peale.1 DESCRIPTIVE GEOLOGY BLACKFOOT BASIN. 561 



latecl ridges in this region. In the low hills east of the head of the 

 creek draining* into John Gray's Lake, dips to the west and northwest 

 are seen, with what appears to be a curving in the strata towards the 

 sonth and west, as if it were connected with the curving noted on the 

 ridge of Station 60. There is here, therefore, evidently the southern 

 end of an anticlinal . Only a small portion of the eastern side is appar- 

 ent here. It consists of Jura-Trias sandstones in which occur a number 

 of indistinct fossils, among which a Tancredia was all that could be 

 identified. The western members of the anticlinal therefore form the 

 ridge. Xorthwest of this ridge on the west side of the lake, in Mr. St. 

 John's district, is another isolated ridge. In this the eastern members 

 form the ridge. Between the two ridges there is probably a slight curv- 

 ing in the axis. West of this ridge is another, seen by us only from 

 Stations 65 and 66. It is broader but appears to show the same mono- 

 clinal structure, although it is iu reality the western side of another 

 anticlinal. Xorthwest of this is another isolated ridge in Mr. St. John's 

 district, composed mainly of Carboniferous beds, a small area of Jura- 

 Trias showing on the east side. On the west side of the Blackfoot is 

 still another of these isolated mountain ridges on which Station 70 was 

 located. Thi s will be referred to again further on. There can be no doubt 

 that these beds are the same that are exposed south of the Blackfoot, and 

 that these isolated hills and ridges are the eroded remnants of the anticli- 

 nalsthat form the ridges between the southern branches of the Blackfoot. 

 The surrounding valleys are filled with basalt which extends up on the 

 flanks of the ridges. This basalt occupies also the valley of John 

 Gray's Lake. The region southeast of John Gray's Lake is occupied by 

 rocks of Cretaceous? and Jurassic age. There is considerable compli- 

 cation in the region just northeast, which made it difficult to determine 

 the age and relations of these beds in the hurried examination we were 

 obliged to make. Mr. St. John's report on the region immediately north 

 will throw considerable light on the structure of this region, as the cause 

 of the disturbance lies within his district. 



West side of Blackfoot River. — When the Blackfoot emerges from the 

 ridges and comes out into the broad open valley, or "Hollow Hand," as 

 it has been called, it flows in sluggish course through broad meadows 

 and marshes. On both sides are seen the edges of the basaltic plain 

 that forms the " Hollow Hand." On the east side of the Blackfoot the 

 flow appears to have come mainly from a point about five miles back of 

 the river. Here two crater-like hills were seen. Near the north line of 

 the district another crater was observed. 



Southwest of the Blackfoot there are at least two craters. ' On one of 

 these a station was located. It is circular and rises about 500 feet above 

 the surrounding surface. On the summit is a circular concavity 130 

 yards in diameter. It is from 10 to 20 feet deep, and the rim is about 

 50 feet iu width and made up of black, red, and yellow scoriaceous 

 basalt. Basalt surrounds the cone, and on the east side seems to have 

 an inclination away from the cone in a succession of layers. On the 

 west this is not noticed, as the other cone is only a little more than two 

 miles distant. The latter is not so regular, its sides appearing to be 

 much broken down. 



About four miles southeast of these two cones a station was made on 

 a high butte which, judging from the specimens brought in by Mr. Mush- 

 back, is either a crater or the remnant of one. The scoriaceous rocks 

 from the summit were all light colored, having a glassy appearance 

 under low-power glasses. 



Scoriaceous rock is found on the basalts at several places, and a vol- 

 36 G s 



