THE MIOCENE PERIOD 265 



of the great central valley, with the marine Miocene of western Cali- 

 fornia, though such connection cannot be affirmed. The lone forma- 

 tion, probably of late Miocene age, 1 is now found at various altitudes 

 ranging up to 4000, or perhaps even to 7000 feet. 2 This has been inter- 

 preted as a minimum measure of post-Miocene deformation, on the 

 assumption that the lone formation was all deposited at or below sea- 

 level. If part of it was fluvial, the above figures are not to be taken as 

 a measure of subsequent deformation. 



East of the lone and the marine Miocene beds of California, aurif- 

 erous gravels, 3 brought down by streams from the Sierras, were being 

 deposited in the lower courses of the valleys during at least the later 

 part of the Miocene period, and this deposition was continued after the 

 close of the period. These gravels seem to have been deposited on a 

 surface of slight relief, a surface which is interpreted to have been a 

 peneplain developed in the Sierran region in Cretaceous and Early 

 Tertiary (before mid-Miocene) times. 4 The tilting of this plain toward 

 the end of the Miocene seems to have occasioned increased activity of 

 the streams in their upper courses, and the deposition of gravel below. 

 The Sierra mountains are thought to have been at least 4000 feet lower 

 than now when the auriferous gravels were deposited. From some of 

 the gravels of California, thought to be of Miocene age, human relics 

 have been reported, 5 but there seems to be good reason for doubting 

 their authenticity. 



During the later part of the period, sedimentary deposits, usu- 

 ally described as lacustrine, are thought to have extended from the 

 central valley of California northward into Oregon, and eastward between 

 the Sierra and the Klamath mountains, into northeastern California, 

 before volcanic extrusions had blocked the Lassen Peak pass. They 

 may connect with the Miocene beds of terrestrial origin known at 

 many points east of the Sierras between the 39th and 41st parallels. 

 Considering these non-marine deposits as lacustrine, it has been thought 



1 Lindgren, Jour, of Geol., Vol. IV, p. 898. 

 2 Diller, Jour, of Geol., Vol. II, p. 47. 



3 Whitney, The Auriferous Gravels of the Sierra Nevada of Calif.; Turner, 14th 

 Ann. Rept. U. S. Geol. Surv., 1894; Lindgren, Jour. Geol., Vol. IV, 1896, pp. 881-906; 

 Diller, Jour, of Geol., Vol. II, pp. 32-54. See also folios of the Gold Belt of Calif., 

 U. S. Geol. Surv. 



4 Diller, Jour, of Geol., Vol. II, pp. 33-54. 



5 Whitney, op. cit. 



