Chap. XVIII.] THE SURFACE OF THE EARTH. 203 



lateral pressure has continued, whilst the granite 

 was becoming cool and hard on the surface, and 

 that as soon as its surface became too hard and 

 brittle to bend further, it cracked along the top of 

 a bending ridge, the upper edges parting asunder, 

 and leavmg a deep wedge-shaped gorge. The debris 

 from the sides and part adjacent to the crack has 

 filled up the lower part of it, and formed the present 

 bottom of the valley. 



There are some trees which, when young, are 

 covered with a smooth thin bark ; but as the tree 

 grows the pressure from within bursts the first 

 layer, disclosing through the crack a new layer, 

 proportioned to the mcreased growth of the tree. 

 Thus layer after layer forms and bursts ; and, as 

 the old layers continue to adhere to the tree, ridges 

 are formed which as each successive layer is added 

 get higher, and the hollows between them wider and 

 deeper, until at length the bark of the old trees 

 presents that rough, uneven surface, which at first 

 sight looks so strange. Now, glance at the bark, or 

 outer crust of the earth — it also presents a rough, 

 uneven surface, rising and falling in mountains and 

 valleys. The cause of this unevenness in the sur- 

 face of the earth, though diametrically opposite, is 

 no less simple, and no more wonderful than the cause 

 of the unevenness m the surface of the tree. The 

 latter is an expanding force — the former is a con- 

 tracting force. As contraction proceeds in the 



