Mesozoic and Cenozoic Geology and Paleontology. 5) 
THE CANOZOIC AGH OR THERTIARY PERIOD. 
By S. A. Mirrer, Esq. 
[Continued from Vol. iv., page 46.| 
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 Muddy. ‘This may 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 Planorbis, Unio, He- 
lix, Goniobasis, 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, marly clays, with numer- 
ous concretionary layers, differing from the chalky beds of the Bridger 
and Green river basin. This divide probably forms the junction of two 
great fresh-water lake basins, that may have existed contemporane- 
ously. The two great basins may 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 clays 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 
weathered into curiously castellated forms, and bear such names as 
