DESCRIPTION OF THE BEDS 221 



leaves, though some trunks up to 4 or 5 inches in diameter occur. Fresh- 

 water pelecypod shells occur in a few places. 



Above the Popo Agie beds there is about 20 feet of reddish, thin-bedded, 

 much ripple-marked sandstone that is uniform in thickness and color 

 over large areas. One peculiarity of this member is uniformity of joint- 

 ing, running almost at right angles, vertical, and usually 2 to 4 feet 

 apart. With the weathering away of the softer beds beneath these beds 

 are left standing with vertical faces, which are determined by the joint 

 planes. The joints are shown at the left in figure 1, where they appear 

 in striking contrast with the irregular jointing of the nodular beds below. 



Sandstones succeed for about ISO feet, but are interrupted four or five 

 times by beds of chocolate-colored sandy shale, 6 inches to 4 feet in thick- 

 ness, that maintain the same thickness for long distances and are regular 

 in texture and color. 



These sandstones are followed by some 50 feet of a very strikingly 

 cross-bedded sandstone of lighter red color than the other sandstones. 

 The bedding is of a type that originates from wind action,^ which the 

 figures of plate 9 show better than it can be described. The false beds 

 dip in various directions, differing from the type developed by stream 

 work, in which the false beds dip rather uniformly in one direction.^'' 

 This sandstone decreases in thickness to the east and may entirely dis- 

 appear within a few miles, though some of it appears in the small anti- 

 clines some 10 miles east of the outcrops along the mountains. It was 

 probably subaerial in origin ; but if so, waters soon overspread the region 

 again and thick beds of gypsum were deposited not far above it. The 

 gypsum is almost pure from top to bottom, though the beds may thin 

 rapidly from 40 feet to 0. 



If the cross-bedded sandstone is wind-blown, as according to my in- 

 terpretation, there is only slight change in color to separate marine from 

 non-marine. But great change in color is scarcely to be expected. If 

 tlie climate is arid, oxidation has no large effect on the wind-blown sand, 

 and the seas covering thin beds of such sand place them under almost 

 exactly the same chemical conditions as though they had been deposited 

 under the sea. If the coloring matter is due to some impurity deposited 

 with the sands, the wind-blown sand is likely to be less liighly colored, 

 ns it will have a smaller amount of such impurities than the subaqueous 

 deposits. The cross-be* I ded sandstone of the I?ed Beds is lighter colored 

 tlian the other sandstone. 



"Bull. GpoI. Soc. .\ni., vol. 24, pp. 403 and 404. 



10 J. Bancll : Criteria foi- tlic recognition of ancient delta deposits. Bull. Geol. Soc. 

 .\m., vol. 2n. p. 452. 



