ENVIRONMENT OP THE TITANOTHEEES 



55 



and subject to heavy silting of fine sediments from 

 annual floods. These were like the flooded areas of 

 the forest belt in the Amazon delta. Such still-water 

 areas were contemporaneous with areas in the pied- 

 mont regions close to the mountains, where stream 

 erosion was active. The conditions that prevailed in 

 general during Wasatch (Sparnacian) time are nearly- 

 paralleled by those now found in the flood plains of 

 Parana, Paraguay, and Uruguay Eivers, which are 

 carrying down vast masses of gravel, sand, and clay from 

 the mountain chains of Brazil, as reported by John Ball 

 in his " Notes of a naturalist in South 

 America "(1887.1). The annual rain- 

 fall in these mountains ranges from 100 

 to 136 inches, and it rapidly disinte- 

 grates the yieldingrocks and discharges 

 a vast quantity of detrital matter over 

 the broad plains of Argentina and 

 Uruguay. The mountain streams 

 have thus built up wide, level areas 

 in these countries, and the lower 

 rivers, ploughing their channels 

 through the vast deposits over which 

 they must make their way, extend 

 their banks with every increment 

 and thus continually make additions 

 to the outskirts of the formation 

 they are depositing. In this way 

 deposits covering an area of 200,000 

 square miles have been formed from 

 the mountains of Brazil. 



The period of flood-plain and 

 piedmont deposition in the Rockies 

 was followed by the great lacustrine 

 period of Green River time and of 

 Wind River (Ypresian) time, in which 

 the climate was much warmer. In 

 the same region there ensued the 

 flood-plain period of the Bridger. 



Eocene basin deposition of another 

 kind and climatic change are indi- 

 cated in the widespread horizontal 

 ' ' white layers "that divide the Bridger 

 into several geologic and faunistic 

 levels. These white layers indicate 

 periods of lagoon leveling by annual 

 uniform flooding and evaporation, similar to that of the 

 existing playa lakes of the Great Basin in Nevada. 



In middle Eocene time new conditions of foresta- 

 tion and erosion and the presence of volcanic atmo- 

 spheric dust in the Bridger and Washakie Basins are 

 indicated. Sinclair showed (1906-1909) that the 

 Bridger formation was composed chiefly of volcanic 

 material that has been more or less rearranged by 

 stream action, and that clouds of volcanic dust 

 doubtless filled the atmosphere during the Bridger 

 epoch (middle and upper (?) Eocene). This interest- 

 ing discovery was confirmed by thorough analyses 



made by Johannsen in 1914. The rocks of the upper 

 and middle Eocene formations consist chiefly of 

 volcanic tuff. Although the minerals of this tufl' are 

 those of a dacite (quartz andesite), the quartz grains 

 may be of sedimentary origin and the volcanic rock 

 may be andesite (Johannsen, 1914.1, p. 210). The 

 presence of dacite tuffs in the lower Bridger levels (B 

 and C) indicates that the atmosphere was charged 

 with volcanic dust, which also settled upon the con- 

 temporaneous deposits of the Washakie Basin, 100 

 miles to the east, as well as on the Uinta Range, 60 



.25, 26a, 26 b 



STACK MT^ 



CO > 



< / U 



LOWER BROV 



FiGURK 40. — Section of deposits near Barrel Springs, Washakie Basin, southern 

 Wyoming (No. 9, fig. 35) 



Showing alternating beds of tuff, siliceous and calcareous deposits, and sandstone. Johannsen (1914.1), after 

 Granger, with modifications. The numbers refer to lithologic specimens examined by Johannsen. 



miles to the south. Thus during middle and upper 

 Eocene time the atmosphere over the present Bridger, 

 Washakie, and Uinta region was at times charged with 

 volcanic dust. Specimens of lower and basal Eocene 

 rocks indicate sediments of more normal type, and 

 whatever volcanic material they contain is so much 

 altered by re-sorting and mixing with normal sedi- 

 ments that it is not clearly recognizable. 



The manner in which the layers of dacite and glass 

 tuffs alternate -with the heavy river-channel sand- 

 stones is clearly displayed in the analysis of sediments 

 from the Washakie Basin by Johannsen. Tuffs are 



