GEOLOGICAL SURVEY OF THE TEREITORIES. 149 



chalky rock; this is capped with a layer of very hard, purplish-drab 

 basalt of variable thickness. This deposit extends down the valley of 

 the Medicine Lodge six miles, with an average of four miles in width, and 

 I estimated the entire thickness to be 400 to 600 feet. The deposit itself 

 has been lifted up, so as to form a sort of anticlinal, that is, the strata 

 inclining each way from the river channel at an axis, 5° to 8°. The 

 lower portion is very much like the Hot Springs deposits at Gardiner's 

 Eiver, hard and white as snow ; some of it is a pudding-stone, made up 

 of worn pebbles. The upper portion is variable, as if volcanic action 

 had existed at the same time. The limestone in some places passes up 

 into thin layers of a white, fine, calcareous sandstone. As we descend 

 the creek the beds of limestone, volcanic ash, and basalt diminish in 

 thickness, and over all is a heavy bed of black porous basalt. It is 

 probable that during the lake period this valley was the center of one 

 of the most active groups of hot springs on the continent; that the 

 principal time of deposition preceded the last period of volcanic action, 

 when the basalt that covered the Snake Eiver Basin with its huge crust 

 issued forth. We can trace its history step by step by the strata; and 

 although we could discover no sign of any water in the vicinity above 

 the ordinary temperature of river- water, yet there is no doubt that this 

 indicates one of the largest deposits of the kind yet known in the West. 

 W^e may inquire from what source all this calcareous material was 

 derived. If this is a synclinal vaUey, and I so regard it, then the vast 

 thickness of Carboniferous limestones which we see on the sides, and 

 extending to the summits of the highest mountains, at least 3,000 feet 

 in thickness, dips down beneath the valley and rises again on the 

 opposite side. The waters permeating such a mass of limestone could 

 dissolve an unlimited amount of lime. 



The valley of the Medicine Lodge, for fifteen miles above the Snake 

 River Basin, passes through a deep gorge, with walls of basalt and bas- 

 altic conglomerate on either side. At the point where we ascend the 

 hill on the west side of Medicine Lodge, the hot-spring deposits have 

 diminished to about 80 feet in thickness, and, with a flexure like a bow, 

 bend down, beneath the bed of the stream, out of sight. We then 

 have, as the lower portion of the wall, 100 feet of very coarse breccia or 

 conglomerate, capped with a bed of basalt; then 200 feet of yellow ma- 

 terial, like marl, undoubtedly volcanic ashes, &c. This also is capped 

 with a bed of basalt. The valley or canon of the Medicine Lodge is 

 450 to 550 feet below the sloping plain line. All over the jjlains there 

 is great abundance of very rough basalt, full of holes, of quite modern 

 origin. 



We have said enough in this report to show that the portion of the 

 West drained by the Snake Eiver and its tributaries is full of interest. 

 We have examined only two or three of the numbers of little streams 

 that carve deep channels from the divide down into the basin for more 

 than two hundred miles — all of them undoubtedly presenting features 

 of the highest interest. Fold after fold of mountain ranges extend to 

 the westward to an unknown distance, very few of which are laid down, 

 on any of our maps. 



