Dr. T. Sterry Hunt — On Chemical Geology. 55 



Mr. Forbes then proceeds to inform us that "the grand development of magne- 

 sian limestones, dolomites, and gypseous beds, really took place in an epoch when 

 numerous air-breathing animals, both vertebrates and invertebrates, lived upon the 

 face of the globe." Is Mr. Forbes aware that a large proportion of the 4750 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 Upper Silurian, from the St. Lawrence to the 

 Mississippi, consists of dolomite, and embraces great gypsum beds ; and, finally, 

 that immense gypsum deposits, found at intervals from Nova Scotia to the Ohio, 

 lie at the base of the Carboniferous system, in which latter only are found \k\^ first 

 remains of air-breathing vertebrates ? It is dangerous to generalize from the geology 

 of the British Islands, or of a small part of Europe. Moreover, will Mr. Forbes 

 attempt to demonstrate that at the time when Tertiary gypsums were deposited in 

 the Paris basin, there did not yet remain sufficient carbonic acid in the air to modify 

 its chemical action on solutions of bi- carbon ate of magnesia, and give rise to the 

 associated dolomites, which I was the first to discover and to describe in that 

 position ? 



We now, in the language of Mr. Forbes, approach the question of "the igneous 

 origin of eruptive rocks and of granite in particular." I asserted in my lecture the 

 non-igneous origin of granite, maintaining that "the composition of the fused crust 

 would have excluded free silica," and "that granite is in every case a rock of 

 sedimentary origin, as it includes in its composition quartz, which, so far as we 

 know, can only be generated by aqueous agencies, and at comparatively low 

 temperatures. " With regard to the first of these statements, it is to be observed 

 that the primitive crust, holding, as we have seen, in the form of silicates, all the 

 soda, lime, and magnesia which now appear in other combinations, must have 

 been a highly basic rock ; moreover, even were it much richer in silica than we 

 can suppose it to have been, there is no reason to believe that free silica, in the 

 form of quartz, ever did or ever could crystallize from a fused slag, such as this 

 primitive rock must have been. Quartz, in the shape of rock-crystal, or flint, when 

 fused, or even when long exposed to a heat much below its meltmg point, is 

 changed into an isomeric modification of silica, analogous to, if not identical with 

 opal, and distinguished from quartz by a much less specific gravity (about 2*2 

 instead of 2 "65), the absence of crystalline structure, and a much greater solubility 

 in alkalies, and hydrofluoric acid. Silica crystallized in the form of quartz has, 

 it is true, been repeatedly obtained by different reactions, but never hitherto 

 except in the presence of heated water or of watery vapour. Heinrich Rose, to 

 whom we are indebted for a careful study of this subject, records, that Gustaf 

 Rose, having submitted to partial fusion a granite rich in quartz, obtained a glass, 

 or obsidian, in which were enclosed unmelted portions of the quartz, converted, 

 however, into the less dense and more soluble opal-like form. These facts in the 

 history of silica were regarded by H. Rose as decisive against the notion of the 

 igneous origin of granite, which he concludes to be incompatible with the actual 

 state of our chemical knowledge. His paper on this subject, which should be read 

 by every geologist, appeared in Poggendorf's Anitalen for September, 1859, and a 

 careful abstract of it will be found in the L. E. and D. Philos. Mag. for January, 

 i860 [4] xix. 32), The new view of the origin of granite, which is there cited, as 

 maintained " particularly by Mr. Sterry Hunt," is, that all granite rocks are derived 

 from the alteration of sediments, containing, besides feldspathic or argillaceous 

 elements, quartz, derived, as I have explained, from the action of acid solutions 

 at high temperatures on the primitive crust of silicates. 



The geological evidences are multiplied, that gneiss, which does not differ 

 mineralogically from granite, and in its coarser varieties is constantly confounded 

 with it, is the result of the alteration in sitit of sedimentary rocks consisting of 

 quartz with feldspathic or argillaceous matters, the debris of pre-existing rocks. 

 Moreover, it is demonstrable that these stratified sediments have been softened, 

 and while in such a condition displaced or extravasated, and have thus taken the 

 form of exotic or eruptive rocks. Having by this or other means lost the mechanical 

 evidences of their former stratified condition, they are called granites. The same 

 view is, according to me, applicable to dolerites, diorites, and trachytes. Modern 

 lavas have no other origin, but take a different form, because they come to the sur- 



