MOUNTAINS; MINING; FORESTRY 203 



have directed the flow of surface waters, which by erosion 

 have made great changes. Cracks in the rocks give en- 

 trance to air and water, and thus erosion begins before 

 uplifting ceases. Some minerals in the rocks are more 

 soluble than others and some are softer; thus the rocks 

 wear unequally. The result is a group of peaks or ridges 

 of resistant rock with ravines and valleys of different 

 depths between. The Catskills and the mountains within 

 the great canyon of the Colorado in Arizona are illustra- 

 tions of mountain formation by erosion. 



FIG. 104. FORMATION OF BLOCK MOUNTAINS 

 a, 6, c, d = four rock strata, t = fragments of rock called talus. 



1. Name at least three changes which have occurred in these strata 

 since they were made at the bottom of the sea. 2. Account for the presence 

 of the talus. 3. Under what conditions might a lake be formed at I ? 



226. Block Mountains. Another form of elevation was 

 caused by a series of breaks in the strata; one side of the 

 break was pushed up or the other fell. . The result is a suc- 

 cession of ridges having a steep slope on one side and a 

 gentle slope on the other. These are called block mountains, 

 because at the time of the uplift the strata were broken into 

 blocks. The mountains of the Great Basin in Utah are of 

 this origin. 



227. Folded Mountains. A third form of mountain was 

 made by a series of folds. As fragments of rock worn from 

 the land were brought to the ocean and deposited along the 

 continental shelf, they were consolidated by great pressure, 

 and cemented by heated waters which contained dissolved 



