52 THE CANADIAN NATURALIST.;;^ [Marcid 



distinguisting characters of original and derived rocks. The 

 further we go back in geological time, and the older the rocks are 

 which we are called on to classify, the greater is the difficulty of 

 doino' so, and the more divergent the opinions of geologists become 

 as to their origin. The stratigraphical relations of rocks are most 

 effective in determining this, but it will be necessary at present 

 to confine ourselves to considerations of a more purely petrological 

 nature. This is the more easily done, since the lithological 

 characters afford abundant means of recognizing original and 

 derived rocks, and distinguishing them from each other. 



Original rocks are made up of crystalline particles of one or 

 more minerals, principally silicates. These are seldom perfect in 

 crystalline form, are frequently more or less irregular or distorted, 

 and are intimately bound together to a compact whole, without 

 the intervention of any foreign substance as a cementing material. 

 They are thus mutually interlocked to a crystalline mass, which, 

 however, possesses at the same time an average mineralogical and 

 chemical composition. This would seem to indicate that the 

 mass must have been originally liquid, and, to some extent, in the 

 same condition during crystallization, otherwise it would have 

 been impossible for the various chemical constituents to move 

 toward the points where the minerals were being formed into 

 whose composition they enter. On the other hand, this liquidity 

 must have been somewhat limited in degree, for the minerals 

 seem to have pressed against each other, so as to have mutually 

 interfered with their crystalline development, and so as also to 

 have fitted perfectly into each other on complete solidification. 

 The size of the crystalline particles varies from a foot or more in 

 diameter down to that of microscopical minuteness. It is even 

 the case that they become so minute as to occasion a perfectly 

 vitreous structure which even the microscope is incapable of 

 resolving into distinct minerals. In all such cases, although the 

 rock can scarcely be termed crystalline, it remains, what its mode 

 of occurrence plainly shows, an original rock. 



Derived rocks are made up of the disintegrated fragments or 

 particles, and the chemical constituents of previously existing 

 rocks, abraded or dissolved away by water or other agents. 

 These fragments or particles are sometimes angular, sometimes 

 rounded off, and always bound together by means of an interven- 

 ing cememt, which is independent of, and may be altogether diffe- 

 rent in nature from, the enclosed fragments. They vary in their 



