2G6 GEOLOGY. 



that the waters of an extensive and irregular Miocene (Pah-Ute) ! 

 lake, or perhaps series of lakes, cast of the Sierras, connected west- 

 ward with the waters in the valley of northern California, 2 and per- 

 haps northward with the John Day basin 3 of Oregon. It is probable, 

 however, that much of this inland Miocene is of fluvial, pluvial, and 

 eolian origin. The sites of some of these deposits seem to have 

 been areas which were subject to erosion during the Eocene, and 

 then to have been so deformed as to become areas of deposition. 



The terrestrial Miocene formations (the Truckee Miocene 4 of King) 

 are said to reach a thickness of 4000 feet (King) at some points in 

 the vicinity of the 40th parallel. In general, they are made up of 

 sandstones, conglomerates, volcanic debris, infusorial earths, and 

 fresh- water limestones, overlain by great thicknesses of volcanic tuffs. 

 The John Day series, the upper portion of which is perhaps Miocene, 

 is also thick (said to be 3000 or 4000 feet), and is made up largely 

 of volcanic ash and sand, much of which seems to be eolian. 5 The 

 deformed and eroded John Day formation is overlain by lava, which 

 in turn is covered by a late Miocene formation (Mascall, perhaps 

 =Loup Fork). Miocene beds contemporaneous with the Miocene 

 of the John Day basin occur also in western Oregon and Washington. 6 

 In the Mount Stuart region of the latter State, 1000 to 2000 feet of 

 basalt (Miocene) is overlain by 1000-1600 feet of sedimentary beds 

 (Ellensburg formation), largely fluvial 7 (Fig. 445). 



Other areas of deposition, some of them lakes, existed during the 

 Miocene in Nevada and Montana. In the southwestern part of Nevada, 

 the Miocene beds (Esmeralda formation) described as lacustrine, con- 

 sist of the usual sorts of clastic rocks, pyroclastic material, and work- 

 able coal, the latter showing that the formation is not altogether lacus- 

 trine. The formation also carries some sulphur. The remarkable 

 thickness of 14,800 feet (which may include Pliocene beds) is reported 



1 King, Geol. Expl. of the 40th Parallel, Vol. I. 

 2 Diller, 14th Ann. Rept., U. S. Geol. Surv. 



3 The earlier John Day beds were Eocene and Oligocene (Dall, loc. cit.), though 

 the later were Miocene. 



4 Op. cit., pp. 412 and 458. 



5 Merriam, Jour. Geol., Vol. IX, p. 71, and Bull. Dept. of Geol., Univ. of Cal., 

 Yol. II, p. 306. 



"Knowlton, Bull. 204, U. S. Geol. Surv. 



7 Smith, G. O., Mount Stuart, Wash., folio, U. S. Geol. Surv. 



