1914] Buwalda: Tertiary Mammal Beds in West-Central Nevada 357 



movements connected with the extrusions. It is probable, however, 

 that following' the deposition of the lacustral beds there was first a 

 period of active deformation, followed by a period during which 

 erosion and lava extrusion were the dominant processes. 



The physiography of the valleys is not unlike that of other desert 

 valleys, in that the surfaces slope smoothly toward a wash along the 

 middle of each valley. Neglecting the relatively thin mantle of fan- 

 glomerate which lies upon the bevelled surface of the lacustral beds, 

 the gently sloping areas of lone and Stewart valleys are essentially 

 cut, rather than aggraded surfaces, as already noted under the de- 

 scription of the Pleistocene fanglomerate mantle. The mesa areas of 

 the valleys, formerly continuous, are now cut into irregular patches 

 by the dendritic branches of the main washes. The areas between 

 the mesas are undergoing erosion and have a subdued badland topog- 

 raphy. The mesas lie at different levels like terraces (pi. 38. fig. 2). 

 It was at first thought that these disconnected mesas could be grouped 

 into two or three sets on the basis of contemporaneity of formation. 

 It seems, however, that the different mesa areas are genetically more 

 or less independent in the different parts of the territory and are con- 

 nected with the more or less independent activities of the different 

 drainage channels, which head in the adjacent ranges and flow across 

 the lacustral beds to reach the main wash. The different mesas do 

 indicate alternations of erosion and deposition at the different locali- 

 ties, but it is doubted whether they indicate erosion cycles which were 

 general over the whole region. These minor alternations may have 

 been connected more or less with pulsating climatic changes similar 

 to those considered by Huntington 10 in connection with the formation 

 of terraces under arid conditions at Tucson, Arizona. 



While the alternations of erosion and temporary deposition in the 

 valleys may not have been connected with warping of the region, 

 such deformation at a late geologic date is inferred through another 

 line of reasoning. It is practically certain that the period of erosion 

 which produced the old-age surface on Cedar Mountain must have 

 cut a plane, graded surface on the Miocene beds early in the cycle, 

 because the lacustral beds are so much less resistant than the older 

 rocks. The drainage courses over this surface to the basins of depo- 

 sition were then presumably at grade, with little or no dissection 

 going on in the lacustral beds. The change from this condition to one 



io Huntington, Ellsworth, The Climatic Factor as Illustrated in Arid America, 

 Carnegie Inst. Wash. Publ., no. 192, pp. 23-36, 1914. 



