28 GEOLOGICAL SURVEY OF THE TERPJTORIES. 



tlio Bhickfoot Creek emerges into the plains, the basaltic walls on either 

 side are 50 to GO feet high, and higher up the canon the channel passes 

 tlirongh ridges of limestone and qnartzitc at right angles, 1,000 to 1,500 

 feet above the phiin. 



From Blackfoot Creek we traveled over a nearly dead level to 

 Taylor's Bridge, the crossing of Snake Eiver, eighteen miles dis- 

 tant. Far distant to the west the three Uittes can be distinctly seen, 

 like isolated fragments of mountains in the plains; still further to 

 the west can be seen, on a clear day, the dim outlines of the Salmon 

 River Ilanges. To the east are a series of broken or irregular ranges, 

 with low grassy foot-hills in front, usually rising 1,000 to 1,500 feet above 

 the plains, but with here and there a high peak, 2,000 to 2,500 feet in 

 height, covered with snow. That this basaltic outflow occurred at a time 

 when this vast basin was occupied by the waters of a lake, I believe, from 

 the fact that all the lower portion is exceedingly compact and heavy in its 

 texture, and the surface, though sometimes full of cavities, must have 

 cooled under a moderate pressure of water at least. After this basalt 

 ceased to flow the lake continued on, so that a superficial deposit of 

 sand and fine volcanic dust, varying from 10 to 50 feet, covered the great 

 basaltic cap. During the dry season of summer this volcanic dust be- 

 comes a sort of impalpable powder, filling the air with clouds to such an 

 extent as almost to suffocate man and beast. 



At Taylor's Bridge the waters of Snake River rush with great velocity 

 through the narrow gorge-like channel which they have worn out of the 

 basaltic floor. The walls on either side form excellent sections, and in 

 the autumn, when the river is low, expose 100 feet or more of the basalt, 

 with all the varieties of texture. These walls show an irregular columnar 

 structure or jointage, and the decomposition or erosion is greatly aided 

 by this condition. The different layers show clearly the different periods 

 of effusion, and in some instances the lowest portions indicate that, 

 after the great mass had cooled and become solid, fluid basalt had been 

 thrust up, showing a texture and color much like modern lava, only 

 more compact. But the most interesting feature in this locality is the 

 existence of numerous cavities, worn into the solid basalt, which are 

 usually called ''pot-holes.'^ These "pot-holes" occur by thousands on 

 both sides of the river, for miles up and down, varying in diameter from 

 a few inches to several feet. They are very distinct on the walls of the 

 river- channel, where the latter seem to have been split down from top to 

 bottom. Many of them have in them, even at this time, the rounded 

 masses, which by constant agitation of the waters have worn out the 

 cavities. Some of these holes are 2 to 3 feet deep, although not more 

 than 4 to G inches in diameter. The examples of degradation by exfolia- 

 tion are finely exhibited here, so that the basalt itself would seem to 

 have assumed aspheroidal shape in cooling, and is now falling in pieces 

 by concentric layers. 



From Taylor's Bridge we traveled along the west side of the 

 river to Market Lake, a distance of twenty miles. To the east of 

 our camp, near the entrance of Henry's Fork, are two rather high 

 flat-topped basaltic huftesj which, have the appearance of extinct 

 craters. Their summits are GOO to 800 feet above the plains around 

 them. The rim of the south hiitte is much broken down. They were, 

 undoubtedly, centers of effusion for the lava. Far in the distance, 

 seventy or eighty miles a little south of east, the Tetons loom up 

 grandly, Avith the form of shark's teeth. To the north of them, and 

 quite distinctly visible, is Mount Madison, one of the finest peaks in the 

 northern ranges of the Rocky Mountains. To the west of Market Lake 



