bansome.] OUTLINE OF GEOLOGY. 33 
northwestern corner of the Silverton quadrangle. This series is dis- 
tinguished as marking the beginning of an epoch of almost exclu- 
sively rhyolitic eruptions. For a time, at least, eruptions of more 
basic locks were suspended. In Potosi Peak itself, and, indeed, in 
the Telluride quadrangle, so far as this series was observed, the floor 
upon which the earliest Potosi lavas were outpoured was approxi- 
mately level. Hut either the great development of the Silverton 
series to the east or other causes have limited the distribution of the 
Potosi in this (irsl observed simple relation to the other volcanics. 
No rhyolites which might be correlated with the Potosi series occur to 
the east in the Silverton quadrangle until the rugged crest of the con- 
tinental divide is reached. Here several summits, like Sheep Moun- 
tain, show light-colored, Huidal lavas at their summits, but they are 
not. sufficiently like the Potosi rhyolites, as known to the westward, 
bo fully justify ,-it this lime a certain correlation between them. 
The rhyolitic Lavas which occur in t he higher summits of the north- 
eastern portion of the quadrangle seem more nearly related to the 
Potosi type; bu1 further examination in the mountains of the Ouray 
iiiadrangle is necessary to show the true relations of these Lavas to 
pose of the Potosi series. 
Stock eruptions. — At a later period than that of the Potosi rhyolite 
here were in the western San Juan several eruptions of magmas, 
Liffering much in composition, and now seen in the form of rather 
'oarsely crystalline rocks penetrating the whole series of surface 
•oleanics to the summits of the highest mountains. 
In the Telluride quadrangle a series of these large stocks were 
feserved, in Mount Sneffels. and Stony Mountain, Ophir Needles, 
T-rizzly Peak, and Mount Wilson, all of whose summits are in these 
lock rocks. There are also several stocks in the Silverton quadrangle, 
he largest being that of quartz-monzonite, the massive rock of the 
maintains immediately southwest of Silverton. This stock has a 
'ery irregular outline. It extends from Copper Gulch on the west to 
he base of Kendall Mountain on the east, and the site of Silverton 
>3 eroded out of this mass. Tlie northern boundary is very much 
oncealed by talus slopes. The evidence that this stock cuts directly 
cross both sedimentary and surface volcanics is clearly seen in the 
xposures of the southern contact, on Sultan Mountain and Bear 
'eak. Smaller stocks of monzonite occur also in the lower part of 
'unninghain and Maggie gulches. A more basio rock of gabbroitic 
haracter appeal's at the head of Henson Creek, where it is crossed 
y the wagon road leading to the divide. 
Other intrusive rocks. — The Silverton quadrangle is notable for 
jreral masses of a peculiar porphyry. This rock occurs in several 
nail stocks; one of them forms a portion of the summit of Red 
buntain, another is on Round Hill, while a third is seen at the head 
Bull. 182—01 3 
