648 WHITMAN CROSS 
and Paleozoic beds and in many places rest on the pre-Cambrian 
granites and schists, is well illustrated by the Hayden maps. This 
overlap is particularly well exhibited in the southern Elk Mountains 
and some of its details are shown in the Anthracite-Crested Butte 
folio (13). ; 
The fact that no Paleozoic formations are present in the Uncom- 
pahgre Plateau was recognized by Peale and expressed on the Hayden | 
map. If, however, the greater part of Peale’s “Triassic” in that 
area be now referred to the La Plata Jurassic, as has been done in 
the preceding discussion, the question arises as to whether evidence 
of erosional unconformity between the La Plata and the under- 
lying Dolores Triassic exists in that area or not. Our observations 
on this point were quite limited but tend to show that such a break 
does occur. It is certain that in the vicinity of the Unaweep Canyon 
the dark-red Triassic strata are much thinner than in the Dolores 
Valley to the west and this decreased thickness appears to be prin- 
cipally due to erosion of the massive red sandstone forming the upper 
part of the Triassic. 
On the north side of West Creek, which is the western stream - 
flowing out of Unaweep Canyon, Messrs. Emmons and Kay found 
the La Plata to rest on granite near the shore line of the Permian ( ?) 
beds which will be discussed in another part of this article. At the 
head of West Side Creek a few miles south of the Unaweep the pink 
La Plata sandstone rests on thin-bedded sandstones and_ shales 
belonging to the lower part of the Dolores formation, as shown by 
the pressence of the fossiliferous ‘‘Saurian conglomerate.” Near 
the head of Atkinson Creek on the western side of Uncompahgre 
Plateau, the La Plata seems to rest on gneiss, according to the state- 
ment of Peale cited on p. 643. 
These facts and the evident variation in thickness of the massive 
Dolores sandstone, which we noticed at many places, seem to speak 
for a relation of the La Plata and Dolores very similar to that existing 
on the western and southwestern slopes of the San Juan Mountains. 
But much more careful observation is needed in the Uncompahgre 
Plateau to determine to what extent the absence or variable thickness 
of the Triassic beds is due to pre-La Plata erosion. Personally, I 
believe that the Triassic beds were originally deposited over the Uncom- 
