Nov. i6, 1871J 



NATURE 



51 



worms. The significance of these facts was further illustrated 

 by showing that the so-called glauconites differ considerably in 

 composition, some of them containing more or less alumina or 

 magnesia, and one from the tertiary limestones near Paris being, 

 according to Berthier, a true serpentine. * 



These facts in the history of Eo2o6n were first made known 

 by me in May 1S64, in the AmLricaii- yournal of Science, and 

 subsequently more in detail, February 1S65, in a communication 

 to the Geological Society of London, f They were speedily 

 verified by Dr. Giimbel, who was tlien engaged in the study of 

 the ancient crystalline schists of Ba\aria, and who soon recognised 

 the existence, in the limestones of the old Hercynian gneiss, of 

 the characteristic Eozotin Canadense, injected with silicates in a 

 manner precisely similar to that observed by Da\\son and myself:}: 

 Later, in 1869, Robert Hoffmann described the results of a 

 minute chemical examination of the Eozoon from Raspenau, in 

 Bohemia, confirming the previous observations in Canada and 

 Bavaria. He showed that the calcareous shell of the Eozoon 

 examined by him, had been injected by a peculiar silicate, which 

 may be described as related in composition both to glauconite and 

 to chlorite. The masses of Eozoon he found to be enclosed and 

 wrapped around by thin alternating layers of a green magnesian 

 silicate allied to picrosmine, and a brown non-magnesian mineral, 

 which proved to be a hydrous siUcate of alumina, ferrous oxyd, 

 and alkalies, related in composition to fahlunite, or more nearly 

 to jolIyte.S 



Still more recently, in the course of the present year. Dr. Daw- 

 son detected a mineral insoluble in acids, injecting the pores of 

 crinoidal stems and plates in a palccozoic limestone from New 

 Brunswick, which is made up of organic remains. This silicate 

 which, in decalcified specimens, shows in a beautiful manner the 

 intimate structure of these ancient ctinoids, 1 have found by ana- 

 lysis to be a hydrous silicate of alumina and ferrous oxyd, with 

 magnesia and alkalies, closely related to fahlunite and to joIlyte.|| 

 The microscopic examinations of Dr. Dawson show that this 

 silicate injected the pores of the crinoidal remains and some of the 

 interstices of the associated shell-fragments, before the intro- 

 duction of the calcite which cements the mass. I have since found 

 a silicate almost identical with this, occurring under similar con- 

 ditions in an Upper Silurian limestone said to be from Llangedoc 

 in Wales. 



Giimbel, meanwhile, in the essay on the Laurentian rocks of 

 Bavaria, in 1S66, already referred to, fully recognised the truth 

 of the views which I had put forward, both with regard to the 

 mineralogy of Eozoon and to the origin of the crystalline schists. 

 His results are still farther detailed in his Gtoi^nost. Bcschreibiiiig 

 dfs ostbayctisclicn Grciizcgcbirges, 1 868, p. S33. Credner, more- 

 over, as he tells us, IT had already from his mineralogical and 

 lithological studies, been led to admit my views as to the origi- 

 nal formation of serpentine, pyroxene, and similar silicates (which 

 he cites from my paper of 1865, above referred to**), when he 

 found that Giimbel had arrived at similar conclusions. The 

 views of the latter, as cited by Credner from the work just referred 

 to, are in substance as follows : — The crystalline schists, with 

 their interstratified layers, have all the characters of altered sedi- 

 mentary deposits, and from their mode of occurrence cannot be 

 of igneous origin, nor the result of epigenic action. The origi- 

 nally formed sediments are conceived to have been amorphous, 

 and under moderate heat and pressure to have arranged them- 

 selves, and crystallised, generating various mineral species in their 

 midst by a change, which, to distinguish it from metamorphism 

 by an epigenic process, Giimbel happily designates diagenesis. 



It is unnecessary to remark that these views, the conclusions 

 from the recent studies of Giimbel in Germany and Credner in 

 North America, are identical with those put forth by me in 1S60. 



At tlie early periods in which the materials of the ancient crys- 

 talline schists were accumulated, it cannot be doubted that the 

 chemical processes which generated silicates were much more ac- 

 tive than in more recent times. The heat of the earth's crust 

 was probably then far greater than at present, while a high tem- 

 perature prevailed at comparatively small depths, and thermal 

 waters abounded. A denser atmosphere, charged with carbonic 

 acid gas, must also have contributed to maintain, at the earth's 



* Amer, Jour. Sci. II. xl. 360, Report Geol. Survey Canada, 1866, p. 231, 

 and Quar. Geol. Jour. XXI. 71. 



t Amcr. Jour. Sci. II. x.vxvii. 4,1. Quar. Geol. Jour. XXI. 67. 



t Proc. R. Bavar. Acad, for 1866, and Canadian Naturalist, N. S., III. 81. 



§ Jour, fiir Prakt. Chem. May, 1869, and Amcr. Jour. Sci. III. i. 378. 



» Amer. Jour. Sci. III. i. 379. 



It Hermann Credner ; die Gleiderung der Eozoischen Formationsffruppe 

 Nord Amerikas. Halle, 1869. 



** That in the Quar. Geol. Jour. XXI. 67. 



surface, a greater degree of heat, though one not incompatible 

 with the existence of organic life.* These conditions must 

 have favoured many chemical processes, which, in later 

 times, have nearly ceased to operate. Hence we find that 

 subsequently to the eozoic times, silicated rocks of clearly 

 marked chemical origin are comparatively rare. In the meclia- 

 nical sediments of later periods certain crystalline minerals 

 may be developed by a process of molecular re-arrangement^ 

 diagenesis. These are, in the feldspathic and aluminous sedi- 

 ments, orthoclase, muscovite, garnet, staurolite, cyanite, and 

 chiastolite, and in the more basic sediments, hornblendic miner- 

 als. It is possible that these latter and similar silicates may 

 sometimes be generated by reactions between silica on the one 

 hand and carbonates and oxyds on the other, as already pointed 

 out in some cases of local alteration. Such a case may apply to 

 more or less hornblendic gneisses, for example ; but no sediments, 

 not of direct chemical origin, are pure enough to have given rise 

 to the great beds of serpentine, pyroxene, steatite, labradorite, 

 &c. , which abound in the ancient crystalline schists. Thus, while 

 the materials for producing, by diagenesis, the aluminous sili- 

 cates just mentioned, are to be met with in the mud and clay- 

 rocks of all ages, the chemically formed silicates capable of 

 crystallising into pyroxene, talc, serpentine, &c. , have only been 

 formed under special conditions. 



The same reasoning which led me to maintain the theory of 

 an original formation of the mineral silicates of the crystalline 

 schists, induced me to question the received notion of the epigenic 

 origin of gypsums and magnesian limestones or dolomites. The 

 interstratification of dolomites and pure limestones, and the en- 

 closure of pebbles of the latter in a p.aste of crystalline dolomite, 

 are of themselves sufficient to show that in these cases, at least, 

 dolomites have not been formed by the alteration of pure lime- 

 stones. The first results of a very long series of experiments and 

 inquiries into the history of gypsum were published by me in 

 1859, and further researches, reiterating and confirming my pre- 

 vious conclusions, appeared in iS66.t In these two papers it 

 will, I think, be found that the following facts in the history of 

 dolomite are established, viz. : first, its origin in nature by direct 

 sedimentation, and not by the alteration of non-magnesian lime- 

 stones ; second, its artificial production by the direct union of 

 carbonate of lime and hydrous carbonate of magnesia, at a gentle 

 heat, in the presence of water. As to the sources of the hydrous 

 magnesian carbonate, I have endeavoured to show tliat it is 

 formed from the magnesi.an chlorid or sulphate of the sea or 

 other saline waters in two ways : — first, by the action of the bi- 

 carbonate of soda found in many natural waters ; this, after 

 converting all soluble lime-salts into insoluble carbonate, forms a 

 comparatively soluble bicarbonate of magnesia, from which a 

 hydrous carbonate slowly separates ; secondly, by the action of bi- 

 carbonate of lime in solution, which, with sulphate of magnesia, 

 gives rise to gypsum ; this first crystallises out, leaving beliind a 

 much more soluble bicarbonate of magnesia, which deposits the 

 hydrous carbonate in its turn. In this way for the first time, in 

 1S59, the origin of gypsums and their intimate relation with 

 magnesian limestones were explained. 



It was, moreover, shown that to the perfect operation of this 

 reaction, an excess of carbonic acid in the solution, during the 

 evaporation, was necessary to prevent the decomposing action 

 of the hydrous mono-carbonate of magnesia upon the already 

 formed gypsum. Having found that a prolonged exposure to 

 the air, by permitting tlie loss of carbonic acid, partially inter- 

 fered with the process, I was led to repeat the experiment in a 

 confined atmosphere, charged with carbonic acid, but rendered 

 drying by the presence of a layer of dessicated chlorid of calcium. 

 As had been foreseen, the process under these conditions proceeded 

 uninterruptedly, pure gypsum first crystallising out from the 

 liquid, and subsequently the hydrous magnesian carbonate. t 

 This experiment is instructive as showing the results which must 

 have attended this process in past ages, when the quantity of 

 carbonic acid in the atmosphere greatly exceeded its present 

 amount. 



As regards the hypotheses put forward to explain the supposed 

 dolomitisation of previously-formed limestones by an epigenic 

 process, 1 may remark that I repeated very many times, under 

 varying conditions, the often-cited experiment of Von Morlot, who 

 claimed to have generated dolomite by the action of sulphate of 

 magnesia on carbonate of lime, in tire presence of water at a 



♦ .Amer. Jour. Sci. II. vxxvi. 396. 

 t Amer. Jour. Sci. II xxxviii. 170, 365; xlii. 49. 



X Proceedings Royal Institution, May 30, 1867, and Canadian Naturalist, 

 New Scries, III. 231. 



