on American Geology. 103 



gneiss and mica slate, and still more when the elements of these 

 sediments are changed into minerals of high specific gravity, such 

 as pyroxene, garnet, epidote, staurotide, chiastolite and chloritoid. 

 This contraction can only take place when the sediments have be- 

 come deeply buried and are undergoing metamorphism, and is, as 

 many attendant phenomena indicate, connected with a softened 

 and yielding condition of the lower strata. 



"We have now in this connection to consider the hypothesis 

 which ascribes the corrugation of portions of the earth's crust to 

 tiie gradual contraction of the interior. An able discussion of 

 this view will be found in the American Journal of Science (2) iii. 

 1*70, from the pen of Mr. J. D. Dana, who, in common with all 

 others who have hitherto written on the subject, adopts the notion 

 of the igneous fluidity of the earth's interior. 



We have however elsewhere given our reasons for accepting the 

 conclusion of Hopkins and Hennessy that the earth, instead of 

 being a liquid mass covered with a thin crust, is essentially solid 

 to a great depth, if not indeed to the centre, so that the volcanic 

 and igneous phenomena generally ascribed to a fluid nucleus have 

 their seat, as Keferstein and after him Sir John Herschel long 

 since suggested, not in the anhydrous solid unstratified nucleus, 

 but in the deeply buried layers of aqueous sediments which, per- 

 meated with water, and raised to a high temperature, become 

 reduced to a state of more or less complete igneo-aqueous fusion. 

 So that beneath the outer crust of sediments, and surrounding 

 the solid nucleus, we may suppose a zone of plastic sedimentary 

 material adequate to explain all the phenomena hitherto ascribed 

 to a fluid nucleus. (Quar. Jour. Geol. Society, Nov. 1859. 

 Canadian Naturalist, Dec. 1859, and Amer. Jour, Sci.(2)xxx. 136.) 



This hypothesis, as we have endeavoured to show, is not only 

 completely conformable with what we know of the behaviour 

 of aqueous sediments impregnated with water and exposed to a 

 high temperature, but ofl"ers a ready explanation of all the 

 phenomena of volcanos and igneous rocks, while avoiding the 

 many difiiculties which beset the hypothesis of a nucleus in a 

 state of igneous fluidity. At the same time any changes in volume 

 resulting from the contraction of the nucleus would affect the 

 outer crust through the medium of the more or less plastic zone 

 of sediments, precisely as if the whole interior of the globe were 

 in a liquid state. 



The accumulation of a great thickness of sediment along a 



