30 ECONOMIC GEOLOGY OF SILVERTON QUADRANGLE, [bull. 182. 
From the western front of the San Juan Mountains one looks down 
upon a great plateau level which reaches for hundreds of miles west- 
ward. This level is nearly 2,000 feet below the base of the volcanic 
.series. The isolated mountains which modify this plateau country in 
the zone bordering the San Juan Mountains are in all cases due to 
igneous intrusions which have in a measure protected the softer sedi- 
mentary beds from erosion. 
The removal of the western part of the original volcanic complex 
uas revealed a complicated structure in the underlying older forma- 
tions, showing that the general center of volcanic activity has also 
loeen the site of several great disturbances in earlier geologic times. 
About a center not 3^et very well located, because covered by the 
remaining volcanics, the sedimentary formations of Paleozoic and Mes- 
ozoic times have been upturned and eroded at several periods. This 
structure is well shown in the zone bordering the present western 
projection of the San Juan Mountains and in the canyons cutting 
through the volcanics. 
The first quadrangle in this area to be resurveyed was the Telluride, 
lying directly west of the Silverton. The main western front of the 
San Juan Mountains traverses this quadrangle from north to south in 
.an irregular line, and the fine exposures of the high peaks and deep 
canyons permit a clear understanding of the principal elements in the 
mass of the volcanic series, in this portion of the area at least. The 
sedimentary formations below are also very clearly shown in the Tel- 
luride quadrangle. The somewhat detailed text of the Telluride folio 1 
must be referred to for much general information which is applicable 
to certain parts of the Silverton quadrangle. 
GEOLOGY OF THE SILVERTON QUADRANGLE. 
Location with regard to the volcanic area. — The Silverton quadrangle 
lies almost wholly within the volcanic area, but two streams, belonging 
to the drainage of the Colorado River, have cut through the volcanics 
and disclosed the underlying formations. 
On the north the Uncompahgre River with its tributaries, Canyon 
■Creek and Red Creek, reveal the Algonkian quartzites and the Paleo- 
zoic and Mesozoic formations underneath the volcanics. The lower 
members of the section appear in the Silverton quadrangle. From 
the south the Animas River has penetrated far into the quadrangle, 
and below Silverton it flows in a canyon excavated below the volcan- 
ics. This stream has cut a deep gorge through the Needle Mountains, 
which rise immediately south of the Silverton quadrangle to eleva- 
tions of over 14,000 feet. These mountains are composed very largely 
of Archean and Algonkian rocks. On the west no stream has eaten 
back into the area of this quadrangle with the exception of the two 
1 Geologic Atlas U. S., folio 57, 1899. 
