54 ECONOMIC GEOLOGY OF SILVERTON QUADRANGLE, [bull. 182. 
Assuring within the various volcanic rocks of the quadrangle. It is 
veiy rarely possible to study in any one mine the passage of a given 
fissure from one rock into rock of a different sort, and thus the kind 
of observations which could best establish such characteristic differ- 
ences, if they exist, is wanting. The massive lavas and indurated 
flow-breccias seems to have a general tendency toward comparatively 
simple Assuring, such as results in fissure veins of moderate size and 
regularity or in sheeted zones. In the softer tuffs and volcanic brec- 
cias the Assuring tends to be irregular, resulting in stringer or breccia 
lodes. Irregular Assuring appears to be favored also by alternations 
of harder and softer members of the nearly horizontal volcanic 
series, as at the North Star (King Solomon) mine. In this instance 
the softer rocks were regarded by the miners as the more favorable 
for ore. According to Purington, 1 in the northeast corner of the 
Telluride quadrangle the fissures passing from the San Juan formation 
and massive andesite up into the Potosi rhyolite series contain there 
less ore, and appear to have originally formed with less open space 
than in the underlying andesitic rocks. There are no workings in the 
Silverton quadrangle which afford an opportunitj^ to verify this state- 
ment, but the Potosi rhyolite is generally regarded by the miners as 
unfavorable to ore in the region where it prevails. Near the head of 
Porphyry Gulch, however, a vein carrying galena and sphalerite ore 
up to 10 inches in width has been prospected in the Potosi rhyolite. 
The vein strikes N. 50° E. It can be followed for a short distance tea 
the southwest, over the bare rhyolitic surface, when it either dies out 
or is cut off by a numerous series of small, nearly east-and-west frac- 
tures filled with white quartz and often showing excellent illustrations 
of linked-vein structure on a small scale. 
In the intrusive stocks of monzonite, such as that of Sultan Moun- 
tain, the fissures are, as a rule, of simple regular character and 
moderate width. They are usually occupied by fairly simple fissure 
veins, subject to local contractions or enlargements, but seldom losei 
their simple linear character. As examples, may be cited the fissures 
of the North Star, Hercules, and Little Dora veins in Sultan Moun- 
tain, and of the Hamlet vein near Middleton. 
The foregoing generalizations are admittedly based upon scanty 
data, and are to be considered rather as suggestions indicating lines 
of future inquiry than as authoritative statements of facts. 
As a rule, local geological structure has had veiy little discovera- 
ble infiuence upon the Assuring. The lack of any regular relation 
between Assures and schistosity has already been pointed out. Con- 
tacts between rocks of different kinds and ages also appear to have 
had but little effect upon the fracturing and subsequent veining. 
This is partly due to the fact that the contacts are frequently nearly 
horizontal, wdiile the Assuring took place along nearly vertical planes. 
Eighteenth Ann. Rept. U. S. Geol. Survey, Pt. Ill, 1898, p. 774. 
