I22 UNIVERSITY OF COLORADO STUDIES 
The Granite Foothills.—These are well rounded hills furnishing an 
example of old topography. Gulches cut by stream erosion are deep 
but mostly broad with rather gently sloping sides, in marked contrast 
to the abrupt walls of the gorges in the sedimentary rocks. Disin- 
tegration of the exposed granite extends for a few inches or a few feet 
in depth and cracks as well as soft streaks in the rock permit the roots of 
shrubs and trees to penetrate as far as need be for moisture. On level 
ground, especially in lower places where an accumulation of soil has 
occurred, there is a well-developed plant covering, but on steep slopes 
and on the tops of hills only trees and shrubs with roots far down in 
rock crevices can exist with any degree of security. The weathering 
of the rock is continually loosening the foothold of herbaceous plants. 
(See Figs. 6, 8, 9.) 
Soil.—A large part of the rock ridges is entirely without soil. Where 
soil accumulates it is rather fine grained, bakes hard, and dries quickly. 
It is very difficult for shrubs and trees to gain a foothold here and so 
these places are generally covered with grass and herbaceous plants. 
Such grassy patches do not occur on the granite hills for when soil 
accumulates there it leads to development of shrub growth. The soil of 
canyons shows an accumulation of humus due to the decaying leaves of 
deciduous trees and shrubs which grow along the stream-banks. Moist 
meadows, with abundant humus, and even swamps, are found in the 
valleys of the granite hills. Such meadows do not occur so frequently 
in the region of sedimentaries, but occasional meadows are found: 
mention may be made of a broad meadow east of Owl Canyon where 
a thin layer of humus was seen to overlie the sandstone bed rock. Layers 
of gypsum and other calcareous material occur at different points in the 
sedimentary series and when these weather out they form a soil which 
will support very little vegetation of any kind. 
Climatic Conditions.—All of the region studied is arid. At Ft. 
Collins the average annual rainfall is 14.4 inches. No other records 
from stations near at hand are known to the writer. However, it can 
scarcely be doubted that the precipitation is greater at higher altitudes 
than at lower and that all of the district studied has a greater rainfall 
than that recorded for Ft. Collins. The difference is probably not more 
