440 ELIOT BLACKWELDER 
Colorado and Southern Railroad advantage has been taken of this 
depression. The bottoms of these saddles now lie from one to three 
hundred feet above the present creeks. ‘They have no very clear 
relation to the present stream courses. While it has not been possible 
to trace these valleys directly into the open head-water courses which 
represent the Leslie cycle in the type locality, both have the same 
general relation to the upper or Sherman surface and to the modern 
canyons. 
As stated above, the Laramie basin seems best explained as a 
depression excavated in the Sherman peneplain. In this connection 
I have considered the possibility that the basin is a modified downwarp 
between two upwarps or horsts; but this view seems to necessitate a 
series of remarkable coincidences by which several small upwarps 
affected only the somewhat irregular areas of resistant rocks and not the 
softer strata now exposed in the basin. On the other hand the present 
conditions are such as would be produced by the deeper excavation 
of a plain in which both hard and soft rocks are exposed. Rejuvena- 
tion readily explains this excavation. ‘The soft Mesozoic beds were 
rapidly cleaned out of the Laramie syncline while the hard Paleozoic 
limestones and pre-Cambrian rocks were left in relief. If running 
water was the agent of denudation, the downward cutting was checked 
at the level of stream-grade and then planation at a slowly subsiding 
level ensued. If wind has been the dominating agent, then these 
nice adjustments are unnecessary. ‘There is clear evidence that both . 
processes are in operation, and since the Laramie river is perennial 
the water erosion probably dominates. 
On first consideration one may experience some difficulty in explain- 
ing on this hypothesis the steep slope by which the Laramie basin is 
separated from the mountainous region on the west. Sheep Moun- 
tain and Jelm Mountain rise abruptly out of the plain. Both are 
partially flanked by faults, but the faults appear to be of ancient 
date (Cretaceous-Eocene) like those elsewhere in the district, for they 
have no topographic expression where they run out into the sedimen- 
tary rocks. From this fact it appears improbable that they are 
recent scarps. The faults have brought the soft Cretaceous shales 
squarely in contact with the much harder pre-Cambrian rocks— 
in the case of Sheep Mountain, a firm granite. The weaker strata 
