﻿236 



Canadian Record of Science. 



These deposits are accompanied by a number of volcanic 

 rocks, with which, on account of the shortness of our 

 time, we cannot further concern ourselves here. 



Out of such various kinds of rocks are formed moun- 

 tains, hills and plains. 



It was formerly believed that mountains w^ere formed 

 by elevation — that some sort of power forced them up 

 from below and then left them standing in the form of 

 folds, blocks, plateaux, etc. To-day it is known that such 

 is not the case. The majority of geologists are of opinion 

 that mountains cannot originate in this way, that nature 

 does not exert such a power, but that the causes of the 

 relief are to be found primarily in the shrinkage wdiich 

 the earth has as a whole undergone during the long time 

 of its cooling. An actual shortening of the radius takes 

 place, small indeed as compared with the diameter of the 

 earth, but which is the cause of two kinds of phenomena : 

 in some places portions of the crust fall in producing, for 

 instance, the abysses of the oceans ; in other places it may 

 be observed that owing to the contraction of the interior, 

 portions of the crust have become too large, and have 

 therefore crumpled up, as we find in the Alps. Fre- 

 quently these folds are pressed against more rigid portions 

 of crust, which do not fold with them. 



Mountain folds are sometimes turned completely over. 

 In many cases they form a series more or less regular 

 synclinals alternating with anticlinals. Denudation, that 

 is, the destructive action of running water, of frost, of ice, 

 etc., has cut into this folded mass, producing the relief as 

 it exists to-day, and in which may still be distinguished 

 the anticlinals and the synclinals. Thus, we see upon the 

 surface of the^ earth table-lands and plateaux such as 

 Central Eussia and the Sahara, and highly folded masses 

 such as the Alps and the Pyrenees. The folded areas are 

 generally very long, and follow definite curved lines 

 called axes. 



