UNAWEEP COPPEE DISTEICT, COLO. 
21 
essentially of quartz, alkali feldspar, and muscovite. In the larger 
dikes the individual crystals of the minerals may reach several inches 
in greatest dimension, and lenses of quartz that extend a few feet 
along the strike and are 6 inches or more in thickness are not uncom¬ 
mon. The granite is also cut by dikes of diabasic rock. The 
largest of these dikes observed, in Taylor Gulch, is said to have 
an average thickness of 200 feet or more. The writer did not con¬ 
firm this measurement, but the dike seems to have fully this thickness 
at some points. The dike on the Chance and Bell claims is 12 to 14 
feet thicky and the one on the McKinley claim is reported to be of 
the same thickness, and is generally supposed to be the same dike. 
There are numerous other dikes of various sizes in the district. 
The composition of the dikes varies somewhat, but they are all dia¬ 
base composed essentially of plagioclase, augite, and magnetite. The 
rocks have been considerably altered, and an accurate determination 
of the composition of the feldspar is not possible, but it is probably 
close to andesine. In most of the dikes the augite has been largely 
serpentinized. 
SEDIMENTARY ROCKS. 
The sedimentary rocks of the area consist of shales and sandstones. 
Lying immediately on the granite is 50 to 70 feet of red ferruginous 
shale which breaks down readily, forming a talus that obscures the 
immediate contact. In some places there seemed to be a few inches 
of a rather fine conglomerate immediately above the granite, but at 
no point observed was the actual contact of granite and sediments 
well exposed. Overlying the red shale is 200 feet of*red sandstone 
with beds of fine conglomerate. Above the red sandstone are beds 
of white to gray sandstone, the thickness of which was not estimated. 
No fossils were collected in the district, and the formations were 
not traced into areas where the age of the sediments has been deter¬ 
mined. The beds above the pre-Cambrian farther west in Unaweep 
Canyon are considered by Cross ^ to be equivalent to the Dolores and 
La Plata formations of the San Juan region, and the rocks in the 
Unaweep district are probably to be correlated with these formations. 
RELATION OF SEDIMENTARY AND IGNEOUS ROCKS. 
The sedimentary rocks at all points where they were observed 
appear to have been deposited on the granite, and as no dikes were 
found in the sediments all the igneous rocks appear to be older than 
the sedimentary rocks. The exact age of the crystalline rocks can 
not be stated, but there is little doubt that they are of pre-Cambrian 
age and that they had been deeply eroded before they were covered 
by the sediments.^ 
1 Cross, V liitman, Jour. Geology, vol. 15, No. 7, p. 648, 1907 
2 Idem, p. 676. 
