56 Dr. T. Sterry Hunt — On Chemical Geology, 



face, and are rapidly cooled, instead of being slowly solidified under the pressure 

 of superincumbent strata. The fact that every eruptive or exotic rock (with the 

 exception of certain rapidly cooled lavas) has its mineralogical equivalent among 

 indigenous crystalline rocks, that is to say among sedimentary strata of chemical 

 or mechanical origin, is a powerful argument in support of the view here put for- 

 ward. In connection with this, I have shown that a combination of chemical and 

 mechanical agencies naturally and inevitably leads to the division of aqueous sedi- 

 ments into the two great types to which lithologists refer all eruptive rocks, namely, 

 the acid, granitic or trachytic and the basic, doleritic groups, which are supposed 

 to form the two zones of igneous rock imagined by Phillips, and since insisted upon 

 by Durocher, Bunsen, and Forbes. As all of these crystalline rocks are, according 

 to my hypothesis, ancient sediments, it follows that water has been present among 

 them from their first deposition, and during all the subsequent processes of their 

 heating, softening, crystallization, and ejection, — a view constantly insisted upon by 

 me, and in accordance with the ideas maintained by Scheerer and subsequently by 

 Sorby. This theory of igneous rocks, although suggested by Keferstein in 1834, 

 and by Sir J. F. W. Herschel in 1837, has been elaborated by me in various papers 

 for the past ten years. 



See Theory of Igneous Rocks and Volcanos, Canadian Journal^ March, 1858 ; 

 Some Points in Chemical Geology, Quart. Journ. Geol. Soc.^ Nov. 1859 ; Chemistry 

 of the Earth, Cojuptes Rendiis,]\xne gth, 1862; Chemistry of Metamorphic Rocks, 

 Diiblm Quatt. Journ.^ July, 1863 ; Contributions to Lithology, Part i, American 

 Journ. Science^ March, 1864. 



In view, then, of my theory of the derived and sedimentary origin of all eruptive 

 rocks, what does Mr. Forbes mean when he inquires whether I am aware of the 

 immense masses "of volcanic rocks (trachytes) scattered all over the face of the 

 globe, which contain abundance of free quartz" ? If he will refer to my Contribu- 

 tions to Lithology, just cited, he will find that I have insisted upon the presence 

 of quartz in trachytes, and also upon the fact that such trachytes pass into granites, 

 from which they differ only in structure {American yoiirn. Science [2] xxxvii. 260). 

 The obvious conclusion to be drawn from the presence of quartz in granites and 

 trachytes is, that neither during nor subsequent to crystallization have these rocks 

 been subjected to a temperature sufficiently elevated to alter the quartz in the 

 manner observed by Rose. In the paper last cited, I have devoted two pages to 

 an analysis of the beautiful researches of Sorby on the microscopic structure of 

 crystals, about which Mr. Forbes talks, though evidently without any conception 

 of their geological bearing. Mr. Sorby, (who makes of the cavities partially filled 

 with watery solutions, which occur in many crystals, thermometers which registered 

 the temperature at which these crystals were formed,) concludes that the quartz, 

 mica, feldspar, and tin-stone of the Cornish veins "were deposited from water 

 holding various salts and acids, at temperatures varying from 200" centigrade to a 

 low red heat," about 340° ; while, for some minerals from Vesuvius, which present, 

 besides cavities holding liquids, others filled with stony and glassy matters, he 

 deduces a temperature of from 360° to 380°, and concludes them to have been 

 formed "at a dull red heat, under a pressure of several thousand feet of rock, 

 when water, containing a large quantity of alkaline salts in solution, was present, 

 along with melted rock and various gases and vapors. I therefore think," he says, 

 " we must conclude provisionally, that at a great depth from the surface, at the foci 

 of volcanic activity, liquid water is present along with the melted rocks, and that 

 it produces results which would not otherwise occur." {Quart. Journ. Geol. Soc. 

 xiv. 483). One of those results, as is evident from the above citation, is the reduc- 

 ing of rocky matters to a melted condition at a dtdl red heat, a point to be borne in 

 mind when Mr. Sorby speaks in his paper of igneous fusion in this connection. A 

 true igneous fusion of such matters, zuitkout water, would, as every one knows, 

 require a vastly higher temperature ; and I have elsewhere, after Scheerer, described 

 this softening of mineral matters under the combined influences of water and heat, 

 as an igneous fusion. 



Mr. Sorby has, moreover, calculated the temperature at which the quartz crystals 

 in the trachyte of the Ponza Islands, cited by Forbes, were formed, and finds it to 

 be 360°, they being, in fact, generated under like conditions with those of the 

 quartz of granite veins, from which Mr. Sorby rightly concludes to a similarity of 

 origin between trachytes and granites. That both have crystallized at temperatures 



