60 ECONOMIC GEOLOGY OF SILVERTON QUADRANGLE, [bull. 182. 
possible bearing upon secondary enrichment of the primary ore 
deposits will be discussed in another part of this report. 
To briefly recapitulate, while many of the fissures in the Silverton 
quadrangle, including some which differ widely in direction, were 
formed at substantially the same time, there have been later periods 
of Assuring, also followed by vein deposition. The oldest fissures 
known have a course somewhat east of north, and appear to have 
been successively cut by later fissures approaching more and more to 
an east-and-west strike. This generalization, however, should not be 
taken too rigidly, as it is very probable that whenever a prominent 
set of nearly parallel fissures were formed, other fissures intersecting 
the dominant set at various angles were produced at the same time. 
Lastly, there has been Assuring not followed, as far as known, by any 
deposition of quartz or ore in the resulting fractures. 
The evidence afforded by the intersections as to the possible conju- 
gate character of the dominant northeast-southwest and northwest- 
southeast fissures is unfortunately inadequate, owing to lack of satis- 
factory exposures of such junctions to determine the question one 
way or the other. 
GEOLOGICAL AGE OF THE FISSURES. 
Beyond the statement that the ore-bearing fissures of the Silverton 
quadrangle are of Tertiary and probably of late Tertiary age, it is 
not at present possible to fix the exact geological time at which the 
first considerable fracturing took place. More precise knowledge 
waits upon a determination of the exact age of the Telluride conglom- 
erate and the San Juan and later volcanic formations. The Telluride 
formation, Avhich, when present, underlies the volcanic rocks, has 
been provisionally referred to the Eocene by Cross. 1 It is difficult to 
conceive of its being older than this, as it has been found by Cross to 
rest unconformably on the Colorado Cretaceous shales. The Telluride 
formation and the younger volcanic series have been cut by several 
great intrusive masses of monzonite, such as the Sultan Mountain 
stock, which is thus the most recent rock in the quadrangle and can 
hardly be older than the Neocene. Since the lodes occur in the mon- 
zonite as well as in other rocks of the area, their formation prob- 
ably does not antedate the latter part of the Tertiary, and may, 
indeed, have extended into the Pleistocene. 
PROBABLE DEPTH AT WHICH THE PORTIONS OF THE FISSURES NOW 
EXPOSED WERE ORIGINALLY FORMED. 
According to Cross, 2 the maximum thickness of the volcanic rocks 
in the Telluride quadrangle was, in round numbers, 5,000 feet. 
Owing to the varying thickness of different members of these series 
Geologic Atlas U. S., folio 57, Telluride, Colorado; also Proc. Colo. Sci. Soc, Vol. V, 1894-1896, 
PP. 835-241. 
2 Telluride folio. 
