232 THE CANADIAN NATURALIST. [May 



stratified rock of mechanical or of chemical origin. These would 

 now so deeply cover the partially cooled surface that the amount 

 of heat escaping from below is inconsiderable, although in earlier 

 times it was very mnch greater, and the increase of temperature 

 met with in descending into the earth must have been many times 

 more rapid than now. The effect of this heat upon the buried 

 sediments would be to soften them, producing new chemical 

 reactions between their elements, and converting them into what 

 are known as crystalline or metamorphic rocks, such as gneiss, 

 greenstone, granite, etc. We are often told that granite is the 

 primitive rock or substratum of the earth, but this is not only 

 unproved, but extremely improbable. As I endeavoured to show 

 in the early part of this discourse, the composition of this primitive 

 rock, now everywhere hidden, must have been very much like that 

 of a slag or lava ; and there are excellent chemical reasons for 

 maintaining that granite is in every case a rock of sedimentary 

 origin — that is to say, it is made up of materials which were 

 deposited from water, like beds of modern sand and gravel, and 

 includes in its composition quartz, which, so far as we know, can 

 only be generated by aqueous agencies, and at comparatively low 

 temperatures. 



The action of heat upon many buried sedimentary rocks, how- 

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

 disengagement of gases, such as carbonic and hydrochloric acids, 

 and sulphur compounds, all results of the reaction of the elements 

 of sedimentary rocks, heated in presence of the water which every- 

 where 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 disengagement 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 like granite and 

 basalt. 



This theory of volcanic phenomena was put forward in germ by 

 Sir John F. W. Herschel thirty years since, and, as I have dur- 

 ing the past few years endeavoured 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 sedimen- 

 tary deposits of the earth, and not in its primitive nucleus, 



