may bo supposed to be maintained by the slow condensation of its mass; a dim- inution by jo'oo^^' *** "^^^ present diameter being sulllch'nt, acc»)rding to Eclmholtz, to maintain the present supply of heat Jbr twenty-one thou- sand years. § 8, This hypothesis of the nature of the sun and of the luminous ])roccss going on at its surface is the one lately put forward by Faye, CHEMISTRY OF THE EARTH. and, althouf;h it has met with opposition, aiipears to be tliat which ac cords best with oni])resentknowledife ol'tlio chemical and physical cou- ditious of matter, such as we must suppose it to exist in the condensing gaseous mass whicli, according to the nebular hypothesis, should Ibrm the center of our solar system. Taking this, as we have already done, for granted, it matters little whether we imagine the different planets to luive been successively detached as rings (hiring the rotation of the primal mass, asis generally conceived, or whether we admit with Cha- cornac a process of aggregation or concretion operating within the primal nebular mass, resulting in the prochiction of sun and plau(?ts. In either case we come to the conclusion that our earth must at one time have been in an intensely heated gaseous condition such as the sun now presents, self-luminous, and with a process of condensation going on at first at the surfaee only, until by cooling it must have reached the point where the gaseous center Mas exchanged for one of combined and liquefied matter. § 9. Here commences the chemistry of the earth, to the discussion of which the foregoing considerations have been only preliminary. So long as the gaseous condition of the earth lasted, we may suppose the whole mass to have been homogeneous ; but when the temperature be- came so reduced that the existence of chemical compounds at the center became x)ossible, those which wore- most stable at the elevated tempera- ture then prevailing, would be first formed. Thus, for example, while compounds of oxygen with mercury, or even with hydrogen, could not exist, oxides of silicon, aluminium, calcium, magnesium, anvoportion to the pressure. As expressed by Mr. Sorby, mechani- cal is thus converted into chemical force, f § 15. lieverting now to the solidified globe, in whose superficial por tious and in the surrounding gases and vapors were present all the chejuical elements with which we have to do, it is necessary to consider briefl}' its physical and its chemical conditio:! at this early period. The formation of a crust at the surface of the viscid layer which still enveloped the solidified mass of the globe, as conceived by Hopkins, is readily admissible; but that this process commenced when the remain- ing onveloi)e of liquid matfer was yet so deep that the refrigeration up to the }U'esent time has not been sufficient for its entire solidification is not i)robable. Such a crust on the cooling superficial layer would, from tiie eontraction consequent on the further refrigeration of the liquid stratum beneath, become more or less depressed, corrugated, and *Tho elevated tenipeiuturo of the iiitorior of the globe would probably oflbr uo ob- stacle to the development of nuigiietisra. In a recent cxpertinent of M. Tiiive, com- miuiieiited by M. Fayo to ilie Frtnich Academy of Seieneea, it was found that niultou Ciutt Iron when jtoured into a mold, r.iirrouuded by a helix which was travereed by an oloctric current, became a tstronj^ magnet when litjuid at a tcmpciatnre of 1,:{0IP C, and retained its maguetism while cooling. {CoimUvs-Ucndua de I' Academic dim iSticHcw. Febrnary, Uli'J.) t T. S. Hunt, Thoughts on Solution, American Journal of Scionoo, [2,] xix, 1(H). t Bakerian Lecture tor ld«53, L. E. aud D, Philouophical Magazine, Fobjuary, 18G4 8 CHEMISTRY OF THE EARTH. brokon ii|), tlins causing the extravasation of tlu .vet niisolidilicU portion, wliicli wouhl contribute to the va.st amount of mhicnil matter I )rou{;ht within tlie eliemical intiuenccs of the surrounding atmosjpliere. Further contrnetion from cooling wouhl ren«h'r this material more or less porous and permeable, preparing it for thitt process of condtined mechanical and chemical disintegration wliich would result from the action of the acid lifiuiils afterwards to be precipitated from the atmos[»here. § IG. We ha\'e next to consider the chemical constitution ot this irregu- lar surfaced and broken-ui) crust of anhydrous and i)rimitive igneous rock, M'hich is now everywhere ouried beneath the products of its disin- tegration. It is eviden hat, with the exception of those which were still in a gaseous form, ii juust have contauied all the elements which now make up the knou n rocks of the earth's crust. If we conceive these, together with the air, the ocean, and its dissolved salts, no\\ to react upon each other under the intiuonce of an intense heat, it will enalde us to form some notion of the chemical relations of the elements of the globe at the time when thej' were cooling down from that condition of igneous vapor which we supjiose to have been that of our j)ianet at an early stage in its history. To the chemist it is evident that from such a i)roc(>ss ap])lied to our globe would result the oxidation of all carbona- ceous matter, the conversion of all carbonates, chlorides, and sulphates, into silicates, and the separation of the carbon, chlorine and sulphur in the form of aciowerlully aided by the temperature. The formation of chlorides of the various bases, and the separation of silica, would go on until the affinities of the aci«l were satisfied, while there would result a sea-water holding in the solution, besides the chlorides of sodium, calcium, and magnesium, salts of aluminum and other metallic bases. At a later period the gradual combination of oxygen Avith sidphurous acid would eliminate this from the atmosphere in the form of sulphuric acid. The atmosphere being thus deprived of its volatile compounds of chlorine and sulphur, would ap- proach to that of our own time, but difter in its much greater amount of carbonic acid. It will be remarked that from thci affinities which would come into play in the conditions above 8ui>posed, all the elements, with the exception of the noble metals, nitrogen, chloi'inc, the related haloids, and the hydrogen combined with these, would be united with oxygen. The volatility of gold, silver, and j)latinum, would ke(?]) them still in a gaseous condition at temperatures where silicon, and with it the baser metals, wen^ precipitated in the tbnn of oxides. § 17. The proces.s .just described ceased with the separation from the air of the compouruls of sulphur and chlorine, and then commenced the second stage in the action of the atmosphere on the earth's crust, by wliich, under the intlueuc(5 of carbonic acid and moisture, its com- l)lex aluminous silicates are converted into a hydrated silicate of alum- ina or clay; while the separated lime, magnesia, and alkalies, being •changed into bicarbouates, are conveyed to the sea in a state of solution. CHEMISTRY OF THE EARTH. 9 The first effect of these dissolved carbonates would be to precipitate the dissolved alumina and the lu'avy metals, after which tame the de- composition of the fhloride of calcium and the formation of vurbonate of lime and chloride of sodium. This action of carbonic iicid is still {^oing on at the earth's surface, slowly breaking down and destioying the hardest rocks, and, aided by mechanical processes, transforming them into clays ; although the action, from the comparative rarity of carl)onic acid in the atmosphere, is now less energetic than in earlier times, when the abundan<'c of this gas, and a liiglier temperatuie, favored the chemical decomposition of the rocks. IJut now, as then, every clod of clay fornuMl from the decay of a crystalline rock corr(\sponded to an equivalent of (uirbonic acid abstracted from the atmospUore, anath the tropics llouri^jing within the polar circles. Many ingenious hy])othesos have been proposed to account for the warmer climate of earlier times, but are at best unsatisfurctory, and it would ai>pear that the true solution of the problem may be found in the constitution of the early atmosphere, when considered in the light of Dr. Tyndal's beautiful researches on radiant heat. He has found that the presence of a few hundredths of carbonic acid gas in the atmosphere, wliile ofi'ering ahnost no obstacle to the passage of the solar rays, would suffice to prevent almost entirely the loss by radiation of obscure heat. The aqueous vapor which our atmosphere contains exerts a powerful * T. S. Hunt. Contvihutious to the Chomistr}' of Natural Waters, Ainoricau Joiuual of Scionco, [2.J XXXIX, 184. 10 CHEMISTRY OF THE EARTH. influence of the same kind, allowing the sun's rays to reach the earth, but preventing to a great extent tlie loss ^y radiation of the heat thus communicated. When, however, the suppiy of heat from the sun is iu- terrui)ted at niglit, the radiation which goes on into space causes the l)recipitatiou of a great part of the ^vatery vapor from the air, and the earth, being thus deprived of its protecting shield, becomes more and more rapidly cooled. If now we could supi)ose the atmosphere to be mingled with some permanent gas which should possess an absorpti^^ l)ower like that of aciueous vai)or, this cooling process would be in a great measure arrested, .and an eft'ect would be produced similar to tliat of a screen of glass, which keeps up the temperature beneath it, both directly by preventing the escape of radiant heat, and indirectly by hindering the condensation of the aqueous vapor in the air confined he- neath. Such a gas is carbonic acid, and the large amount of it which existed in tlie atmosphere during former geological periods must havo aided greatly to maintain the elevated temperatures which then existed at the eartli's .siu'fice. Without doubt the greater extent of sea ami the absence or rarity of high moinitains contributed mn«'h to the mild cli- mate of former geologic; ages; but to these must be added the inlhience of the whole of the carbon since condensed in the forms of carbonate of lime and coal, which then exi:sied as a transparent and permanent gas mingled with the atmosphere surrounding the earth, and protecting it like a dome of glass. To this effect of carbonic acid il is possible that other gases may have contributed. The ozone which is mingled Avith the o::ygen set. free from growing i)lants, and the marsh-gas which is now evolved from decomi)osing vegetation, may, by their absori)tive powers, wliich are far greater than that of carbonic acid, have contributed greatly to maintain a high temperature at the earth's surface in early times.* § 19. The part which vegetation has played in the chemical history of the globe has not been limited to tlie purifl(n\tiou of the atm()si)here. It seems to have been the great agent through which solar force has elfect oxygen neces- sarj' for the oxidation of the <'arbon. Inaanuuth as the ores of these old rocks, in their present forms of hematite and n)agnelit(^, are very insolu- ble, and represii ii' so nnu^h iron withdrawn JVom the terrestrial eircida- tion, it i.s evident that the proportion of this element, existing in a dif- •T. S. Hunt, On tho Eartli's Climate, cto., Amerioaii Journal of Bcionco, [y,] sxxYi,396, 1863. CHEMISTRY OF THE EARTH. 11 tused and oxidized state in recent aedimeuts, must be less in those of more r(3inote times.* To the chemist the presence of grraphite, or of a metallic sulphide in a rock, affords dear evidences of the intervention of organic life ; and these indirect evidences are met with not only in the oldest known strati- fied rocks, those of the Laurentian system, but in the eruptive diorites, which rise from beneath them, and are pyritifi^rous. The presence of graphite, native iron, and sulphurets in most ai'rolites, not to n>ontion the hydrocarbouaceous matters which they 8onietin)es contain, tells us in unmistakable language that these bodies come from a region where vegetable life has performed a i)art not unlike that which still jjlays on our globe, and even lead us to hope for the discovery in them of organic foiiiia vvuicu mii^ give us some notion of life in other worlds than our own. §20. Animal life has played in the chemicai history of our planet a part nnich less important than vegetation, since it is entirely depend- ent for support u])on the products elaborated for it by ])Iants, and by chemical forces. Thus, although many limestones are made up chieliy, and even wholly of the calcareous remains of marine animals, these did no more than appropriate from the water the carbonate of lime generated by the chemical actions explained in §17. If the waters ot the i)re8ent ocean do not deposit carbonate of lime, it is simply becaUvSe the amount of it now generated by the slow decomoosition of the solid rocks is not more than is required for the living organisms whi<-h it con- tains. Let these become extinct and the supply of carbonate of lime, which AV(mld still contitnu^, would soon cause deposits of preeipitated carbomite of lime. Such a condition of things existing in past ages, in Hmited basins, has given rise to sediments of this kind, which constitute some of the finest statuary marbles. The waters charged with the products of the sub-aerial decay of rocks, convey to the sea, as we have seen, bicarbonates of alkalies, lime, ami magnesia; but from the reaction of these on the chloride and snlphali^ of calcium in the ocean waters carbonate of lime alone sei)arates, siiuic bi- carbonate of magnesia decomposes chloride of calcium with fornuition of maguesian chloride. When, however, in a closed sea basin all of the chloride of calcium is decomposed, the chloride of magnesium is atta<'ked by the alkaline carboiuites, and the resulting carbonate of magnesia is sei)arate(l, mixed with the carbonate of lime which had accompanied these. When into a similar closed basin, or an evai)orating sjilt lake in a dry region, holding sulphate of magnesia, there is conveyed a water charged with biearbonate of lime, thero results a double decom])osition, giving rise to suli)hat.e of lime and bicarbonate of nmgnesia. The latter, being the more soluble salt, renuiins dissolved, while the sulphate of linuM'.rys- tallizes out in the form of gypsum, but at alateri>eiiod is deposited as a hydiated carbonate of magnesia, generally mixed with carbonate of lime. Toettect this reaction it is necessary thai there should be pr«\sent such an excess of carbonic, acid as to keep tiie inagm^sia in the condition of bi- carbonate initil the gypsum has crystallized out, inasmuch as dissolved sulphate of lime is readily decomposed by carbonate of magnesia. This condition can only he attained by especial i)recautions in th(mtnu)Si)hero of our period; but by operating in an atmosjdiere more highly charged with <'arbonic acid, the ]M'od action of gypsum aiul nuigiu'sian carbonate by this reaction is readily ett'ected. We iiuiy hence conclude that it was the more highly carbonated atmospljcre of early perioils which • Geology of Couado, 18C3, p. 573. 12 CHEMIBTRY OF THE EARTH. favored the accumulation of the great heds of gypsum and raagnesian limestones which generally accompany the salt deposits of past geologi- cal periods. The hydrated maguesian carbonate, whether the con- comitant of gyi)sum, as in this case, or of chloride of sodium, as in the former reaction, unites chemically with the associated carbonate of lime, and gives rise to dolomite or maguesian limestone.* § Ul. The iu'tion of carbonated alkaline waters on the salts of the sea under ordinary conditions thus gives I'ise to carbonate of lime, and it is only under peculiar circumstances that maguesian carbonate is sep- arated. The case is, however, changed with silicated alkaline waters coming from deep seate«l silicated rocks, which undergf* a decomposi- tion without the intervention of the atmospheric air, and hold dissolved silicates of nlkalies and of lime. These rejict.ing on the maguesian salts di.ssoived in sea-water give rise to mugnesian silicates, which are very insoluble. Uence we frequently find deposits of maguesian silicates hi sedinuMits, while silicates of lime are comparatively rare. In the solubility of bicarbonate of nuignesia and the insolubility of the cor- responding lime salt, and in the insolubility of maguesian silicate and the solululity of silicate of lime, we ttiul a simple exi)lanation of the geo- logical relation of calcareous ai)d nnignesian silif!ates aiul carbonates.t § 22. The relations of the alkalies, potash and soda, require some consideration in this connection. The silico-aluminous compouvids of potash jtossess a much greater degree of stability than those of soda. This is exemplihed in the qase of rocks which contain, side by side, ortbodase and albite, or oligodase, when it is often found that the soda- feldspar has undergone decomj>ositiou from a loss of a i>ortion of its alkali and partial . t T. B. Hunt, Amorlcau Journal of Science, [2,] xl, 49. CHEMISTRY OF THE EARTH. 13 certain portion of alkali and silica. The soda-feldspars, l)eing more easily decomposed and disintegrated by atmospheric intlnences, are broken np by meehani(!al a<»encies more readily than the potash-feldspar. The same is true of silicates like hornblende and pjToxene, which are less hard than the feldspars. From the mechanical and chemical disintc{;ration of ordinary crystalline rocks, which consist chiefly of these various minerals, together w ith quartz, there will result a coarse sandy sediment, in which quartz witli more or less orthoclase will i)revail, while the liiu'r mud will contain only the more minutely divided particles of these, together with ])artially decomposed soda-feldspar, clay, and the comminuted hornblende and pyroxene. § 24. This process is evidently one which must go on in the wear- ing away of rocks by .aqueous agency, and ex])lains the fact that while quartz, or an excess of combined silica, is, for the most part, wanthig in rocks which contain a large portion of alumina, it is generally abundant in those rocks in which potash feldspar predominates. The coarser and more sihcious sedinu'iits are readily permeable to inlilt rating waters, which gradually remove from them the !;;oda, lime, and irkagnesiu which they still coutaiu, and, if organic matters intervene, the oxide of iron, leaving at last little more tlian silica, alumina, and potash, the elements of granite, trachyte, gneiss, aifd miiui-schist. On the other hand, t lie liner sediments^ whose origin, simultaneous with the coarser, Ave have just ♦explained, resisting the penetration of waters, Avill retain all their soda, lime, magnesia, and iron-oxide, and containing an exe<\ss of alumhm, with a small amount of silica, may, by their metamorphism, give rise to basic lime and soda-feldspars, and to pyroxene and hornbleudi^ — the elements of diorites and dolerites. § 25. The disintegration of alkaliferous rocks, however, frequently takes i»lace vuider such coiulitions as to be more mechanieal t!ian ehemi- eal, and it may often happen that sediments still retaining a considerable amount ol' condjiiied soda become mingled with carbonates of lime and magnesia. The reaction which then goes on between the libeiated alkaline silicate and these earthy carbonates gradually ell'ects the eon- version of these into silicates, while the alkali is eliminaied in the form of sohible<'arb(tnate of soda,.giving rise to alkaline mineral waters, wliieh, as 1 have shown, are abundantly generated in sediments where reldsj)atiii(; matters and earthy carboimtes are intermingled. It is only from rocks destitute of these carl)onates that silicated alkaline waters can issue. § 2(). A decomposition more exclusively chemical is obseived particu larly among the crystalline schists of tn^pieal and semi-tro]»ical regions, where a ])rocess of disint<'gratit»n often destroys the coliesion of the I'ocks to a considei'abh' (le[>th. Tiiis change, which has been but inqx-r- fectly studied, is probal>ly dependent in great part on the actitm of the soluble products of vegetable d(.'compositiou, aided by llie eh'\aled t<'m- ])«uature. It, however, requires careful investigation; and a c(Uisi(h'i'a- tion of tiie causes which have indiu'cd it, and the extent to which it may have in tbrmei- peviods prevailed (»n the earth'ssurfatu', is ol' great geolo- gical inqtortance, since the immens«> erctsion (►f which geognosy all'ords us evidence, and which seems so ditlicult to explain if we coiu'cive the rocks to have been as liard as we now find theiu in nnmy regions. Ix'ctMnes more easily inlelligible if we suppose tlM> cohesion of the crystalline rocks to have been previously much weakeiu'd by decay. § 27. The operation of the mechanical andchemical agencies which pre- side over the disintegiation of pre existing rocks naturally divides the insoluble jn'oducts into two types, appr«)achingin chemical conqjosition. as we have shown, to granites, gueiss, and mica-schist, ou the one hantl, 14 CHEMISTRY OF THE EARTH. and to dioritfis and dolerites on the other. Tliese correspond to the two classes of igneous rocks designated by Bunsen as the trachytic and pyroxenif, types. § 28. There is, however, a third source of silicated rocks, to which some alhision has already been made in speaking of the production of magnesian silicates by direct precipitation, as the result of chemical changes in solutions. In this way have been formed, besides these and related protoxide silicates, other silicates, including alumina. This base in certain conditions as yet but imperfectly understood, passes into solution in water, and has given rise to complex silicates, including pro- toxide bases. cVs I have elsewhere expressed it, not 0|nly steatite, pyrox- ene, hornblende, and serpentine, but chlorite anil, in many casos, garnet and epidote, have had their origin in the crystallization and molecular rearrangement of natural silicates, generated by chemical processes in aqueous solutions at the earth's surface. To these must be added other silicates, containing alkalies, chiefly potash, such as glauconite, and a hydrous silicate of alumina and potash which has the composition of pinite or agalmatolite and forms beds in the sedimentary rocks of dif- ferent geological periods. Evidences abound of the solution of aliunina, and of the generation, as chemical precipitates, of various alunnniferons silicates. These, like the similarly-formed protoxide silicates, are in ■iiost, if not all cases, highly basic, and moreover, from the mechanical conditions of their production and deposition, are found associated and even intermhigled with the flnelj -divided basic sediments of mec^.anical origin. The aluminous silicates thns formed, though mineralogically important, are probably small in amount when compared with the great mass of argillaceous sediments. § 29. The chemical changes which are wrought in the silicated rocks during their mechanical disintegration are, as we have seen, chiefly the elimination of the alkalies, especially the soda, in a soluble form from its alumin(ms compounds, and the separation and accumulation of the oxide of iron. The decomposition of the silicates of lime and magnesia which takes phice is, to a great extent, compensated for by the regene- ration of similar compounds by the reaction already explained, bnt the mean comi)osition of the argillaceous sediments of any geological epoch will depend not only upon the ago of a formation, but upon the number of times Avhich its materials have been broken up, and the length of the periods during which they have been exposed, in an unmetamorphosed condition, to the action of water, carbonic acid, and \'egetation. If, how- ever, we may assume that this action, other tnings b(nng ebio Rocks, Dublin Qnarti'vl.v Journal orScinnco, Julj, 18G3; olao Geology of Ciuiiwln, 18G3, pngo 5G1, and cliai>. XLX, of the same work. 16 CHEMISTRY OP THE EARTIf. chemical reajjonts might be necessary, and that water impregnated with alkaline carbonates and silicates would, at a temperatnre not above lOO^ centigrade, produce chemical reactions .among the elements of many sedimentary rocks, dissolving silica and generating varions insohUjle silicates.* Subseqnent experiments by Danbree contirmed these residts of mine, and both together showed the agency of heated alkaline waters to be snthcient to eticct the metamorphism of sediments by the two modes already mentioned, namely, by molecular changes and by chem- ical reactions. § oJ. l)anbr»n'tion8 are posited around the nuclei already formed, which are thus augmented at the expense of tlie smaller particles, until these at length dis;ippear, being gathered to the crystalline centers. Such a process, which has been studied by 11. Deville, sufiices, under the influence of the changing tem- l>erature of the seasons, to convert many flue i>r(^cipitates into crystal- line aggregates, by tiu^ aid of liquids of slight solvent powers. A* simi- lar agency may be supposed to have efl:ectetl the ciystallization of buried sediments, and changes in the solvent power of tlie permeating water might b<> due either to ^'ariations of temi)erature or of i)ressure. Simul- taneously with this process one of chemical union of heterogeneous element^s may go on, and in this way, for example, we nniy suppose the carbonates of lime ami magnesia become united to form dolomite or magnesian limestone. (§ 20.) § 34. WIjcu the sedimentary strata have thus been rendered crystal- line by metamorphism, their permeability to water and their alterability thereby become greatly diminished; and it is only when again broken down by mechanical agencies to the condition of soils :ind sediments that they once more become subject to the chemical changes which have • T. S. Hunt, Amoi'icixu Jouruiil of Science, [2,] xxiii, 407 ; xxv, 287-437. CHEMISTRY OP THE EARTH. 17 beeu described in § 23. While the crystalline stratified rocks are but slightly porous the unaltered strata hold large quantities of water in their I)ore8. The mean of thirty-six determinations upon sandstones, shales, limestones, and dolomites from twentytive different localities among the unaltered paleozoic sediments of Canada showed that 7.75 volumes of water were held in 100 volumes of the thoroughly-moistened rock. The proportion varied from less than 1.0 per cent, in the more compact limestones, to 10.0 and even 21.0 per cent, in the sandstones, an amount which is greatly exceeded in some more recent limestones.* A large proportion of the ocean's waters is thus imprisoned in the vast volume of unaltered sediments, and set free when these become metamorphosed, a process which is attended with a corresponding reduction of volume. In addition to this, moreover, the clays and other hydrated silicates lose a large part of their chemically-combined water during metamorphism, and become changed into crystalline compounds of increased dei'sity. This becomes obvious when we compare the specific gravity of such species as garnet, epidote, chloritoid, staurolite, andalusite and kyauite with that of the unaltered sediments in the midst of which they are gen- erated. From this ccmdensation, then, as well as from the mechanical contraction consequent upon the ex])ulsion of water, the metjimorphism of sediments is attended with a very considerable diminution of bulk, which is not without geological significance. It results from the exper- iments of Sorby (§ 14) that such chemical changes as are accompanied by condensation or diminution of volume are favored and ai^celerated by pressure, which may thus become a direct agent in promoting meta- morphism as well as solution. § 35. The crystallization whicii takes place in sedimentary rocks not unfrequently effaces more or less completely the traces of their stratified and sedimentary origin, as is seen, for example, in many gneisses, which are scarcely distinguishable from granite. The studj' of such rocks, moreover, affords abundant proof that this alteration has been attended with such a softening that the material has been molded by pressure, forced into fissures or openings in less fusible or less heated strata, and thus taken the form of what is designated as eruptive rocks. The action of heat upon sedimentary rocks is not, however, confined to condensa- tion, crystallization, and softeni g; strata in which carbonates, sulphates, chlorides, and carbonaceous suostances are mingled with silicious and argillaceous matters, will, at a sufBciently-elevated temperature, in the presence of water, undergo such changes as must liberate carbonic acid, hydrochloric sicid, and sulphuretted hydrogen, which are the common gaseous accompaniments of volcanic action. From these considerations we are led to a rational theory of volcanic and eruptive rocks, which wo conceive to have their seat, not in an uncongealed portion of tlie once liquid globe, but in the more deeply-buried portions of that disinte- giated crust whose origin has been explained in § 14. § 36. The history of this theory forms an interesting chapter in geology. As remarked by Humboldt, a notion that volcanic phenomena have their seat in the sedimentary formations, and are dependent on the combustion of organic substances, belongs to the infancy of geology. To this period l)elong the theories of L^mery and Breislak, {Cosmos, v. 443; Ott<>'s translation.) Keferstein, in his NaturfjeHeMchte dcs Erdlcor- pers, published in 1834, maintained that all crystalline non stratified rocks, from granite to lava, are products of the transformation of sedi- mentary strata, in part very recent, and that there is no well-defined " GooJogica 1 Report of Cauacla, 1866, p. 283. — Amoricaii Joiirual of Scieuoo, [2,] Xixix, 183. 18 CHEMISTRY OF THE EARTH. line to be drawn between neptunian and volcanic rocks, since they pass into each other. Volcanic phenomena, according to him, have their origin, not in an igneous fluid center, nor in an oxidizing metallic nucleus, (Davy, Daubeny,) but in known sedimentary formations, where they are the result of a i)eculiar kind of fermentation, which crystallizes and arranges in new forms the elements of the sedimentary strata, with an evolution of heat as a result of the chemical process, {Nainrgeschichfe, vol. i, p. 109; also Bulletin de la Societe Q6ologiqm de France, flj, vol. vii, ]). 197.) In commenting upon these views, {American flournal oj Science, July, 18G0,) I have remarked that, by ignoring the incandescent nucleus as a source of heat, Keferstein has excluded the true exciting cause of the chemical changes which take place in the buried sediments. The notion of a subterranean combustion or fermentation, as a source of heat, is to be rejected as irration.al. § 37. A view identical with that of Keferstein, as to the seat of vol- canic phenomena, was soon after put forth by Sir John Uerschel, in a letter to Sir Charles LycU, in 183G, {Proceedings of the Geological Society of London, ii, 548.) Starting from the suggestion of Scropo and Babbage, that the isothermal horizons in the earth's crust must rise as a conse- quence of the accumulation of sediments, he insisted that deeply-buried strata will thus become crystallized by heat, and niay eventually, with their included water, be raised to the melting point, by whi(;h process gases would be generated, and earthquakes and volcanic eruptions follow. At the same time the mechanical disturbance of the equilibrium of pres- sure, consequent upon a transfer of sediments, while the yielding surface reposes on matters partly liquified, will explain the movements of eleva- tion and subsidence of the earth's crust. Herschel w as probably ignorant of the extent to which his views had been anticipated by Keferstein; and the suggestions of '>he one and the other seemed to have passed unnoticed by geologists until, in March, 1858, 1 reproduced them in a i>aper read before the Canadian Institute, (Toronto.) being at that time acquainted with Herschel's letter, but not having met with the writings of Kefer- stein. I there considered the reactions which would take place under the influence of a high temperature in sediments permeated with water, and containing, besides siliciousand aluminous matters, carbonates, sul- phates, chlorides, and carbonaceous substances. From these, it was shown, might be produced all the gaseous emanations of volcanic dis- tricts, wliile from aqueo-igneous fusion of the various admixtures might result the great variety of eruptive rocks. To quote the Avords of my I)ai)er just referred to : " We conceive that the earth's solid crust of anhydrous and ])rimitive igneous rock is everywhere deeply concealed beneath its own ruins, which form a great mass of sedimentary strata, permeated by water. As heat from beneath invades these sediments, it produces in them that change which constitutes normal metamorphism. These rocks, at a suflicient depth, are necessarily in a state of igneo- aqueous fusion ; and in the event of fracture in the overlying strata, may rise among them, taking the form of eruptive rocks. When the nature of the sediments is such as generate great amounts of elastic fluids by their fusion, earthquakes ami volcanic eruptions may result, and these — other things being equal — will be most likely to occur under the more recent Ibrmations." {Canadian Journalj May, 1858, vol. iii, p. 207.) § 38, The same views are insisted upon in a paper '' On some points in Chemical Geology." (Quarterly Jounial of the Geological Society, Lon- don, November, 1859, vol. xv, page 594,) and have since been repeatedly put forward by me, with further explanations as to what I have designated CHEMISTRY OF TIIK EARTH. 19 above, the rwin.s of the cnist of anhydrous and primtfire igneous rod-. Tliia, it is concfivod, must, by contnu'tioii in cooliiij^, have becM)iii(> porous and l)erineabl(', lor ii considerable depth, to the waters afU'rwanl precipitated upon its KurtVice. In this way it was prepared alike for niechanic^al disin- tegration, and lor tne (tiiemical action of the acids, which, as sliown in § IG, must have been present in the air and the waters of the time. It is, more- over, not improbable that a yet nnsolidified sheet of molten matter may then have existed beneath the eaith's crust, and may have intervened in the volcanic i)henomena of that early period, contributing, by its extra- vasation, to swell the vast amount of mineral matter then bi"on<;ht within aqueous and atmospheric inlluenctes. The earth, air, and water thus made to react upon each other, constitute the lirst matter from which, by mechitnicalandehemical transformations, the whole mineral worldkuown to us has been produced. § 39. It is the lower portions of this great disintegrated and waterim- pregnate«l mass which form, according to the i)resent hypothesis, the semi-liquid layer supposed to intervene between the outer solid crust and the iimer solid and anhydrous nucleus. In order to obtain a correct notion of the condition of this mass, both iji earlier and later limes, two points must be especially considered, the relation of temperature to depth, and that of solubility to i)ressure. It bring conceded that the increase of temperature in descending in the earth's crust is due to the transmis- si -"> and escape of heat from the interior, Mr. Hopkins showed mathe- matically that there exists a constant proportion between the ettect of internal heat at the surface and the rate at which the temperature in- creases in descending. Thus, at the present time, while the mean tem- l)erature at the earth's surface is augmented only about one-twentieth of a degree Fahrenheit, by the escape of heat from below, the increase is fouiul to be equal to about one degree for each sixty feet in depth. If, however, we go back to a period in the history of our globe when the heat passing upwards through its ths, sediments should, therefore, by the effect of the water which they contain, accjuire a certain degree of liquidity, ren- dering not inqirobable the suggc^stion of Scheercr, that the presence of ftvo 01' ten per cent, of water may sullice, at temperatures approaching red- BBBi 20 CHEMISTRY OF THE EARTH. nesa, to jji vo to a granitic mass a liquidity partaking at once of the diarac- ter of an igneous and an aqueous fusion. The studies by Mr. Sorby of the cavities in crystals have h'd liim to eon( hide that the constituents of gi'anitic and trach.'s'tic rocks have crystallized in the presence of liquid water, under great i)res8ure, at temperatures not .above redness, and con- sequently very far below that required for simple igneous fusion. The intervention of water in giving liquidity to lavas, has, in fact, long been tauglit by Scrope, and notwithstans of eruptive rocks like peridotite, phonolite, leucitophyre, and similar rocks, which are so many exi't^ptions in the basic {jroup of Bujjsen. As, however, they are rejin'sented in the sedi- ments of the earth's crust, their appenriince as exotic rocks, consequent u]>on a softcninjj and extravasation of the more easy liii|ueHal)le strata of deeply buried forunitions, is readily and simply explained. §44. In this connection a few words may be said about the popular notion which makes granite the substratum of all stratified foruuitious, and even identifies it with the supi)oscd i)rimitive crust of the globe. That this crust is everywhere concealed beneath its own ruins, and, more- over, that its composition must have been very ined alkalies in these ancient rocks. The presence of quartz, an essential element alike in gneiss and granite, would suftice to show that giauite is in all cases a secondary or derived rock, formed under aqueous influences — even had Sorby not shown that the minute crystal-cavities in the quartz of granitic rocks contain liquid water which must have been introduced at the time ot crystallization. (Quartz has not only never been met with as a result of igneous fusion, but it is clearly shown by the experiments of Kose that a heat even much less than that reipiired for the fusion of quartz ) these metals have been in great pnit with- drawn from the terrestrial circulation. The frequent occurrenci' of these metals in waters which are alkaline from the presence of carbonate of soda, is signilicant, when taken in connection with the metalliferous character of certain dolomites, which ])rob!'bly owe their origin to the action of similar alkaline springs upon basins of sea-water, (§20.) The intervention of intense heat and fusion or sublimation to explain the origin of metallic ores is uncalled for. The solvent i)owers of water and of various saline, alkaline, and sulphuretted solutions ;>:t high tem- peratures, in (!oniu'ction with the notions above enunciated, Avill sutllce to form the basis of a rational theory of metallic dei)Osits.t § 48. Th(^ consideration of the nature and origin of endogenous rocks has led to a digression to discuss the theory of metalliferous veins, which the pl.m of this i^ssay did not permit us to treat before. AVc now resume the line oi" inipiiry followed from § ;5(> to § 4.'i, and [)roe('ed to consider the phenomena of vohinoes and earthquakes in accordance with the notions already put forward. Violent n>ovementa of the earth's crust are conlined to certain regions of the globe, which are at the same time characteiized by volcanic} * Of this a r«Mniirknl>]o oxnmi»lo wft8 afibnlod in 18()() at Go«ievich, in Ontario, wlioro, at a of tlii.s, I saw a ciiIm^ of jiun^ salt, (inc-lonith of an inch in diaiiu'tcr. wliicli Iiatl foiincd niton and around a. pht- j('('tin;j; \)oint of an iron valvt' in ( lif ]>nniii, al>ovc tlic snrfaro of tlic i;ronnd. 'I'lui liqnid 'ijcncaili a iiiosMun- ol' 1,0(1(1 feet of tirino, (((nal to alioiU l,'2(t(l f(>(>t of water, or ',]C> at- nios))lM'r<"s. iiaxiii^; talii'i\ up n\ori> saJt tlian it cotild liold at tlio ordinary proMHurc, dny materials assumt^ a more solid and crystalline condition, and resemble the older eru[»tive locks fiuuul in regions not now volcanic. These include granites, trachytes, dolerites, basalts, &'-., and are masses of rock whicli, thougii extrava- sated after the nnuiner of lavas, became consolie the chain of the Urals, an area in central Germany, and one in the Ihitish Islands are apparent, and in North America there appeal' to have been but two volcanic regions in the ])aleozoic period — oiu? in the basin of Lake Superior, and another, which may l)e described as occurring ahmg either side of the Appalachian chain to the northeast, inchuling the valleys of the lower St. Lawrence, Lake (Jham- plain, the Hudson and Connecticut Kivers, and extending still further southward. The study of the various eruptive rocks of this region shows that volcanic activity in dift'erent i)arts of it was prolonged Irom the beginning of the paleozoic period till after its close. § 52. The theory of Kefersti'in aud Ilerschel, explained in § 37, shows in >vhat manner voU;anh; phcuoinena may be directly dependent on the accumuhition of sedimeutary strata. It has already been shown that both temi)erature ami pressure combijie ti i)roduce in the lower portions • It is a iiKiHt signiliciint fact that tliia rojarjon is nearly co-extoiiHivo with that occiuiioil for age.H by lli<^ great (^ivili/iiig races of the world, t'roiu th(( ])lateaii nt' Central Asia, througliout their weatwaril migration to the ])illiirH of thircnle.s, the ln(lo-10nro|H!au nations were familiar witli llu^ volcuuo and tiio eartii<|nake: and that the iSemitio race were not Htrangv;is to the same phisnomenii. the wlioh^ poetic imagery of the Hebrew ScriptnrcH 1)earH amjile evide\iee. In tlio huignage o[ their writers, the monntaiiis are molten, tliey Deity, when the melting tiro bnrnotii. The Inry of UIh wrath \h ]»onred forth like tire; He toneheth tlie IhIIh and they smoke ; while lire ami snlithnr come down to destroy the doomed cities of the plain, whose fonndation is a molten tlooil. Not less does the poetry and llu' mythology of tlreeee and lioini! hear the iinjiress of the nether realm of fu'o in which (lie volcano and the cart lupmko have their scat, and their inllnence is conspioions throiighont the imaginative lileratnre and Ihe religions systems of the Indo-lOnroiiean nations, wiioso contact wit !i th(^se terrible manifestations of nnseon forces lu^yond their foresight or control could n()t fail to act strongly on their moral and intellectual development, which would have doubtless iiresciited \ cry dilfcreiit phases had the early home of theso races iieen the Australian or t!ie eastern side of the .iincricaii continent, Avhero volcanooB are unknown and the earthi|iiake is scarcely felt. (From a luuturo bcibre tUu Aiiior, (jcographical Ssooloty, April, ld(iS».) CHEMISTRY OF THE EARTH. 25 of tho sodimentary miitcriiil a condition of iij^noo-aquoons fusion. It would be foreijjn to our plnu to discuss iu this place tlu; ageucies \a hicli, from early geologic periods, have been effecting the transfer of sedi- ments, alternately Avasting and building-uj) continents. (J)ne, however, requin^s notice in this connection, namely, the contraction of sediments consequent u])on chemical changes, as already explained in § 34, which must result in subsidence. Such an effect may also result IVom the ex- travasation of great volumes of liquetied rock, and in either case the depressed portion of the surfiu-e becomes a basin, in which sediuu'uts may subsequently accinnulate, and by their Mcight upon the yielding stratum beneath (continue the i)rocess of subsidence. While the lower and more fusible strata becouu's softened, the great mass of the nu)re silicious rocks, losing their porosity, become cemented into a comparatively rigid mass, and iinally, as a result of the earth's contraction, or to counter- balance the dej)ression of some other region, are uplifted as a harilened and corrugated eontinental mass, from whose irregular erosion results a mouutainous region.* § 5;{. Those strata which, from their composition, yield, under the conditions just described, the most lifpiid jn-oducts, are, it is conceived, the source of all plutonic and volcanic rocks. Accomi)anied by water, and by diftieultly coeniible gases, they are either forced among the lis- sures which form in the overlying strata, or lind their way to tlu^ sur- face. The variations in the composition of lavas and their accom})anying gases in different regions, and e\ en from the sanu' vent at different times, are strong contirnialions ol' the truth of tiiis view. As explained in § 3!), the semi-liquid layer of water inq>regnated material constitutes a plastic bed, upon which the strut ilied sediments rei)ose. Tluvse, by their irregular distribution over dill'ercnt i)ortions of the earth, determined, alter a lai)sei of time, in the regions of their gT<'atcst accnmidaticui, vol- canic and plutonic phenomena. It now remains to show the observed relation of these j)henonu»na, both in the earlier and later times, to great accumulations of sediment. § 51. If we look at the North American <'ontinent, wo lind along its northeastern iKUtion evidences of gr«'at subsidence anpi valley are probably less than 4.0110 feet in thirkness. Conlbrmably with this, tlu're are no traces of i)lutoMic or volcanic outbursts IVom (lie northeast n'gion just men- tioned tin-oughoiit this\ast jialeozoic basin, wit!i the evception of the region of Lake Supei ior, w hen', we find the early portion of the paleozoic age marked by a great accumulation of sediments, comparable to that occurring at the same time iu the region oi' N(>w I'-ngland, and followe