506 ALLEN DAVID HOLE 
stocks whose dimensions are to be expressed in miles, and which 
have given rise to many of the loftiest and most rugged peaks to be 
found within the area, as for example the Wilson group (Fig. 2) and 
Grizzly Peak. 
With the exception of a few small outlying remnants, the bedded 
volcanic rocks are found only in the eastern half of the quadrangle 
where they form the lofty and rugged peaks and ridges of the San 
Fic. 2.—The Wilson group of mountains, an eroded stock. Looking south of 
west from elevation about 11,200 feet at the mouth of Alta basin. The body of water 
shown in the foreground is a small temporary glacial lake formed by unequal distri- 
bution of drift. 
Juan mountain front. The lowest member, the San Juan series, 
consists of andesitic tuffs, breccias, and agglomerates, cemented to 
some extent by minerals such as calcite deposited by circulating | 
waters, so that in favorable positions steep, clifflike slopes are 
formed. Its maximum thickness is 2,000 feet. The intermediate 
series, named, after the publication of the Telluride Folio, the 
Silverton series, consists of alternating andesitic and rhyolitic flows 
with which are mingled sheets of tuff and breccia of andesite and 
