THE MATERIALS OF MOUNTAIN CHAINS. 1 9 



These folds once formed remain for all time. 

 They may b r, or depressed beneath 



the sea, and new rocks laid down upon them; 

 but as those ancient folds increase in intensity 

 with the slow - ssi >n of geoloj . the 



newer : \i\vd with their folds, and 



the folds run in the same direction. 



In such puckered and crumpled rocks as moun- 

 tain chains exhibit on their denuded heights, there 



Jmost invariably evidence of a Crystalline tex- 

 ture. This may be attributed to the influence of 

 the heat produced by the mechanical power, trans- 

 formed by the resistance which the rock mass of- 

 fered to compression. 



The rocks which form mountains are chiefly 



5, with here and there Some 



lite masses or sheets of vol >ck. They 



have only been laid bare by the removal of vast 



thicknesses of water-formed rock which once ex- 

 tended above them. If the crystalline materials 



not th- products of the upward 



thrust of the mountain chain and adjacent land 

 which supports it, it may be difficult to account 

 for the uniform ch of the rocks of which 



the durable central masses of mountain chains are 



built. There ai s in this process of change. 



The thinks of a mountain range commonly shom 



the hue mil 3 LC crystalline texture of slate, 

 while the central masses show the coarse crystal- 

 line texture of schist, or granite. 



Slate. — The part which slate plays in the forma- 

 tion of mountain n - well seen in the struc- 

 ture of the mountainous regions of North and 

 Central Wales, in parts of the Lake District in 

 Westmoreland and Cumberland, and in the south 

 of Scotland. It is certain that slate was originally 



