THE PLIOCENE PERIOD. 299 



certain parts of Colorado (North Park, North Platte, etc.). In some cases their 

 areas are large, though their boundaries are undetermined. They have been 

 assigned thicknesses ranging up to 1400 feet, and they contain much volcanic 

 debris. They are said to be unconformable on the Miocene, which they over- 

 lap in all directions. The later auriferous gravels 1 of California (Fig. 460) 

 already referred to under the Miocene, belong to this class. Their deposition, 

 begun in the Miocene, was continued into the Pliocene, and probably even 

 into the succeeding period. Deposits of similar origin probably abound 

 throughout the western mountains, but, except where the latter are of glacial 



Fig. 460. — Section showing auriferous gravels, Kg, overlain by rhyolite tuff, AV, and 

 andesite, Na. Length of section lj miles. (Lindgren, Nevada City, Cal. Special 

 folio, U. S. Geol. Surv.) 



or fluvio-glacial origin, they have few characteristics which distinguish them 

 from later deposits. 



Sedimentation (Rattlesnake beds) appears to have continued during the 

 Pliocene in the John Day basin, 2 where the aggregate thickness of the Tertiary 

 beds is said to exceed 10,000 feet. 3 Pliocene beds arc also reported from 

 Idaho (Idaho formation), where they overlie the Payette (Eocene) formation 

 unconformably 4 from New Mexico, 6 Arizona," and Mexico 7 (Sonora). Non- 

 marine sedimentary beds are also said to be of common occurrence in the 

 southern Coast ranges of California/ and are reported from the coastal plain of 

 northern Alaska, where the sequoia grew 9 in latitude 70°, or thereabout. 



East of the Rocky mountains, on the border of the Great plains, 

 deposits of this class have been noted at many points, but a demon- 

 strative interpretation is as yet generally lacking. Some of them 

 have been referred to the Pleistocene, but many so referred are prob- 

 ably older. In many places these formations show by their constitution 

 that the source of their material was in the western mountains. In 



1 See references to Auriferous Gravels under the Miocene, p. 274. 

 2 Merriam, Bull. Dept. of Geol., Univ. of Cal, Vol. II, p. 312. 

 3 Merriam, Bull. Geol. Soc. of Am., Vol. Nil, p. 496. 



4 Lindgren and Drake, Nampa and Silver City folios, U. S. Geol. Surv. 



5 Reagan, Am. Geol., Vol. NNNI, p. 84. 



8 Blake, Sci., Vol. XV, N. S, p. 413, and Durable, Am. Inst. Min. Eng., Vol. NXXI, 

 p. C9d. 



7 Dumble, Trans. Am. Inst, Min. Eng., Vol. NNNI, p, 696, NNIX, p. 691, 125. 



8 Fairbanks, Jour, of Geo!., Vol. VI, p. 565. 

 9 Schrader, Bull. Geol. Soc. of Am., Vol. XIII, p. 249. 



