XIV REPORT UNITED STATES GEOLOGICAL SURVEY. 



either during or just prior to the present period, as there has been but 

 little if any change in the surface since the eruption. On the Portneuf 

 River a narrow tongue of basalt extends almost to Snake Eiver Yalley. 

 The surface of this basalt slopes somewhat, but not so much as the pres- 

 ent bed of the Portneuf. The lower valley of the Portneuf is interesting 

 from the fact that it is probably one of the ancient outlets of the great 

 lake that once filled the Salt Lake Basin and extended across into Cache 

 Valley. In Cache Valley and Malade Valley, modern Tertiary deposits 

 are found jutting against the mountains, and in the central portions of 

 the valleys they seem to pass gradually into the more modern deposits. 



The interesting soda-springs at the bend of Bear River were carefully 

 examined. 



Although the area surveyed by the Green River division was large, 

 comprising about 13,000 square miles, large collections of fossils (many 

 of them new) were made. Kotes were obtained for the preparation of 

 a geological map of the area, and data collected for the elucidation of 

 many interesting i^roblems in relation to the age of the mountains. 



Coal-outcrops were noted at a number of localities, on some of the 

 branches of the Upper Bear River and of Green River. The famous salt- 

 works on a branch of Salt River were also examined. 



Professor St. John rexDorts the Teton district to be one of great inter- 

 est. He found extensive areas covered with rocks of igneous origin, 

 basalts, and trachytes. 



The Snake River plains are everywhere floored with basaltic rocks. 

 They extend up the valley of the Snake as far as the lower basin, where 

 they are succeeded by other volcanics, mainly trachytes. The latter are 

 observed inclining at greater or less angles and appear to be more 

 ancient than the basalts. The Blackfoot Valley and the valley depres- 

 sions between the Blackfoot Mountains and the Caribou Range are 

 floored with basalts in every way similar to those occurring in the Snake 

 River plains. These extend southward into Dr. Peale's district. 



Rhyolitic products were found at a few localities. In one instance the 

 eruptive matter appears as a dike in the crest of a low, short ridge be- 

 tween the Blackfoot and Caribou Ranges, its eruption having tilted the 

 sedimentary deposits into an anticlinal ridge. 



In Caribou Mountain, also, interesting phenomena were observed. The 

 mountain is a monoclinal ridge, made up of sedimentaries, between 

 whose strata the igneous material is intruded, appearing from a distance 

 like veritable beds of deposition, while the bulk of the west portion of 

 the mountain appears to consist of an enormous mass of eruptive mat- 

 ter thrust up from below. This mountain would therefore appear to be 

 another instance of local outburst, similar to those brought to light by 

 the survey in Western and Southwestern Colorado. 



Extensive areas of the district are occupied by sedimentary or strati- 

 fied rocks, which were referred to the Lower Silurian, Carboniferous, 

 Jura-Triassic, Cretaceous, and Tertiary ages. These rocks have been 



