of the Structure of the Basin Region. 263 



Basin ranges, for example those of S. E. Oregon, were indeed 

 wholly formed at that time and in the manner already ex- 

 plained. But the Sierra, the Wahsatch and many of the Basin 

 ranges existed before that time. The Sierra was born from 

 the sea by the folding and ups welling of thick sediments at 

 the end of the Jurassic. Many ranges in the Basin region 

 were formed at the same time and in the same way. The 

 Wahsatch was similarly formed, probably about the end of the 

 Cretaceous. But at the end of the Tertiary the greatly eroded 

 land surface previously formed in this region was arched and 

 broken and readjusted, forming these ranges in their present 

 condition, as already explained. 



In an article published in 1872,* entitled " Theory of the 

 formation of the greater features of the earth's surface." I 

 showed that mountain ranges were formed by lateral pressure 

 acting upon thick sediments folding and swelling up the mass 

 along the line of yielding. In another article published in 

 1878,t I further developed the same views and tried to show 

 that even the Basin ranges — claimed by Gilbert as belonging to 

 a different type and formed in a different way — were no excep- 

 tion ; that they also were formed by lateral pressure ; only 

 that in this case the crust of the earth being rigid would not 

 yield by mashing, but only by arching — the blocks of the 

 broken arch readjusting themselves to form the orographic 

 features already described ; and therefore that mountain 

 ranges are all of one type and formed in one way, viz : by 

 lateral pressure. I now feel compelled to modify this state- 

 ment. It is evident from the character of the faults i. e. 

 normal instead of reverse faults, that the arch was not formed 

 by lateral pressure but by tension of lifting. Therefore, I 

 now believe that mountain ranges are of two types : (1.) Those 

 formed by lateral crushing and folding, and (2.) those formed 

 by adjustment of crust-blocks. The one produces reverse 

 faults, the other normal faults. The best types of the one are 

 the Appalachian, the Alps, and the Coast ranges of California ; 

 the best types of the other are the Basin ranges. Very often 

 the two types are mixed, or one is superposed on the other — 

 the one or the other predominating. This is the case with the 

 Sierra, the Wahsatch, and to some extent with many of the 

 Basin ranges. 



Note. — Since writing the above my attention has been called 

 to the fact that Hopkins, in his "Researches in Physical Geology" 

 (Phil. Trans, for ] 842, p. 53), gives a similar explanation of normal 

 faults in the case of wedge-shaped blocks, but he says nothing of 

 the far more common case of rhomboidal blocks. 



* This Journal, vol. iv, p. 345-460. 

 f This Journal, vol. xvi, p. 95. 



