Mesozoic and Camozoic Geology and Palceontology. 235 



Before reaching Bridger station the strata on either side of the 

 road are horizontal, or nearly so. A long, flat ridge extends down a 

 little east of north from the Uinta mountains, between Black's Fork 

 and the Mudd3\ This ma}^ be regarded as the geological divide be- 

 tween the waters of the Great Salt Lake Basin and the drainage of 

 Green river. The Muddy is one of the branches of Black's Fork, 

 which flows into Green river, and west of this stream we have what is 

 called the eastern rim of the Great Basin of Salt Lake. If we were 

 to travel southward to the foot of the Uinta mountains, from the rail- 

 road along this divide, we should be able to detect no well-marked line 

 of separation between the Green River Group and the Wasatch Group. 

 Bridger's Butte, as well as the entire eastern portion of this divide 

 fronting the valley of Black's Fork, exhibits a large thickness of the 

 somber, indurated sands, clays, and sandstones of the Bridger Group, 

 passing down into light buff, chalky layers, with Fkmorbis, JJnio, He- 

 lix, Goniohasis, etc. Within a distance of ten miles to the west of this 

 butte the little streams cut through the pinkish beds of the Wasatch 

 Group, then pass up into whiter, indurated, marl}- claj's, with numer- 

 ous concretiouar3' layers, differing from the chalky beds of the Bridger 

 and Green river basin. This divide probabh' forms the junction of two 

 great fresh-water lake basins, that may have existed contemporane- 

 ouslv. The two great basins ma}' have been connected with each 

 other at different points at some stages of their growth, but there is 

 an abrupt, persistent, very marked difference in the character of the 

 sediments of the two basins. While the Green River and Bridger 

 Groups abound with fossils, the Wasatch Group, like all the rocks of the 

 west that are characterized by brick-red coloring matter, is compara- 

 tively quite barren. At Bridger station, and from Bridger to Aspen, 

 which is about 24 miles, the ochreous beds of the "Wasatch Group are 

 well exposed on both sides of the road, and the valley through which 

 the road passes from Piedmont to Aspen is carved out of this Group 



The tunnel at the head of Echo canon is cut through the reddish 

 and purplish indurated sands and cla3"s of the Wasatch Group. It is 

 770 feet in length. The valley of Echo canon is one of erosion, and 

 on either side the rocks rise wall-like 500 to 1,000 feet, or have been 

 wefithered into curiously castellated forms, and bear such names as 



