6 GEOLOGY OF ASPEN MINING DISTRICT, COLORADO. 
which are afterwards enlarged by solution of the quartz. The secondary 
quartz cement appears to be more easily dissolved than the detrital grains, 
so these grains become partly isolated, and the rock in some degree returns 
to its original condition of sandstone. Some of the smaller grains which 
are thus isolated are carried out of the rock mechanically, so that the 
cavities continually widen, until the rock becomes so cellular as to be 
hardly able to hold together, and finally crumbles into soil. The iron 
which gives the brown color to the weathered rock does not seem to be 
derived from the rock itself, but to be secondary. It is a yellow hydrated 
oxide, very small in amount, which is deposited in thin coatings on the 
walls of the cavities and in interstices. In places, however, especially 
toward the top of the beds, there are many irregular nodules and seams of 
hard hematite, which microscopic study shows to have been formed by 
actual replacement of the quartzite, the quartz having been gradually dis- 
solved to make way for the iron. In this process the quartz cement is first 
dissolved, so that the ore contains isolated bits of quartz, which are some- 
times fragments consisting of several grains, but usually single grains 
which have been stripped of their cement, and the ragged and corroded 
outlines of the grains themselves show that they are also undergoing 
replacement, although more slowly. The source of the numerous iron 
nodules in this horizon is probably the oxidation of glauconite in the beds 
immediately above. 
Glauconitic grit—A bove the altered dolomitic sandstones comes a thin bed 
which presents in the field certain striking characteristics that readily 
separate it from the beds above and below. The bed is not more than 
15 or 20 feet thick, compact, and of a peculiar reddish color, mottled 
with yellow on weathered surfaces. It is of fine grain, but contains many 
greenish crystals of calcite, which, though small, have a porphyritic 
appearance; there are also greenish fragments of detrital feldspar, so that 
the rock resembles in appearance an altered eruptive rather than a sedi- 
mentary. It has a granular texture, an irregular fracture, and is distinctly 
heavy. 
Under the microscope the rock is seen to be made up, in large 
measure, of detrital grains of quartz, feldspar, and mica. The feldspar is 
sometimes quite fresh, but is oftener altered to a muscovitic aggregate; the 
mica is colorless. Apatite and zircon are found in perfect crystals inclosed 
