ransome.] OUTLINE OF GEOLOGY. 31 
short tributaries of the San Miguel heading in Ingram and Savage 
basins. On the eastern border the Lake Fork of the Gunnison River 
has exposed a mass of granite which is entirely surrounded by vol- 
canics, but as its boundaries are at least in part fault lines, it is pos- 
sible that dislocations on these planes have produced this outcrop by 
local upthrust. The structures observable in the formations beneath 
the volcanics show the Silverton quadrangle to be near the center of 
an area of repeated orogenic disturbance in Paleozoic and Mesozoic 
times. Some of the igneous phenomena seem also to center here, but 
it is as yet premature to conclude that the principal center of the 
entire volcanic activity was in this vicinity. There may have been 
several centers of eruption whose products combined to form the great 
complex known. 
All of the principal members of the volcanic series thus far identi- 
fied are exhibited in the Silverton quadrangle. There are also a 
number of intrusive rocks, most of them having been previously 
observed in the Teliuride quadrangle. The principal elements of the 
igneous complex will now be briefly described. 
San Juan formation. — The earliesl volcanic formation of this series 
revealed to the present time has been termed the San Juan forma- 
tion. It is a more or less perfectly stratified series of tuffs and 
agglomerates of andesitie rocks, no lava flows having been observed 
within it. The character of the San Juan tuff and agglomerate in its 
greatest known development is very clearly exhibited on either side 
of Canyon Creek. It reaches there a thickness of fully 2,500 feet 
(PI. IV). In many parts the San Juan seems to be a water-laid 
deposit, so fine is the stratification. The agglomerates are more or 
less chaotic, but exhibit distinct stratification when seen in large 
exposures. These tuffs and agglomerates are also exposed in typical 
development in the mountain group southwest of Silverton, of which 
Sultan Mountain and the Grand Turk are the most notable summits. 
The thickness is here, however, but 1,500 feet. On the east side of 
the Animas the San Juan tuffs are exposed in very irregular thick- 
ness on the slope of gneisses and granites descending from the Needle 
Mountains. Apparently, erosion was here extremely active in the 
period succeeding the San Juan fragmental eruptions. 
On the eastern side of the Uncompahgre River the floor upon which 
the San Juan rests and also its upper surface dip rather abruptly 
southward, bringing the succeeding formation abruptly down to the 
level of the river, and the San Juan does not reappear in the canyon- 
like valleys of the streams in the eastern portion of the quadrangle. 
It is plain that in the entire valley of the Animas from Silverton to 
Animas Forks the volcanics rest upon a surface much lower than that 
wiiich constitutes their base in Sultan Mountain. Whether this fact 
is due to an original depression in this area or to subsequent sinking 
nas not yet been determined. 
