262 



GEOLOGICAL SURVEY OF THE TEREITOEIES. 



succeeds, from five to seven miles across, reaching np to the foot of the 

 Ivigli vertical limestone-walls of the main divide (?), whose ragged crest 

 shows plainl}^ from the month of the valley, and from whose many 

 gorges come the numerous small spriug-branclies which form the main 



stream. Besides the main 

 fork, seven or eight large 

 creeks come in from either 

 side, showing that the basin 

 as a whole occupies a large 

 area, and its amount of wa- 

 ter makes this one of the 

 most important tributaries 

 of the Upper Snake. At 

 this season, indeed, it car- 

 ried fully two - thirds as 

 much water as the Snake 

 itself. About the mouth 

 of the valley, the slopes are 

 covered with partially-ce- 

 mented Post-Tertiary sands 

 and gravels, with occa- 

 sional exposures of white 

 marly clays, supi^osed to 

 be of the same age, though 

 no fossils were seen. The 

 gray sand-stones of the 

 canons are plainly the con- 

 tinuation of the Tertiary (?) 

 beds of Barlow's Eiver 

 directly to the northward ; 

 and the limestones of the 

 high cliffs at the head of 

 the valley are probably of 

 Quebec Group age, perhaps 

 capped with Carboniferous. 

 A small bit of fine-grained, 

 compact sandstone, found 

 loose on the rubbish-slope 

 near the top of the moun- 

 tain by Mr. Brown, con- 

 tains fragments of some 

 thirty trilobites of the gen- 

 era Conocoryphe and JDicel- 

 locephalm (?). 



Snake River escapes from 

 Jackson's Lake at its south- 

 eastern angle and runs off 

 eastward to the valley of 

 Buffalo Fork before renew- 

 ing its southerly course. 

 Yet, from the top of the 

 butte at the mouth of the 

 lake, it is plainly seen that 

 a broad valley extends 

 directly southward from 

 the southern extremity of 

 the lake : and it at once 



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