32 DE. THOMAS STBKRY HUNT ON THE 



1st. Ail gueisses, petrosilexes, hornbleiidic and micaceous schists, olivines, serpentines, 

 and in short, all silicated crystalline stratified rocks, are of neptunian origin, and are not- 

 primarily due to metamorphosis or to metasomatosis, either of ordinary aqueous sediments 

 or of volcanic materials. 



2nd. The chemical and mechanical conditions under vi'^hich these rocks were deposited 

 and crystallized, whether in shallow waters, or in abyssal depths (where pressure greatly 

 influences chemical affinities), have not been reproduced to any great extent since the 

 beginning of paleozoic time. 



3rd. The eruptive rocks, or at least a large portion of them, are softened and displaced 

 portions of these ancient neptunian rocks, of which they retain many of the mineralo- 

 gical and lithological characters. 



§ 63. In a subsequent paper, in 1880, it was said, with reference to the subaerial decay 

 of rocks: " The aluminous silicates in the oldest crystalline rocks occur in the forms of 

 feldspars, and related species, and are, so to speak, saturated with alkalies or with lime. It 

 is only in more recent formations that we find aluminous silicates either free or with 

 reduced amounts of alkali, as in the argillites and clays, in micaceous minerals like musco- 

 vite, margarodite, damourite and pyrophyllite, and in kyauite, fibrolite and andalusite ; all 

 of which we regard as derived indirectly from the more ancient feldspars." In connection 

 with this important point, which I had already discussed elsewhere, I added the following 

 note, referring at the same time to the propositions of the preceding paragraph : "- " It 

 is a question how far the origin of such crystalline aluminous silicates as muscovite, 

 margarodite, damourite, pyrophyllite, kyanite, fibrolite and andalusite, is to be sought in a 

 process of diagenesis in ordinary aqueous sediments holding the ruins of more or less com- 

 pletely decayed feldspars. Other aluminous rock-forming silicates, such as chlorites and 

 magnesian micas, are, however, connected, through aluminiferous amphiboles, with the 

 non-aluminous magnesian silicates, and to all of these various magnesian minerals a very 

 different origin must be ascribed." 



In a farther discussion of this subject, in 1883, it was noted " that decayed feldspars, 

 even when these are reduced to the condition of clays, have not, in most cases, lost the 

 whole of their alkalies." •^'^ This was shown by the analyses made by Sweet, of the 

 kaolinized gi-anitic gneisses of Wisconsin, from which it appears that " the levigated clays 

 from these decayed rocks still hold, in repeated examples, from two to three hundredths or 

 more of alkalies, the potash predominating." 



§ 64. The question of the source of the matters in aqueous sohition which, according 

 to the hypothesis before us, gave rise to granitic A^einstones, naturally comes up at this 

 stage of our inquiry. As we have seen, the granitic substratum of igneous origin, the 

 existence of which is postulated by most modern geologists is, since the time of Scrope, 

 Scheerer and Élie de Beaumont, generally conceived to be impregnated with a portion of 

 water, conjectured by Scheerer to equal perhaps fiA^e or ten hundreths of its weight ; and 

 through the iuterA'ention of this to assume, at temperatures far below the point of liquefac- 



*^ The Chemical and Geological Relations of the Atmosphere, Amer. Jour. Science, xix, 354. See farther, for the 

 stratigraphical relations of the various aluminous silicates, (whicii were first set forth by the author in 1863), Chem. 

 and Geol. Essays, pp. 27 and 28 ; also Report E, Second Geological Survey of Pennsylvania, (1878) p. 210. 



^ The Decay of Rocks Geologically Considered, Amer. Journal Science, (1883) xxvi, 194. 



