220 E. B. BRANSON ORIGIN OF RED BEDS OF WESTERN WYOMING 



not made up of shells of animals. Woodruff's theory could apply for only 

 very small areas, where the beds are irregular in thickness. All evidences 

 seem to indicate an origin from chemical precipitation, and the absence 

 of fossils is not peculiar with the waters so highly charged with mineral 

 matter. 



Bed sandstone continues for 80 to 90 feet above the limestone and is 

 then succeeded by a heterogeneous member called by Williston^ the Popo 

 Agie beds. As the lower contact is always covered, the exact thickness 

 of these beds has not been determined, but seems to vary from less than 

 20 feet to more than 60 feet. For the most part the member consists of 

 sandy shales and mudstones. At the top a mudstone that shows no sign 

 of bedding ranges up to 15 feet in thickness and consists of nodules of 

 purplish, argillaceous sandstone. The mud seems to have been cracked 

 by the sun and the cracks filled in by wind-blown or flood-washed ma- 

 terials and the bedding destroyed, as described by Barrell.^ Much of the 

 formation contains rounded white spherules, usually about 0.6 millimctei- 

 in diameter, that appear like grains of oolite and that have concentric 

 structure. 



In color the Popo Agie beds are usually red to yellow, but range 

 through various shades of green, brown, purple, and orange, with occa- 

 sionally white beds and uow and then carbonaceous bands. Not infre- 

 quently included fossil bones are black, owing to carbonization. 



Here and there a conglomerate, varying in composition from pebbles 

 of various kinds of rocks to pieces of bone and teeth of reptiles and 

 amphibians, occurs among the other rocks, and this thickens and thins 

 remarkably in short distances. Beds of sandstone or shale 20 feet thick 

 may give way to something entirely different within a few feet. The 

 colors change abruptly and in a way that gives striking effects. The top 

 was eroded before the deposition of the overlying formation, giving rise to 

 a slight unconformity, that may be observed at most places Avhere the 

 contact is well exposed. 



Fragments of bone and teeth are common near the upper part of the 

 member, but articulated bones are extremely rare. Nearly all of the 

 bones have been worn by being washed about, but the writer and his 

 party found one nearly complete crocodile skeleton and two or three 

 others more or less complete, and Mr. N. H. Brown collected two almost 

 complete amphibian skulls. These remains showed no signs of having 

 been transported and were probably fossilized where the animals died. 



Plant remains are common and most of them consist of lanceolate 



TJour. Geol., vol. 12, 1904, p. 688. 



*Bull. Geol, Soc, Am., vol. 23, 1912, p. 426. 



