50 ECONOMIC GEOLOGY OF SILVERTON QUADRANGLE, [bull. 182. 
easterly. There arc several other fissures in this vicinity which do not 
strictly belong with any of the systems noted. Several have a nearly 
east-and-west strike, and might, perhaps, be grouped together or 
possibly included with veins of the Golden Fleece system. 
In the canyon of the [Jncompahgre, north of Mount Abrams, the 
thick masses of San Juan breccia and the underlying Algonkian schists 
and quartzites are cut b} T numerous nearly parallel fissures with a 
general trend of about N. 5° TV. and a dip of about 80° to 85° to the 
east. These fractures, which do not, as a rule, contain prominent 
veins, have divided the San Juan breccia into huge, nearly vertical 
slabs (PI. VII), and have thereby contributed to the preservation of 
the lofty cliff faces of this part of the canyon. A second, less promi- 
nent, system of fissures, usually carrying quartz stringers, has a 
general strike of about N. 60° W. 
At the head of Porphyry Gulch, near the western border of the 
quadrangle, the Potosi rhyolite and underlying Silverton series are 
traversed b} r a conspicuous series of nearly vertical fissures, striking 
N 8° W., which do not contain workable ore bodies, so far as known. 
A similar series of fissures is also very prominently shown in the 
steep walls of Canyon Creek northeast of the mouth of Richmond 
Basin. The general strike of these fissures appears to be about N. 
35° W., and they usually dip steeply to the northeast. They belong 
genetically with the northwest-southeast system of lodes as developed 
in Silver Lake and Richmond basins. So far as is known they do not 
guyyj an} T workable ore bodies. 
On the eastern side of Ironton Park are several fissures having a 
general northeast-southwest course and a steep southeasterly dip. 
Their formation was accompanied by considerable normal faulting. 
The parallel coordinated fissures or sheeted zones hitherto described 
all possess considerable width and are characterized by the division 
of the country rock into slabs of relatively great thickness as compared 
with the resulting fissures. But this is not always the case. The: 
fissures may sometimes be verj 7 small and also very closely spaced, as 
in the col separating Lake Como from Cement Creek, where the thor- 
ough manner in which the rock, a much-altered andesite or rhyolite, 
has been sheeted is beautifully shown. This minor sheeting is gen- 
erally parallel in strike with the larger lodes in the vicinity, and at 
the particular jDoint where best shown the fracture planes dip west- 
erly at about 60°. As many as fifty such planes may frequently be 
counted within the width of a foot. There has been some infiltration 
of silica along these minute fissures, which causes them to stand ou1 
on weathered surfaces as little parallel ribs. The local strike of this 
sheeting is about N. 35° E , but it is sometimes disturbed by irregulai 
later cross fractures. No regularity or rhythm could be detected ii 
their spacing. 
Sometimes several nearly parallel fissures of considerable extent 
