iy ee 
G. K. Gilbert—Ancient Outlet of Great Salt Lake. 257 
like passes, depressed somewhat below the level of the shore line, 
and leading to other basins. Following the Detroit P% e 
would be led to the Huron basin and would find there a shore 
line so nearly on a level with the Erie that he could not readily 
determine which was the higher. Following the Buffalo Pass 
he would find a continuous descent for many miles to the Onta- 
rio basin, and in that basin he would find no water-line at the 
level of the Erie shore. In each case he would learn from the 
form of the passage that it had been the channel of a river, 
and in the latter case he would learn from the direction and 
continuity of descent, and from the absence of corresponding 
shore lines, that it had been the channel of an outflowing river. 
in regard to Lake Bonneville. To discover its outlet it 
was necessary to find a point where the Bonneville shore line 
was interrupted by a pass of which the floor was lower than the 
shore line, and which led to a valley not marked by a continu- 
ation of the shore line. These conditions are satisfied at 
Rock Pass, and, in addition, there is a continuous descent from 
the pass to the Pacific ocean. All about Cache Valley the 
Bonneville shore line has been traced, and it is well marked 
within a half mile of the pass. The floor of the Pass at the 
the conclusion is irresistible that here the ancient lake outflowed. 
At the divide a portion of each wall of the ancient channel 
is composed of solid limestone, and its floor is interrupted by 
knolls of the same material. It is evident, too, that the chan- 
nel has lost something in depth, for Marsh Creek and some 
smaller streams at the south have thrown so much debris into 
it as to divide it into several little basins occupied by ponds 
and marshes. It is not improbable that twenty or thirty feet 
have thus been built upon the floor and that the original bed of 
the channel where it crosses the limestone is 360 or 370 feet 
lower than the highest Bonneville beach. Still we must not 
coexistent level of the lake, but rather that during the exist- 
ence of the outlet its channel was slowly excavated to that ex- 
is sustained in a very striking manner by the phenomena of the 
shore lines. 
