356 



z.v 



PHYSICAL GEOLOGY 



The Uinta Mountains of Utah are formed from a flattened dome 

 or broad arch 150 miles long and 20 to 25 miles wide, which rises 

 about 10,000 feet above sea level. It will be seen from the diagram 



Fig. 342. — A block diagram of a domed mountain, the Black Hills of South Dakota. 

 The investing valleys with their steep, infacing cliffs are well shown. The central 

 mountain mass is granite, and the three isolated mountains are intrusive masses of 

 igneous rocks. 



(Fig. 343) that if all the rock which has been carried away were 

 restored, the mountains would be three and a half miles higher than 

 now. This does not prove that the mountains were ever as high as 

 that, since the denudation of a mountain mass commences as soon 

 as it begins to rise above the surrounding country, and the rate of 

 erosion in all probability is about the same as the rate of upheaval. 



Fig. 343. — A section across the Uinta Mountains, Utah. The range has been 

 formed out of a single broad arch 40 miles wide, which has been greatly eroded. The 

 original surface is indicated by the dotted line, showing that three and one half miles 

 of rock have been removed by erosion. 



Complexly Folded Mountains. — It is to this class that the great 

 mountain systems of the world belong, the Appalachians, the American 

 Cordilleras (the Rocky, Sierra Nevada, Coast, and Cascade moun- 

 tains;, the Alps, Himalayas, Pyrenees, etc. The strata which compose 

 them may consist of a series of gentle anticlines and synclines (p. 254), 

 or may be intricately folded and faulted. Portions of the Jura 



