OF CENTRAL CANADA PART JII. 



175 



FIG. 92. 



overlying step-like and columnar sheets of matter. And in these 

 conditions, they are frequently seen to traverse older rocks of the 

 same class, or to penetrate various stratified formations. They are 

 thus essentially intrusive rocks ; and they are also, in the words of 

 Humboldt, essen- 

 tially endogenous 

 rocks i. e., they 

 come from more 

 or less deeply- 

 seated sources 

 within or beneath 

 the Earth's crust, 

 from whence they 

 have been forced 

 up from time to time through cracks and fissures opened in the 

 overlying or surrounding rock masses. From this it follows, as a 

 general rule, that the intrusive rocks in question must have been at 

 one time in a soft and plastic state, if not in an actually fluid condi- 

 tion. Certain trachytic and basaltic rocks members of this group ; 

 described below cannot be distinguished by chemical or minera- 

 logical characters from ordinary lavas ; and the former existence of 

 many basalts in a molten or highly heated condition is established by 

 the effects produced by veins or dykes of these rocks on coal beds 

 and other strata through which they have been erupted. Coal in 

 contact with dykes of this kind, has been burnt into cinder, or 

 converted into coke ; clays have been baked into brick-like masses ; 

 sandstones rendered more or less vitreous ; and various limestones, 

 to cite no further instances, have been hardened and altered into 

 marbles of crystalline texture. Intrusive veins and masses of granite 

 and syenite are also known to have produced metamorphic effects on 

 the rocks which they traverse. But in many instances no alterations 

 of this kind have followed the intrusion of a vein or mass of unstra. 

 tified rock amongst sedimentary deposits. Hence it is clear that 

 although the intrusive rock must have been in a soft or plastic 

 condition, it could not in these latter cases have been in a molten or 

 intensely heated sta'te. Occasionally also, solid granitic' masses 

 appear to have been thrust up amongst overlying strata, the intru- 

 .sion being followed necessarily by signs of great mechanical 

 disturbance. The condition of the quartz in granite and syenite, is 



