LARAMIE REGION, WYOMING AAI 
must have been rapidly stripped from the granite wall, while 
only insignificant ravines were being cut in the face of that wall. 
Furthermore, where there are no faults, as around the hills southeast 
of Jelm Mountain, the pre-Cambrian rocks rise equally abruptly out 
of soft sandstones and shales. ‘The massive limestone formation, 
which on the east side of the Sherman arch gives the sharp hog-back 
ridges, is replaced in the space of less than forty miles by an entirely 
different series, in which there are no very resistant members. The 
absence of the hog-backs is thus explained. 
The highest surfaces of the broad Laramie plain are certain mesas, 
well represented by the rim of the Great Hollow. The tops of these 
mesas descend very gently from an elevation of 500 feet above the river, 
near the mountains, to about too feet above it, near the city of. Lara- 
mie. At first glance they might be mistaken for outliers carved from 
a plain of alluvial aggradation, for the surface is very even and rock 
exposures are rarely visible at any distance. When carefully exam- 
ined, however, it is found that inclined strata, chiefly of Mesozoic age, 
are exposed in many places throughout the plain; and, although they 
are covered by a layer of gravel, sand, and loam, this covering is never 
deep. Near the mountains on the west the sheet of alluvium, only 
a few feet in thickness, consists of coarse gravel, which is gradually 
replaced farther outward and down-stream by finer and finer materials. 
The deposits are such as are left by shifting streams, graded only 
for coarse material, which slowly reduce the level of their basins, 
while at the same time they widen them much more rapidly. The 
Laramie basin then is not a filled basin, but a cut plain; and the 
Laramie River is at present engaged in continuing the planation at a 
slightly lower level. 
In the Laramie basin one finds no distinction between open valley 
heads and canyons below. All is open. I interpret this to mean 
that the Leslie cycle still reigns in that part of the district, the wave 
of rejuvenation, whatever its cause, not having worked up the Lara- 
mie River far enough to be felt within the basin. 
The Hecla cycle oj canyon-making.—Reference has already been 
made to the canyons which are sunk beneath the broad valleys 
of the Leslie cycle. As’ the open valleys in the Sherman uplift 
are traced eastward, they become narrower and are walled in by pre- 
