ORIGIN OF FOLDED MOUNTAINS — PROUTY 295 



Kurile Arc, and tlie Japanese Arc, have their convex side toward the 

 ocean foredeep. 



9. Most folded mountains have thrust faults and nnsymmetrical 

 folds. 



10. All old mountains have been worn down, and most of them 

 have been reelcvated — some many times. 



One of the questions most frequently asked is in regard to the 

 origin of these earth highlands. Before we attempt to answer this 

 question we must understand that inountains are of diil'erent origins. 

 We have mountain peaks of the type of Vesuvius or Shasta, which 

 have been built up from the earth's surface by volcanic ash and lava 

 flow. Others like the Hawaiian Islands have been built up by a 

 succession of lava flows from the ocean floor, in this case to a height 

 of over 30,000 feet. 



We have mountains made of great blocks of the earth's crust, 

 faulted and tilted on edge and raised relatively above the blocks 

 which have been down-faulted. The High Sierras, the Wasatch, and 

 many of the mountains in the Great Basin area are examples of 

 these so-called " block mountains." 



We have the so-called " laccolithic mountains ", which have been 

 formed by the injection of great masses of igneous rock into the 

 earth's crust and the arching of the earth's surface above such in- 

 jected masses, as for example the Henry Mountains of Utah or the 

 Crazy Mountains of Montana. 



All elevated land masses are being constantly reduced by the agents 

 of erosion, chiefly running water and glaciers. The least eroded por- 

 tion of such elevated land masses, irrespective of cause of elevation, 

 are left as so-called " erosional mountains ", as Mount Mitchell, 

 Monadnock, and the Matterhorn. 



The most general type of mountain, however, is the one which is 

 made up mostly of folded rock. All the great mountain ranges of 

 the world belong to this type, though they may be locally modified 

 by injection, faulting, or erosion into other types. 



The origin of these great folded mountains has engaged the atten- 

 tion of many men of science. Strong dift'erences of opinion have led 

 to controversies, and these in turn have led to detailed studies of 

 " Mother Earth." This study has been carried on by men of various 

 sciences, but chiefly geolog}', physics, mathematics, and astronomy. 

 The field is too broad to be mastered by any one grouj) of scientists. 



Before we undertake to discuss the origin of folded mountains, let 

 us review briefly some of the known facts and current opinions con- 

 cerning the nature of the earth's interior: 



1. Under the influence of tidal forces the earth is at present as 

 rigid as glass or as steel, and therefore largely a solid. 



