IMAGE EVALUATION TEST TARGET (MT-3) 1.0 I.I 1.25 -m iiiiM ■ IM IIIIIZ2 2.0 LA. IIIIII.6 V} <^ /} % VI "m VI •>." o /,. /!% # / Photographic Sciences Corporation « 1 r* V t t CHEMICAL GEOLOGY. MR. T. STERRY HUNT'S REPLIES TO THE CRITICISMS or MR. DAVID FORBES. ^m^tm •■P^^^^in TftOT- A NOTICE OF THE CHEMICAL GEOLOGY OF Mr. D. FORBES. BY T. STERRY HUNT, F. R. S. The Geological Magazine for October last contains a criticism by Mr. Davi'l Forbes of certain views put forward by me in a lecture delivered before the Royal Institution of Great Britain on the Slst of May, 1867. Of this lecture a short-hand report appears in this Magazine for August, besides which a condensed report, revised by myself, is published in the proceedings of the Institution, in the Chemical News for June 21st, and in three French translations in the Revue des Cours Scientijiques, Les Mondi, and Cosmos. The Chemical News for October 4th contains a criticism of my lecture by Mr. Forbes, to which I have replied in a communication recently addressed to that Journal. In the lecture in question, I endeavored to bring together the results of modern investigations in physics, chemistry, mathema- tics and astronomy, and to construct from them a scheme which should explain the development of our globe from a supposed i»tensely heated vaporous condition down to the present order of things. I could not pretend to discuss from their various stand- points, all the conclusions arrived at by different investigators, inasmuch as, even had my attainments permitted, the limits of an hour's lecture would have proved far too short. In regard to the structure of the earth I alluded to two views, one of which supposes a liquid globe covered with a thin crust of solidified rock, generally estimated at from twenty to thirty miles in thickness, while the other regards the earth, if not solid to the centre, as having a crust at least several hundred miles in thickness, and of such solidity and rigidity as to be, so far as superficial phenomena are concerned, inert as if in a solid state. To this latter view, I incline, and I cited in support of it the conclusions of Hopkins from the phenomena of precession and nutation, the in- vestigations of Archdeacon Pratt on the crushing effect of ini- mense mountain masses like the Himmalayah, and the deductions of Sir Wm. Thompson from the phenomena of the tides, showing the great rigidity of the earth, as so many concurrent evidences that our planet, if not actually solid to the centre, has a crust far thicker than can be accounted for by the theory of a liquid globe covered only with a crust resulting from superficial cool- ing. This latter view, which was deduced from Cue increilse of tem- perature observed in descending into the earth, is in conflict with the various mathematical and physical considerations above no- ticed, and it becomes necessary to revise the older notions of the conditions of a cooling globe. The investigations of Charles Deville and of Delesse, as well as the earlier ones of Bischof, show that the density of fused rocks is very much less than that of the crystalline minerals of which they are composed. From this we may naturally conclude that the crystalline compounds which would separate by slow cooling from a bath of molten rock would gravitate toward the centre, as Sae- mann has already justly observed, (Bull. Soc. Geol. de Fr. Feb. 4. 1861.) In opposition to this view, Mr. Forbes appeals to the results seen in a small scale in the cooling of melted metals, etc., — where a crust forms over the surface. It must, however, be considered that the conditions presented by a small vessel full of a liquid congealing in an atmosphere greatly below its own temperature, and having a crust growing out from and supported by the sides of the vessel, are widely different from those of a liquid globe slowly cooling beneath a very dense and intensely heated atmos- phere. In such a case, with a bath of materials similar to those forming our present rock-crust, the crystalline minerals of which have been shown by Deville to be from -^g to | heavier than the liquid mass, these, as they separated, would sink as naturally as the crystals which form at the surface of an evaporating basin of brine. The analogy holds good, since the denser crystals formed at the surface, whether by evaporation or by cooling, obey the inevitable laws of gravity. Mr, Forbes next proceeds to some considerations drawn from the mean density of the earth, which being about 5.3, is twice that of the average specific gravity of th3 solid materials known at the surface. Admitting that a solid crust of specific gravity 2.65 were to form at the surface of a liquid of density 2.3, and in obedience to natural laws, to sink therein, our critic conceives that, in its descent, it would meet with a denser liquid stratum. lie supposes a liquid globe " becoming rapidly denser in descending, as the pressure increased by the superincumbent column of liquid mat- ter," and he tells us, in a note, that we may admit a density of " nearly 10.7 for the middle zone and about 18.8 for the centre." . (page 435). Two pages farther on he has completely changed his mind, for he tells us that " experimental research tends to show that a limit is soon reached beyond which the compression or increase of density becomes less and less in relation to the force employed," and concludes that there are strong reasons for believ- ing that the central jmrts of the earth "must consist of much dens- er bodies, such as metals and their metallic compounds," which he farther on exjilains may mean " dense sulphids." To which of these two views does Mr. Forbes mean to hold, that of a rapidl.y and constantly increasing density from i>ressure, or that in which, limiting the condensing effect of pressure, he seeks to explain the density of the earth by a nucleus of heavy metallic compounds? The latter is seemingly an after- thought of the critic, suggested by some notion of the jirinciple involved in the augmentation by pressure, of the fusing point of bodies which expand m melting. As was shown by James Thomp- son, the effect of pressure upon ice (and naturally ujion such me- tals and metallic alloys as, like it, contract in melting,) would be to reduce its melting point, a fact which has been exjjerunentally established for ice. Reasoning from the same principle. Sir Wm. Thompson deduced the conclusion that a reverse effect should result from pressure for all such solids as expand in melting, that is to say, that their points of fusion would be raised, a conclusion verified by the experiments of Bunsen and by those of Fairbairn and Hopkins. From some apparent irregularities in these results, and from the fact that certain of the substances submitted to experiment were bodies of the carbon series, which Mr. Forbes calls " organic," he argues against the conclusions which depend upon a well defined physical law. In the case of the fusible alloys tried 6 by Mr. Hopkins, it is to be remarked that most of these bodies, like ice, expand in cooling, and consequently should not have their melting points raised by pressure. For the memoirs of James and William Thompson, see Trans, Royal Soc. Edin. XVI, part 5, and L. E. D. Philos. Mag. [3] XXXVII, 125. A simple and popular exposition of the principle, and of Mr. Hopkins's argument there- from, for the solidity of the globe, will be found in the fourth of Tyndal's lectures on Heat as a Mode of Motion. See also Sorby's Bakerian lecture for 1863, cited farther on. Mr. Forbes must con- sider that just so far as he admits the condensing power of the pres- sure of the superincumbent mass, he increases the difficulty of maintaining that rocky mass in a liquid state. The condensing effect of pressure was by Dr. Young estimated to be sufficient to reduce a mass of granite at the earth's centre to one-eighth its bulk at the siuface, which would give to the earth a mean density equal to twelve or thirteen times that of water. This consideration has led a recent writer in the London Athen- (Bum to conclude with Herbert Spencer, that our earth .and the other planets may be only shells of varying thicknesses, enclosing a central cavity filled with vaporous matter, by which hypothesis we may explain the apparently feeble density. See Mr. Spencer's essay on the Nebular Hypothesis in the Westminster Review for July 1858. It may be observed that his view which supposes con- densation to have resulted in the formation of a solid shell around a gaseous nuclent, is not incompatible with my scheme, which is simply opposed to a liquid interior. See also the note of Mr. Barker in this Magazine for September last, page 426. Leaving Mr. Forbes to settle these vexed questions, we may remark that in case we suppose condensation of the gaseous globe to have com- menced either at the centre or around a gaseous nucleus, it is probable that solidification from pressure must have taken place long before the liquefaction of earthy matters was complete. But if we adopt Mr. Forbes' s second hypothesis that pressure would not materially augment the density nor raise the melting point of the fused mass, what grounds has he for assuming, as he does, that there occurred a separation of the liquid into zones of different densities ? That metallic sulphids could be formed at an elevated temperature, by condensation from an atmosphere containing an excess of oxygen, is contrary to all that we know of chemical affini- ties ; sulphurous acid and metallic oxyds would be the results so soon as the temperature fell below that of dissociation. As for the noble metals, whose compounds with oxygen are decomposed at elevated temperatures, their great volatility, as compared with earthy and metallic oxyds, would keep them in the gaseous form till the last stage of precipitation of earthy oxydized matters, wlien by far the greater part of the globe was probably solidified. Hence we now find them in the earth's superficial crust, instead of being, as Mr. Forbes would suppose, carried to the centre of the planet. Judging from what we know of chemical affinities, and of the proportions of the elements now existing in the superficial parts of the globe, we cannot conceive anything else than the production of a homogeneous oxydized silicated mass, upon which, at a late pe- riod, would b .recipitated the noble metals. From this mass, while yet liquid, there might take a separation of various crystalline com- pounds, by a process analogous to that by which pui-e lead separ- ates from the bath of the argentiferous alloy in Pattinson's process, as Fournet has already suggested, (Gcol. Lyonnaiso, 18G2, page 398). The last congealed and lighter portion of our glooo, with which alone we have to do, was, probably, a sort of mother-licjuor from which, during its slow cooling, compounds of various consti- tution and density may well have crystallized. In furnace opera- tions, it is true, we may obtain, besides silicated slags, a dense stra- tum of reguline metals, sulphids or arsenids on the one hand, and a lighter one of saline sulphates or chlorids on the other. But neither of these classes of compounds was possible in the cooling globe, the reguline matters for reasons just given, and the saline com- pounds, for reasons yet to be explained. I have in my lecture set forth that the earth's superficial crust must have been composed of silicates of the metallic, earthy and alkaline bases, surrounded by a dense acid atmosphere of hydro- chloric, sulphurous and carbonic acids, besides watery vapor, ni- trogen and oxygen. These chemical combinations are such as would naturally result from the affinities brought into play at the elevated temperatures then prevailing, in virtue of which all those elements capable of forming fixed and stable compounds with oxygen would be precipitated as oxyds. In these conditions, as already said, no metallic sulphids would be formed, and the whole of the sulphur would be found as sulphurous acid. In like man- ner the production of alkaline chlorids under such conditions, is inconceivable, since in the conjoined presence of oxygen, hydrogen, and silicon or silica, an alkaline silicate and hydrochloric acid would result. Even, if, as Mr. Forbes supposes, chlorid of sodium were to be formed in the heated atmosphere, it would be precipitated into a bath of fused silicates, covered by an intent ely heated at- 8 mosphere containing water, or mingled oxygen nnd hydrogen gases, and would immediately undergo the same decomposition that takes place when the vapors of common salt arc diffused through a potter's kiln, or, as in Mr. Qossage's new soda-process, are passed with steam over red-hot flints. In both cases silicates of soda are formed with separation of hydrochloric acid. These considerations lead to the conclusion that after all the more fixed elements were precipitated, tht hole of the chlorine would finally remain in the partially cooled atmosphere as hydro- chloric acid, and the whole of the sulphur as sulphurous acid, to- gether with a large proportion of oxygen, since we find this element in the form of sulphate and not as sulphite in the sea-waterS. Mr. Forbes does not, it seems, believe that an excess of oxygen could exist in an atmosphere highly charged with sulphurous acid, and elsewhere (in the Chemical News), he tells that it is, " if not impos- sible, at least, highly improbable that such a heated atmosphere containing sulphurous acid, hydrochloric acid, with oxygen and nqiieous vapor could exist," the elements being in his opinion, incompatible. He is aware that at certain temperatures sulphur- ous acid and oxygen unite, in the presence of water, to form oil of vitriol, but he forgets that at a higher temperature this com- pound is again resolved into water, sulphurous acid and oxygen ; and that one of the best processes for preparing the latter gas on a large scale, is by this decomposition of sulphuric acid, and the subsequent removal of the sulphurous acid from the cooled gaseous mixture. In the opinion of Mr. Forbes, as set forth in the Chemical News, the sulphurous and hydrochloric acids would decompose each other, in the presence of watery vapor (though every chemist's experience teaches him the contrary ; ) another reason for holding that my supposed atmosphere was impossible. Unfortunately for his opinion, however, it happens that large quantities of precisely such an atmosphere are disengaged from various volcanic vents. To cite one among many examples examined by Charles Deville and Leblanc (Ann. de Ch. et Phys. [3]LII. pp. 5-63) &fumerolle of Vesuvius yielded in June, 1856, a mixture of highly heated steam, hydrochloric acid, sulphurous acid and air containing 18.7 per cent of oxygen. The sulphurous acid was equal to 2.6 per cent, of the air, and the amount of hydrochloric acid was about five times as great. Traces of sulphuric acid were found in the water condensed from this steam, doubtless formed by the slow combination of the sulphurous acid and oxygen, and I may state for the information of Mr. Forbes, that it was doubtless by a similar reaction that the sul. WNI mmmm 9 phurous acid became eliminated from the primeval atmofiphere. We have here, I may remark, an illu.stration of the fact upon which I have elsewhere insisted, that volcanoes reproduce, on a limited scale, the conditions of the primeval earth, not only in their solid but in their gaseous products. Mr. Forbes next asserts that, according to me, " the hydrochloric acid in the i)rimeval atmosphere was derived from the mutual reactions of sea-salt, silica and water," (page 438) and then charges the author with the folly of <' supposing the pre-existence of com- pound bodies in a case where he had previously informed us that there were only dissociated elements engaged." Mr. Forbes knows better than this, or at least did know better when he wrote his criticism on my lecture in the Chemical Neivs of Oct. 4, for he here quotes my own words, when, in descrfbing the cooling globe, the conditions through which it must have passed, and the affinities brought into play, I say the products must have been '^just what would now remit if the solid land, sea, and air, weremade to react upon each other under the influence of interne heat.^^' It is so difficult to characterize properly such a wilful perversion of an author's words, that I must leave the task to my readers. What follows in Mr. Forbes" s paper as to chlorids, etc., I have already discussed and dis- posed of. The theory of the constitution of tlie solid globe next put forward by Mr. Forbes, borrowed from Phillips, Durocher and Von Waltershausen, is also, as I conceive, met by the argument in the previous pages. When, however, he comes to the atmosphere sur- lounding his primitive globe, Mr. Forbes puts forward a scheme which is strikingly original. He supposes around the ''solidified crust " a dense vapor consisting chiefly of chlorid of sodium, " above this a stratum of carbonic acid gas, and then of water in the form of steam, whilst the oxygen and nitrogen would be ele- vated still higher," (p. 439), probably, also, separated in the order of their densities. In explanation of this order he tells us in a note that the zone of carbonic acid gas would be heavier than that of steam, and should therefore come below it, but he forgot that oxygen and nitrogen (or atmospheric air) are also both heavier than steam, and should consequently be placed below the zone of watery vapor. The specific gravities of carbonic acid and steam are respectively 1.525, and 0.624, air being l.O(M). But, apart from this absurd mistake, what shall be said to a man who ignores com- pletely the laws of the diffusion of gases? WillMr. Forbes kindly ex- plain why, in our present atmosphere, the same elements, namely, oxygen, nitrogen, carbonic acid gas and watery vapor, are com- £ mmmmmm W^ 10 mingled, instead of being, as he would have them, arranged in separate zones ? I have said in my lecture that the first ocean waters would hold in solution salts of alumina and the heavy metals, all of which would be precipitated before the separation of carbonate of lime commenced. In such event, says Mr. Forbes ^^ geologisti^, thovgh as yetunsuccesufulin doing so, might stiJl hope io Jindbeds of almnina or of the metallic oxyds or carbonates alluded to, in the older strata, vis no beds of such character are known to occur in nature,^' he re- gards my view with distrust. Known to Mr. Forbes ! Has he nev«r heard of beds of emery, which are chiefly crystalline alumina, and which occur in the crystalline limestones of Asia Minor,and in the old crystalline schists of New England ? Is he ignorant that the beds of bauxite, so abundant in the Mediterranean basin, and used in the manufacture of aluminium, consist chiefly of hydrated alumina ? "To console Mr. Forbes however, I will say that I believe these beds of emery and of bauxite to have been formed by secondary and subse- quent reactions, and that we have Uvnvhere exposed to view the first, dei^osited beds, which are everywhere destroyed or burietl under more recent strata. When he remembers that the oldest known series of rocks, the Laurentian, consists of quartzites, lime- stones, and gneiss, evidently of sedimentary origin, and derived from still older sedimentary rocks, ne will understand why he cannot hope to discover the first deposits of alumina Jor metallic oxyds. These, however, in most cases, have doubtless, by mechanical sub- division, or by solution, been subsequently diffused, and enter into the composition of later rocks. In a note to this iniragrapii, Mr. Forbes inquires what became of the sulphurous acid of the early atmosphere ; as I have already told him, it doubtless became changed into sulphuric acid and passed into the sea. He then says ''it may safely be asserted that there is fully as much (if not more) sulphur than chlorine" in nature, and that according to my hypothesis, the sea would become a solution of sulphate of soda. Very safely asserted indeed, since Mr. Forbes takes care to tell us that the sulphur in the form of dense metallic sulphids went to the centre of the oartli, which 1 liave shown, I think, good reasons for not believing. As it is, we have only to consider the (Uiantities of sulphids and sulphates in the rocks and waters to see the absurdity of his remarks. lie next proceeds to discuss the theory of the origin of carbo- nate of lime. I have said that with the exception of that derived from the subauia) decomposition of primitive calcareous silicates, ip 11 all of the carbonate of lime of the earth's surface has been formed from the decomposition of the soluble lime-salts of the sea, by- carbonate of soda (and other soluble carbonates.) I, moreover, lay- down the proposition that "animals can only appropriate the car- bonate of lime already formed." In the face of these quotations, cited by Mr. Forbes, he says, as if charging me with holding the view, that if limestones were "formed by precii)itation, they would have, from the moment of their deposition, a tlecided crys- talline structure," while "Sorby's microscopical researches prove satisfactorily that all limestones, from the most ancient up to the most recent, are solely formed of the debris of organisms;'^ this will probably be surprising news for Mr. kSorby, and a decisive blow for those who question the organic nature of Eozoon. I am prepared to go as far as any reasonable man in asserting the or- ganic origin of limestones, and have, as every one must see, implied the intervention of organic life, wlien I say, "animals appropriate the carbonate of lime, etc." The question is, however, wneuce comes the carbonate of lime to supply the wants of these animals ? Mr. Forbes declares that " zoologists believe that marine animals can utilize the other salts of lime existing in the ocean," evidently the sulphate or the chlorid of calcium once so abundant there. Will Mr, Forbes or the zoologists explain what has become of the acids once combined with the lime which has built w\-> the thousands of fees of limestone, chiefly fossilifevous, which are found in the earth's crust? The only plausible chemical ex])lanation is that wliich I have given, namely : that the chlorid of calcium has been decom- posed by carbonate of soda derived from decaying feldspathic rocks, giving rise thereby to common salt and to the carbonate of lime which has supplied the marine animals. As regards the question of the origin of dolomites, which Mr. Forbes next proceeds to notice, he will do well to consult my pajier on the subject in the American Journal of Science for July, 1866, ([2] XLll, 4'.)). In this, at §112, he will see that, apai't from the for- mation of stratified sedimentary dolomites, 1 insist upon tlie frequent occurrence of dolomite as a mineral of secondary deposi- tion, lining drusy cavities, filling veins, and even the moulds of fossil shells. To such cases the observations of .Sorby may possibly refer, I can find no other account of his researches than the brief note in the Proc. of the Brit. Assoc, foi- 1856, cited by Mr. Forbes. Although I have a great respect for Mr. Sorby as an investigatoi-, I have very little for the old theory of dolomitiziition of sedimen- tary limestones. No one who has carefully studied, as I have done for 12 I years, the distribution and association of the great beds of dolomite wliich occur in the Lower Silurian rocks of Canada and New Eng- land, can for a moment admit that these are the products of subsequent alteration. Repeated alternation of pure blue lime- stones with reddish ferruginous dolomites, interrupted beds and patches of these enclosed in the former, the line of demarcation sharply drawn, and finally conglomerates in which pebbles of pure limestone are enclosed in beds of dolomite, are incontrovertible evidences against the theory of the dolomitization of limestones, and in tavor of the deposition of dolomites as magnesian sedi- ments. (Geology of Canada, 1863, page 612). Mr. Forbes, in a note, insinuates that I am unaware of the various speculations and theories which have been put forward to explain the supposed origin of dolomite by alteration. Although the stratigraphical relations of dolomite, as described above, com- pletely contradict this h,yi>o thesis of its origin, at least in the great majority of cases, Mr. Forbes will find tliat the observations and speculations of Haidinger, VonMorlot, Marignac, and others, on this subject have been fully discussed and made the subject of multi- plied experiments by me in a memoir published in 1859, {Amer- Jour. Science, [2] XXVIII. 170, .30"),) and later in the paper quoted above, and that I have shown by many experiments that the action of sidphate of magnesia on carbonate of lime, alluded to by llaitUnger and Von Moi-lot before Ilarkness or Regnault, does not give rise to dolomite, but to carbonate of magnesia, wliich remains mechanically intermiii/^led with sulphate of lime ami any excess of cai'bonate of linie. Some of the results of my prolonged study of certain of the salts of lime and magnesia, which are, for the most part, set forth in the papers j'ust referred to, were, says Mr. Forbes, by me considered worthy of being presented to the French Academy {Comptes lien- dus, April 22, IHfiT), although he declares the reactions therein des- \cribed, to have been for more than twenty years in general application, on a large scale in (ireat Britain for the manufacture of magnesia salts. Here it becomes difficult to admit the plea of ignorance wliich suggests itself for most of Mr. Forbes's previous error.s and mis. statements. I have, in the note to the French Academy, above referred to, pointed out the following facts, dis- covered by my investigations of the salts of lime and magnesia: — 1st. That bicarbonate of lime, at ordinary temperatures, de- composes solutions of sulphate of soda and suljthate of mag- nesia, with formation of sulphate of lime and bi-carbonates. ip m 13 2nd. That from mingled solutions, of sulphate of magnesia and bi-carbonate of lime, there separates by evaporation, crystalline gypsum, and, subsequently, a hydrous carbonate of magnesia ; the bi-carbonate, of this base being, as is well known, very much more soluble than either the sulphate or the bicarbonate of lime. 3rd. That this separation of gypsum is favored and rendered more complete by an atmosphere impregnated with carV)onic acid gas ; and 4th, that mixtures, in due proportions, of precipitated carbo- nate of lime and hydrous carbonate of magnesia, when gently heated under pressure, and in the presence of water, unite to foim the anhydrous double carbonate, dolomite. These are the re- actions which I described to tlie French Academy as new, and as forming the basis of a reasonable theory of the origin of gypsums and of dolomites. I now demand Mr. Forbes to make good his bold assertions to the contrary, oi' to show that any one of them has been employed for the last twenty years in the manufacturo of magnesian salts. Mr. Foi'bes then proceeds to inform us that " the grand develop- ment of magnesian limestones, dolomites, and gyi)seous Vjeds, really took place in an ej)och when numerous air-breatliing animals, both vertebi*ates and invertebrates, lived upon tlie face of the globe." Is Mr. Forbes aware that a large proportion of the 4,750 feet of limestone measured by Sir William Logan in Canada, and constituting the three great limestone formations of the old Laurentian system, is magnesian, and often, through great thicknesses, a pure dolomite, that a large part of the Lower Silurian system, and nearly the whole of the LTpper Silurian, from the St. Liiwrence to the Mississippi, consists of dolomite, and embraces great gypsum beds ; and, finally, that immense gyp- sum deposits found, at intervals, from Nova Scotia to the Ohio, lie at the base of the Carboniferous system, in whicli latter only are found the Jir.st rcmaiiui of air-breathing vestebrates? It is dangerous to generalize from the geology of the British Islands, or of a small })art of Europe. Moieover, will Mr. Forbes attempt to demonstrate that at tlie time when Tertiary gyitsums were deposited in the Paris basin, there did not yet remain suffi- cient carbonic aci