LETTER TO THE SECRETARY. XIII 



Scarcely any vegetation has sprung up on the light glacial soil, and the 

 •haracteristic distribution of erratic material bears every evidence of 

 " freshness." 



All along the Sweetwater River the characteristic Sweetwater group 

 of the Tertiary was found, continuing northward to the hills opposite 

 Seminoe Pass. These hills were found to be projections of granite that 

 during the Tertiary epoch, and probably long before, existed as islands 

 ill a widely extended sea. 



On the south side of the Sweetwater, in the Seminoe Hills, the older 

 sedimentary formations were noted. In the region between the Seminoe 

 Hills and Rawlins, on the Union Pacific Railroad, an interesting group 

 of mud-springs, analogous to the mud-pufts of the Geyser region, were 

 Been. About four hundred of these curious springs were found and ex- 

 amined. 



After finishing his work with the field-party, Dr. Endlich visited the 

 eoal-mines near Evanston, Wyo., to examine the coal-bearing rocks of 

 that region. 



The Green River district lies directly west of the Sweetwater district. 

 With the exception of a small area of granite, along the southwestern 

 side of the Wind River Mountains, and some basaltic flows in the north- 

 western portion of the district, the rocks are sedimentary. The Green 

 River Basin was the first area surveyed. The prevailing formation was 

 found to be the Green River group of the Tertiaiy, underlaid by the 

 Wahsatch. Toward the south, buttes of the Bridger clays rest on the 

 Green River marls and sandstones. They are the northern outliers of 

 the extensive Bridger areas found farther southward. 



On the northeast, the Tertiary beds rest on the granites of the Wind 

 River Mountains, the line of junction being considerably obscured by 

 morainal material. One of the most interesting points noted in the 

 Green River Basin was on its west side, where the Wahsatch Tertiary 

 is seen resting uncomformably on Jurassic and Cretaceous strata. At 

 one point Carboniferous fossils were obtained fi'om bowlders of lime- 

 stone found in a conglomerate in the Wahsatch group. These were 

 derived without doubt from the Carboniferous limestones of the mount- 

 ains that stand a short distance to the westward, and which must once 

 have formed a portion of the shore-line of the lake filling the Green 

 River Basin. 



An arm of this Tertiary lake extended up Ham's Fork of the Green 

 River, where Green River and Wahsatch beds are found in horizontal 

 position. In the Green River shales, at several localities, good collec- 

 tions of fossil fish and insects were found, among which were many new 

 species. 



The region of the Blackfoot River, in the northwestern part of the 

 district, is covered in all its lowest portions with flows of basalt which 

 had their origin in craters that still show between the Blackfoot, Bear, 

 and Portneuf Rivers. The pouring out of this basalt must have occurred 



