CENOZOIC HISTORY AND THE VOLCANICS 345 



and in complicated series. Mere lithological similarity or even identity 

 is not alone a sufficient criterion of contemporaneity of extrusion. 



It will have been noticed that all the faults which appear in the sec- 

 tions, photographs, or descriptions as breaking through the volcanics, 

 including the Sierra Nevada fault just described, show simple* fault 

 scarps. In his discussion of the fault hypothesis Spurrf says, " The 

 writer has undertaken to show * * * that the ascertainable faults 

 are very rarely attended by simple fault-scarps," and he gives a number 

 of sections showing faults determined by stratigraphic discordance in 

 the Paleozoic rocks, which show either no fault scarp or an erosion scarp. 

 None of these sections include Tertiary or later rocks. The only sections 

 which he presents showing later rocks are figures 1 and 4, plate 24, and 

 in both of these the deformations are shown to be in ascendency over 

 the erosion, the first illustrating faulting, and the second, folding. From 

 the fact that he would not accept a scarp or any other physiographic 

 indication of faulting, and as most of the ranges studied by him carried 

 very little or no post-Jurassic rocks, it is easy to see how Cenozoic faults 

 would be largely overlooked. 



In the same way it is said, "According to the accumulated record of 

 observation, ranges consisting essentially of a single monoclinal ridge 

 are exceedingly rare." By this is meant monoclinal in respect to the 

 bedrock ; but if, as in Humboldt Lake mountains, a period of folding 

 and erosion precedes the block faulting, the bedrock could only be 

 expected to show a monoclinal structure under rare circumstances, and 

 changes from monocline to anticline or syncline should be not uncom- 

 mon in the same range. As regards the Tertiary volcanics, the Hum- 

 boldt Lake range is a monocline, and so also, we might say, would it 

 have been topographically a monocline, even if there had been no post- 

 Jurassic rocks on its surface. 



In the paper referred to, the writer has evidently not distinguished 

 pre-Cretaceous from post-Jurassic and especially late Tertiary faulting, 

 folding, or other earth movements. The great Cretaceous-early Tertiary 

 erosion period has, of course, obliterated all of the ancient topographic 

 features, but great faults and other deformations of later Tertiary or 

 Quaternary time are commonly preserved, with but comparatively slight 

 alteration. 



Directions for visiting Geologists 



In the belief that many geologists crossing Nevada by the Southern 

 Pacific railroad would be pleased to visit the district above described if 



*See note, page 305. 



t Bull. Geol. Soc. Am., vol. 12, p. 265. 



XLV— Bull. Geol. Soc. Am., Vol. 15, 1903 



