1914] Buwalda: Tertiary Mammal Beds in West-Central Nevada 341 



suffered the intense deformation of the underlying older rocks, they 

 have been considerably tilted along the flanks of the range. They 

 dip in general toward the valleys, usually at steeper angles than the 

 lake beds where the latter rest upon them. The irregularity of the 

 contact line indicates pre-lacustral relief in the lava surface, and an 

 erosion interval presumably separated the outflow of the pre-lacustral 

 lavas and the deposition of the lake beds. The latter being upper 

 Miocene in age, it is probable that most of the pre-lacustral lavas are 

 not younger than middle Miocene. 



Stewart and Ione Valleys 

 Stewart and Ione valleys lie parallel to Cedar Mountain, Stewart 

 Valley lying to the west, and Ione Valley to the east, of that range 

 (pi. 33). Lacustral strata of upper Miocene age constitiite in the main 

 the surface rocks of these valleys, as contrasted with the Mesozoic 

 limestones, associated intrusives and later extrusives of Cedar Moun- 

 tain. The deposits in the two valleys are very similar, and are con- 

 nected stratigraphically through the depression between the north end 

 of Cedar Mountain and the south end of the Paradise Range. The 

 physiographic history of the two valleys has apparently also been 

 quite similar. For these reasons it has seemed desirable to discuss 

 the two areas together. 



UNDEELYING EOCKS 



The basement on which the lacustral series was deposited consists 

 of rocks not dissimilar to those of Cedar Mountain. At a number 

 of points in the two valleys they are exposed at the surface, either 

 having had their cover of lacustral sediments removed by erosion or 

 perhaps having never been covered by the Tertiary sedimentary series. 

 Some of the exposed areas are of considerable size, such as the north 

 end of the 6600-foot hill in Ione Valley, midway between Bell Spring 

 and Red Spring. Other patches, such as those lying one to three 

 miles farther north in the middle of Ione Valley and a few masses 

 protruding through the lacustral series in Stewart Valley at consid- 

 erable distances from the valley borders, are of only small extent. 



The north end of the 6600-foot hill in Ione Valley is nearly sur- 

 rounded by the lacustral beds. The rocks exposed in the base of this 

 hill along the road between Bell Spring and Black Spring are older 

 than the lacustral series; they constituted an eminence on the pre- 



