44 THE CHEMISTRY OF THE PRIMEVAL EARTH. [IV. 



position quartz, which, so far as we know, can only be gener- 

 ated by aqueous agencies, and at comparatively low tem- 

 peratures. 



The action of heat upon many buried sedimentary rocks, 

 however, not only softens or melts them, but gives rise to a 

 great disengagement of gases, such as carbonic and hydrochlo- 

 ric acids, and sulphur compounds, all of which are results of the 

 reaction of the elements of sedimentary rocks, heated in pres- 

 ence of the water which everywhere filled their pores. In the 

 products thus generated we have a rational explanation of the 

 chemical phenomena of volcanoes, which are vents through 

 which these fused rocks and confined gases find their way to the 

 surface of the earth. In some cases, as where there is no dis- 

 engagement of gases, the fused or half-fused rocks solidify in 

 situ, or in rents or fissures in the overlying strata, and constitute 

 eruptive or plutonic rocks, such as granite and basalt. 



Tliis theory of volcanic phenomena .was put forward in germ 

 by Sir John F. "W. Herschel thirty years since, and, as I have 

 during the past few years endeavored to show, it is the one 

 most in accordance with what we know both of the chemistry 

 and the physics of the earth. That all volcanic and plutonic 

 phenomena have their seat in the deeply buried. and softened 

 zone of sedimentary deposits of the earth, and not in its primi- 

 tive nucleus, accords with the conclusions already arrived at 

 relative to the solidity of that nucleus; with the geological 

 relations of these phenomena, as I have elsewhere shown ; and 

 also with the remarkable mathematical and astronomical de- 

 ductions of the late Mr. Hopkins of Cambridge, based upon the 

 phenomena of precession and nutation, those of Archdeacon 

 Pratt, and those of Professor Thompson on the theory of the 

 all of which lead to the same conclusion, namely, 

 that the earth, if not solid to the centre, must have a crust sev- 

 eral bundled miles in thickness, which would practically ex- 

 din I- it from any participation in the plutonic phenomena of 

 the earth's surface, except such as would result from its hi^h 

 temperature communicated by conduction to the sedimentary 

 strata reposing upon it 



