562 REPORT UNITED STATES GEOLOGICAL SURVEY. 



canic ash occurs ^itli it on. the Blackfoot. Between the biittes north of 

 the station is a circuhir depression occuioied by a lake. It also has the 

 appearance of an old crater. 



The Great Basalt Plain extends westward to the east base of the Soda 

 Springs Hills, and a flow of basalt passes through a depression in them 

 to Basalt Valley, where it forms the plain between the Upper Portneuf 

 Valley and Gentile Valley, on Bear River. The gap through which the 

 basalt poured is southwest of the craters. It is cut through Carbonifer- 

 ous limestones and is only about a mile in width. Southward the flow 

 extended via Soda Springs Valley into the valley of Bear Eiver, connect- 

 ing along that stream with the Basalt Valley flow. 



There is no drainage outlined on the basalt in the basin of the Black- 

 foot. All the streams in this region sink beneath the crust. A few 

 streams are seen to begin on the basalt, but they can be traced only a 

 short distance, when they disappear. There have probably been several 

 outpourings of basalt, but our limited time did not permit us to follow 

 the succession of the rocks sufficiently to determine them. 



North or a little west of north from the volcanic craters is a group of 

 hills occupying an area of about 18 square miles. On the highest point 

 •of this group Station 70 was located on an outcrop of limestone, beneath 

 which Avere sandstones similar to those of Stations 89 and 90. The out- 

 crops were obscure, but the dip appeared to be to the west or south- 

 west. The angle could not be determined. On the ridge leading down 

 northward from the -station, a dip in the opposite direction was noted. 

 These hills are, therefore, a remnant of an anticlinal axis, the western 

 members of which form the major 2>art of the elevation. On the Black- 

 foot, about Ave miles northwest of Station 70, the northeastern dip is 

 seen again in the caiion wall, a point of the sedimentaries rising above 

 the basalt, limestones and shales outcropping. On the north side of the 

 river this anticlinal rises into a high ridge that continues northwestward 

 into Mr. St. John's district. The line of the strike of the beds here 

 and those of Station 70, when prolonged, falls on Aspen Eidge. There 

 is therefore every j)robability that it is the same fold. Basalt covers 

 the country surrounding the hills of Station 70. It is generally desti- 

 tute of timber. Sometimes a few aspens and low brush are seen on 

 some of the creeks. The limestones on Station 70 contain numerous 

 specimens of Terebratula augusta. The sandstones below are filled with an 

 Aviculopecten, among which^ probably, two new species exist. The beds 

 are all undoubtedly of Jurassic or Triassie age. 



Lander's road crosses the Blackfoot almost on our north lijie. The 

 liver here is broad and very sluggish and full of vegetation. The cross- 

 ings are few and generally muddy. Below the crossing of the road the 

 river turns abruptly and flows southwest into a canon, the walls of which 

 are principally basalt. On the south, as I have already noted, lime- 

 stones and shales outcrop at one locality. The basalt is horizontal, and 

 appears to follow the canon of the river a long way to the northward. ■ 



In the caiion, a short distance below the outcroj) of limestones, there 

 are calcareous spring deposits. Onlj^ one active spring was noted, with 

 very little water escaping. The temperature at 7 a, m. was 82"^ F., the 

 air being at 55'^ E. On tasting the water the presence of carbonic acid 

 ^as and iron was recognized. The deposits were calcareous as far as 

 «een. 



Between the head of the Portneuf and the Blackfoot, outcrops of light- 

 colored sandstones and conglomerates were noted, aiopearing to occupy 

 a basin which extends northward into Mr. St. John's district, and which 



