56 THE ORIGIN OF MOUNTAINS. V. 



Professor Rogers has endeavored to explain by his theory of 

 earthquake-waves propagated through the igneous fluid mass 

 of the globe, and rolling up the flexible crust. We shall not 

 stop to discuss this theory, but call attention to another agency 

 hitherto overlooked, which must also cause contraction and 

 folding of the strata, and to which we have already elsewhere 

 alluded. (Am. Jour. Sci. (2), XXX. 138.) It is the conden- 

 sation which must take place when porous sediments are con- 

 verted into crystalline rocks like 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, staurolite, chiastolite, and chloritoid. This contrac- 

 tion can only take place when the sediments have become 

 deeply buried and are undergoing metamorphism, and i.s, 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 the gradual contraction of the interior. An able discussion 

 of this view will be found in the American Journal of Science 

 (2), III. 176, 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 Hennessey that the 

 instead of being a liquid mass covered with a thin en; 

 essentially solid to a great depth, if not indeed to the centre, 

 so that the volcanic and igneous phenomena generally ascrilxxl 

 to a fluid nucleus have their seat, as Keferstein and, after him, 

 Sir John Herschel long since suggested, not in the anhydrous 

 solid nucleus, but in the deeply buried layers of aqueous 

 ments, which, permeated with water, and raised to a hL'h 

 temperature, become reduced to a state of more or less com- 

 plete igneo-aqueous fusion. So that beneath the ouU-r 

 of sediments, and surrounding the solid nucleus, we may sup- 

 pose a zone of plastic sedimentary material adequate to explain 

 all the phenomena hitherto ascribed to a fluid nucleus. (Quar. 



