266 STRUCTURE COMMOX TO ALL ROCKS. 



boundary of the Basin region, the Wahsatch was at the same time also 

 heaved up on its western side, forming there one of the greatest faults 

 known. Therefore, so far as their present forms are concerned, the 

 Sierra and Wahsatch may be said to belong to the Basin system. It 

 is not difficult to imagine how the whole system may have been formed. 

 At the end of the Tertiary the whole Basin region, including the Sierra 

 on one side and the Wahsatch on the other, was lifted probably by 

 intumescent lavas into an arch, and by tension split into great oblong 

 crust-blocks. The arch broke down, the crust-blocks readjusted them- 

 selves, as explained on page 233, to form the Basin ranges, and left 

 the abutments, viz., the Sierra and the Wahsatch, with their raw faces 

 looking toward one another across the intervening Basin. The pro- 

 cess and result are shown in the ideal diagram (Fig. 232). It must not 

 be imagined, however, that this took place at once as a great cataclysm, 

 but rather that it took place very slowly — the lifting, the breaking 

 down, and the readjustment, all going on at the same time. 



/\ S 1 N REG I O N 



Fig. 232. — Diagram showing Probable Origin of the Basin System. 



Thus, then, there are two types of mountains strongly contrasted, 

 mountains of the one type are formed by lateral pressure and crushing, 

 of the other type by lateral tension and stretching. The one gives rise 

 mainly to reverse faults, the other always to normal faults. Mountains 

 of the one type are formed by ups welling of thick sediments, those of 

 the other type by irregular readjustment of crust-blocks. Mountains 

 of the one type are born of the sea, those of the other type are born on 

 the land. We find examples of the one type in nearly all the greatest 

 mountains everywhere, but especially in the Appalachian, the Alps, and 

 the Coast Bange. The best examples, perhaps the only examples, of 

 the other type are the Basin ranges. Some mountains, as the Sierra, 

 the Wahsatch, and certainly some of the Basin ranges, belong to both 

 types. In their origin, they have been formed in the first way, but 

 afterward have been modified by the second way. Thus the first is the 

 fundamental method, and the second only a modifying process.* 



Mountain Sculpture. 



As soon as a mountain-range lifts its head above the general level 

 of sea or land, it is immediately attacked by erosion. All the grand 



* On this whole subject see papers by the writer, American Journal of Science, vol. 

 xix, p. 170, 1880; vol. xxxii, p. 167, 1866. 



