WASHAKIE BASm. 217 



arenaceous, interstratified with thicker or thinner beds of coarse gray sand- 

 stone. They are extremely loosely agglomerated, and the material of 

 which they are composed is very fine-grained and homogeneous. The 

 characteristic feature of their peculiar topography is the fact that the eroded 

 faces are practically vertical, and without the usual talus-slopes which 

 result from the disintegration of ordinary beds. 



The explanation of this may be looked for partly in the loosely agglomer- 

 ated character of the beds themselves, which seem to be, in great part, merely 

 mechanical admixtures of clay and sand, but principally in the climatic 

 conditions of the region, in which erosion, at the -present day at least, is the 

 result ratherof the action of air than of water. The region is exposed to no con- 

 tinuous rains; during the summer months, there fall only occasional violent 

 showers, while in the winter the snows, which accumulate upon the surface, 

 are driven into sheltered spots by the violent winds which sweep across the 

 plains, and in the early summer are suddenly melted, and send down vio- 

 lent torrents of comparatively short duration. The more clayey beds 

 crumble and disintegrate in the dry air, and under the influence of the 

 expansion and contraction produced by the great diurnal changes of 

 temperature from midday to midnight. The sandy strata are thus under- 

 mined, and the waters of the intermittent rains only serve to sweep out and 

 deepen the existing gullies, without degrading their sides. 



The view given in the Frontispiece shows some of the characteristic 

 forms of erosion of this region. Here the bed of coarse sandstone, some 15 

 or 20 feet in thickness, which probably was locally indurated, has preserved 

 isolated remnants of the beds from degradation. The thickness shown in this 

 view is a little under 200 feet; the aggregate thickness of these beds, how- 

 ever, it was impossible to estimate, owing to the horizontal position of the beds 

 and their want of continuity. That of the Western or Bridger Basin, as will 

 be seen later, is between 2,000 and 2,500 feet; it seems probable that in 

 this region, however, the thickness is considerably less. From these beds, 

 great quantities of vertebrate remains have been obtained, which have been 

 made the subject of a special report by Professor Marsh in another volume 

 of the series. Their horizon has been determined by him to be that of the 

 Lower or Middle Eocene, and in general features to correspond with those 



