ransome.] OUTLINE OF GEOLOGY. 29 
bodies of these mines appear to be in place. Apparently the land- 
slide action is rather superficial, and the siliceous outcrops of the 
larger ore bodies have remained stationary, while the fractured and 
mineralized rock around them have slid in considerable masses down 
the slope. Obviously such sliding renders superficial prospecting very 
unsatisfactory. It is quite possible that some ore bodies have been 
covered by landslide material and entirely concealed. For further 
account and discussion of landslide areas in this and other portions 
of the quadrangle the reader is referred to the heading " Landslides" 
(p. .'37), in the outline of geology given by Mr. Whitman Cross. 
OUTLINE OF GEOLOGY. 
By Whitman Cross. 
SAN JUAN VOLCANIC AREA. 
The complex of volcanic rocks within which the Silverton quad- 
rangle is situated is one of the most extensive in the Rocky Moun- 
tains. It consists of a scries, several thousand feet in thickness, of 
tuffs, agglomerates, and lava flows. The more or less distinctly hor- 
izontal surface volcanics have been penetrated by later stocks of vari- 
ous rocks, ranging in composition from gabbro almost to granite, and 
by numerous small dikes of a considerable variety of rocks. The 
eruptions began with the Tertiary and continued during the greater 
part of that era. 
The area covered at the present time by these volcanic rocks is in 
general well shown by the Hayden map of Colorada. The main peaks 
of the San Juan Mountains are situated in the western center of the 
area. From this point the volcanic series extends northward across 
the Gunnison River to the West Elk Mountains, eastward to the San 
Luis Valley, and southeasterly a broad arm passes a considerable 
distance into New Mexico. 
The former extent of the San Juan volcanics was very much greater 
than at present. There is evidence of enormous erosion, which was 
greatest along the western and southern sides, and it appears that 
very possibly the high mountains of the Telluride and Silverton quad- 
rangles are near what was the center of the volcanic area at its maxi- 
mum. Certainly the former border of the volcanic pile must have 
been located many miles farther west and south than at present. 
The Hayden map of this region is only of general value. There is 
great complexity within the area of surface lavas and agglomerates, 
and the map shows in part quite incorrectly the relations of sedimen- 
tary and other formations on the borders of the volcanic rocks. The 
resurvey of this extensive tract has not progressed sufficient^ to per- 
mit the preparation of a satisfactory outline of San Juan geology. In 
the Telluride folio, however, an attempt has been made to present 
the results of the more recent work in their 
