236 WHITMAN CROSS 
ual’’ except the comparatively thin bed. The mapping of these 
lithologic units, however, would involve topographic maps of 
large scale and great accuracy, and an expenditure of time which, 
in view of the present condition of the survey of Colorado, 
renders such detailed work inadvisable. But assuming that the 
mapping of each “‘lithologic unit”? were feasible and that it had 
been carried out, without some grouping of these units the map 
would be merely a lithological map. It seems improbable that 
even Mr. Willis would be content with such a map, in view of 
the range of this section. The problem of grouping the units 
is, however, the same as that of dividing the entire section for 
the purpose of representing the main ascertainable facts of geo- 
logic development. 
It is known by fossil and structural evidence that the section 
in question embraces practically all that is present in the region 
of the Carboniferous (including Permian) and Triassic systems. 
In earlier reconnoissance surveys and reports touching the prov- 
ince, this general distinction has been drawn. The upper half, 
which has a reddish color, has been distinguished as the ‘Red 
Beds” of assumed Triassic age; the lower half, which has no 
prevalent reddish color, has been referred to the Carboniferous 
from scanty fossil evidence. In the recent areal work of the 
geological survey in charge of the writer, it has been found that 
the reddish color line is both indefinite, through gradations, and 
variable as to horizon. For any but crude reconnoissance work 
the red color line is not a practical guide. Through careful 
study of the section in several quadrangles, it has been found 
that the lower two thousand feet of the complex contains in its 
limestones and shales a persistent Lower Carboniferous inverte- 
brate fauna. While extensive collections have been made from 
various horizons, the paleontological opinion rendered by Mr. G. 
H. Girty is to the effect that no faunal groups are indicated 
warranting a suggestion that any particular one of the many 
planes of lithologic change represents an interval of unrecorded 
events, or gives grounds for a division, for the expression of 
correlation with structural members of the Upper Carboniferous 
