2G6 J. E. SPURR — ORIGIN AND STRUCTURE OF THE BASIN RANGES 



rapid. But folding, <and especially faulting, are of irregular speed, while 

 erosion is more uniform, so tliat local deformation ma}' oiftstrip erosion 

 for a time, but in any long period is likely to be beaten itself. Thus of 

 the faults actually observed in the Great basin, among them even some 

 post- Pliocene faults, nearly all have no di7'eci expression in the tojiog- 

 raphy. Nevertheless, some Pleistocene faults are expressed in the to- 

 pography, and these have sometimes a very great displacement, as in 

 the Colorado plateau. 



In the case of the Grapevine mountains tlie range appears to have been 

 already in existence in pre-]^]ocene or Cretaceous times. In someof tlie 

 mountains of erosion which have not suffered comparatively recent de- 

 formation, as has the range just mentioned, their ancient origin is still 

 more clear, for the Tertiary beds rest against their bases unconformably, 

 generall}' occup3nng only the present valleys and not being found in the 

 mountains. Therefore the main svstem of ranges was probabl}' laid out 

 in pre-'J'ertiar}'^ times. It was at the close of the Jurassic, so far as we 

 can judge from the rather scant evidence, that the Great Basin region 

 was uplifted above the sea and plicated. During the Cretaceous the 

 region was probably a land-mass, and this was very likely the period 

 when man}' of the ranges were differentiated. Their regularity is no 

 more than one would expect in view of their resulting from parallel 

 folds, and is not so striking as in the Appalachian region. To explain 

 this early dissection we must assume a greater precipitation than now 

 and must conclude that the country was traversed by many rivers. 

 Subsequently the climate became arid and the water sup})ly was not 

 sufhcient to remove the detritus frotn the valleys, which filled up, so 

 that for a long period the reverse of differentiation, or leveling, has been 

 ^oing on. Nevertheless the Tertiary rocks show that warping, folding, 

 and faulting went on continuously all through the Tertiary into the 

 Pleistocene, and is even now progressing. 'I'o this warping, doubtless, 

 the Tertiary lake basins were due. 



According to this conclusion these mountains are not simple in origin 

 and structure. However, the. writer would compare the typical Basin 

 range to the less compressed portions of the Appalachians and the Alps. 

 Among the exceptional types of ranges the Funeral Range type is con- 

 spicuous. This has a structure similar to that of a portion of the Jura 

 mountains, as described and figured by Professor Davis.* 



♦physical Geography, 1899, p. 108. 



