4 GEOLOGY OF ASPEN MINING DISTRICT, COLORADO. 
origin—It was formerly assumed that a part at least of the Archean 
granite of the Rocky Mountain region was metamorphic in nature, having 
been produced by the profound alteration and crystallization of sedimentary 
beds. In the Aspen region, however, there is no evidence of any sedimen- 
tary origin for the granite; its structure shows only that it has consolidated 
from fusion, without hinting whether or not the materials of which it is 
composed were ever exposed to atmospheric influences before the present 
stage in its history. 
CAMBRIAN SEDIMENTS. 
Description —In the Aspen district, as elsewhere in Colorado, there rests 
directly upon the granite a thin bed of conglomerate, which soon passes 
upward into fine white quartzites. The very lowest layer is an arkose or 
granitic grit, made up of the materials of the underlying granite, only 
slightly rearranged before deposition, so that the exact line of demarcation 
between granite and sedimentary rock is often difficult to distinguish within 
a foot or two. This difficulty is increased by the fact that the granite is in 
many places disintegrated for some little distance below the contact. 
Directly above the contact, however, the material begins to be more sys- 
tematically arranged, the fragments of granite disappear, and quartz assumes 
the most prominent position among the minerals. This mineral occurs in 
bluish translucent grains which average about the size of large shot; these 
are generally inclosed in a fine paste, of kaolinie nature, derived from the 
decomposition of the feldspar in the granite. The size of these grains 
diminishes as the distance from the granite increases, so that the rock 
becomes fine-grained, compact, and of a bluish color, being made up almost 
exclusively of small rounded grains of granitic quartz, which have been 
cemented together by secondary quartz since their deposition. 
Microscopic structure—Under the microscope the lowest layers of these beds 
are seen to be made up mostly of granitic quartz and feldspar, the quartz 
being strained and fractured, as is typical in the granites, and the feldspar, 
which is often quite fresh, being mostly microcline. Alteration of the feld- 
spars has produced aggregations of kaolin and muscovite. Among the 
smaller rounded grains which fill the interstices between the larger pebbles 
quartz is most common, with kaolin and finely divided and irregularly 
packed calcite which has the aspect of lime mud. There is generally some 
iron in the section, which from its position is evidently secondary, and is 
