264 J. E. SPURR ORIGIN AND STRUCTURE OF THE BASKST RANGES 



displacement has taken place since the beginning of the great volcanic outbursts 

 in the vicinity of Lassens peak. If we may accept numerous small earthquake 

 shocks as evidence, the faulting still continues." 



Besides the writers above mentioned, the theory of the direct origin 

 of the Great Basin ranges by faulting has been accepted and elaborated 

 by Professor Le Conte and others. 



In opposition to this general acceptation Professor James D. Dana 

 examined the question and found the data insufficient for the proof of 

 the fault hypothesis. He wrote in 1895 as follows : * 



" 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 the 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 Uintah mountains, others 

 in the Wasatch, and still others in the Sierra Nevada, which are referred to the 

 Great Basin system. The fact of such movements extending into recenttime has 

 been urged by Powell, Gilbert, Russell, 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 continuance of one sloi)ing 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 supposed structure ; but until 

 the stratigraphy of the ridges of the whole basin shall have been studied and sec- 

 tions of them represented and the relations of each ridge to those lying on the 

 same northward and northwestward line of strike shall have been thoroughly 

 investigated, general stratigraphic conclusions can not be safely drawn." 



Analysis of the Fault Hypothesis 



The present writer, seeking the foundation of facts on which rests tlie 

 fault theory of the origin of the Basin ranges, has been obliged to fall 

 back almost wholly on the original work of Mr Gilbert, who appears to 

 have conceived the hyj)othesis. The views of subsequent investigators 

 seem to have been based chiefly on the a priori assumption that all the 

 scarps of the region were necessarily fault-scari)s, and much of the lit- 

 erature has been i)re])ared with little or no personal acquaintance with 

 the field. 



* Mauual of Geology, 4th edition, p. 366. 



