TERTIARY CONTINENTAL HYDROCLASTICS 629 



of more abundant rainfall, when the surface of the interniontane 

 basin was prevented from drying out rapidly. The red clays, how- 

 ever, appear to have been formed "during the drier cycles, when 

 the carbonaceous matter of decaying plants was completely oxidized, 

 concentration and oxidation of iron compounds occurred, and ani- 

 mal bones exposed at the surface were weathered and broken be- 

 fore entombment." Conditions of this kind seem to have been 

 widespread, as shown by similar color banding in the Wasatch in 

 other localities. 



In the Wasatch formation along the contacts of red and blue 

 beds or in many of the red beds themselves great numbers of 

 fragmentary jaw^s and scattered teeth of vertebrates have been 

 found. The clays appear to represent the deposits on the dry basin 

 floor over which the bones of these creatures were scattered and 

 weathered before being buried. In the blue clays associated skele- 

 tons are common. These are the remains of animals which were 

 either drowned and rapidly covered beneath fluviatile sediments or 

 were mired in the soft clays. The teeth found in the Wind River 

 area usually have the roots worn away and only the harder enamel- 

 covered crowns are preserved. The Unio beds of the Wasatch are 

 always of limited extent and seem to be confined to the blue clays. 

 The lignite layers in these clays are usually mere dirt bands, but 

 some in the Wind River basin have considerable thickness (52 :ii7). 



The Oligocenic beds of this region contain limestone deposits 

 associated wath wind-laid volcanic tufifs. These limestones, which 

 are found near the top of the series, are a spring deposit forming 

 sheets of tufaceous limestone, or layers of white nodular masses, 

 calcareous without, but containing more or less silica within. Worn 

 quartz, feldspar, and pink granite pebbles are sometimes found in 

 the limestone, which is also partly replaced by silica in the form 

 of opal or chalcedony. No fossils have been found in the lime- 

 stone, which appears to have been formed under relatively dry 

 climatic conditions. 



These Tertiary deposits on the eastern slopes of the Rockies 

 have thus all the characteristics of deposits formed under semiarid 

 conditions. These conditions prevail to-day in this region under 

 the influence of the westerly winds, which, on crossing the Coast 

 Range, where they leave most of their moisture, become still drier 

 on crossing the Rockies. Greater elevation of the mountain ranges 

 would probably increase the aridity of the intermont basins in this 

 region and so reestablish the conditions of Tertiary time. 



Hobbs (31) has recently described a typical torrential forma- 

 tion of great thickness from southern Spain. This ranges in age 



