360 



PHYSICAL GEOLOGY 



Colorado is estimated to be about 25 miles, and that of the Coast 

 Ranges of California, 9 to 12 miles (Fig. 349). 



A careful study of folded regions shows that the strata are often 

 broken and faulted, the folds frequently giving place to thrust or 



Fig. 348. — Folds in the Appalachian Mountains between Harrisburg and Tyrone. 

 (As restored by R. T. Chamberlin.) 



reverse faults, especially where the strong or competent stratum is 

 not deeply buried. As already stated in the discussion of reverse 

 faults (p. 263), the overriding of the strata is sometimes 10 or more 

 miles. In fact, in the southern Appalachians thrust faults are so 

 numerous as largely to determine the positions of the mountain 

 ridges (Fig. 347, p. 358), and the elevation in the Scottish and 



% W "11 



Fig. 349. — Profile of the Santa Cruz Mountains in the Coast Ranges of southern 

 California. (Arnold.) 



Scandinavian Highlands is due, to some degree, to the fault slices piled 

 one on top of another. 



Igneous rock is often associated with mountains and in some 

 ranges is an important factor in the folding and metamorphism of 

 the strata. It often composes the cores of mountain ranges and 

 frequently forms their highest portions (p. 355). (1) The igneous 

 core of mountain masses is often derived from igneous intrusions ; (2) 

 it may be the rock of the floor of the geosynclines ; or (3) it has even 

 been suggested that it is sometimes the lower portion of the sediments 

 of the geosyncline which have been fused as a result of the rise of 

 temperature (p. 348) from the interior of the earth. (Haug.) In 

 each of these cases the igneous core is exposed only after erosion has 

 removed a great thickness of overlying sedimentary rock. 



Experiments in Mountain Building. — Experiments have been 

 performed to determine whether the folds and reverse faults observed 

 in such mountains as the Appalachians can be reproduced. In these 

 experiments a series of layers composed of wax and other substances 

 varying in rigidity and elasticity were prepared to represent rock 

 strata of widely different character, such as shale, sandstone, and 



