366 



DYNAMICAL GEOLOGY. 



cline is stated to be a magnificent arch of Cambrian, Silurian, and Devonian ; the Little 

 Elko, Cortez, Shoshone, Pah-Ute, and other ranges. The same flexed condition of the 

 beds is mentioned by I. C. Russell as existing in the ranges of the Oregon part of the Great 

 Basin. 



The ranges of the Great Basin have many faults as well as flexures, as 

 described by Gilbert in 1876; and these faults are generally downthrow 

 faults. The following are two of his figures ; they illustrate two ridges made 

 up of blocks displaced as described. The dip and the downthrow faults are 

 in opposite directions. 



338. 



East- 



339. 



Fig. 338, section of Pahranagat Range at Silver Cafion, southern Nevada, scale 55^5. Fig. 389, section of 

 Timpahute Range, west of the Pahranagat, scale y^Jni,. Gilbert, '76. 



Gilbert, in view of the great displacements by nearly vertical and largely 

 downthrow faults, designated the system of mountain-forming movements 

 the " Great Basin System." He shows that the displacements are along old 

 fault planes, and also along new planes of fracture made in the course of the 

 Tertiary era, and later. 



Great displacements along old and new fault planes have been shown 

 to have taken place also in the high plateaus of Utah and in the Uinta 

 Mountains, others in the Wasatch, and still others in the Sierra Nevada, 

 which are referred to the Great Basin System. The fact of such move- 

 ments extending into recent time has been urged by Powell, Gilbert, Rus- 

 sell, Le Conte, Diller, and others. 



The ridges of the Great Basin, made thus of upturned and plicated rocks, have been 

 assumed to be each limited by faults, and to have undergone up and down movements, and 

 variously tilting displacements, and thus to have become in effect " monoclinal orographic 

 blocks" in the " Basin System," ■ — each block making by itself a monoclinal mountain, 

 even when not so in its bedding (Russell, 1885). In the ideal sections made to illustrate 

 this hypothesis, the wide intervals of alluvium (that is, of buried and concealed rock) are 

 represented as underlaid each by a block at lower level, or by the subterranean continu- 

 ance of one sloping ridge to the next ; and the actual flexures or lines of bedding have 

 been omitted, and monoclinal lines substituted. They are intended to exhibit the sup- 

 posed structure. But until the stratigraphy of the ridges of the whole basin shall have 

 been studied and sections of them represented, and the relations of each ridge to those 

 lying on the same northward or northwestward line of strike shall have been thoroughly 

 investigated, general stratigraphic conclusions cannot be safely drawn. 



