GLACIATION IN THE TELLURIDE QUADRANGLE 505 
Speaking generally, it may be said that so far as rocks near the 
surface are concerned, the plateau portion of the quadrangle is for 
the most part underlain by sedimentary formations, while the peaks 
and ridges in the mountainous portion are composed very largely of 
igneous rocks. Of the latter there are (1) various phases of intrusive 
bodies which in form include stocks, laccoliths, dikes, and sills, and 
in composition range from basalt to rhyolite, including andesite, 
Fic. 1.—Dike in sandstone on the north side of the San Miguel River, one-fourth 
of a mile northeast of the mouth of Big Bear Creek. Looking northeast from a point 
too feet above the stream. 
gabbro-diorite, diorite-monzonite, diorite-porphyry, monzonite, and 
granite-porphyry; and (2) widespread sheets of bedded extrusive 
rocks of three series named in order from the oldest, the San Juan, 
the Silverton, and the Potosi rhyolitic series. 
The intrusive bodies vary in size from dikes whose maximum 
thickness is to be expressed in feet and inches, and which at best 
have contributed only in the most insignificant degree to the 
physiographic and structural features of the region (Fig. 1) to 
