36 SEOLOGY OF ASPEN MINING DISTRICT, COLORADO. 
with the accessory materials. In the coarser varieties of the rock there is 
very little indication of water action, the fragments being large and angu- 
lar, and the different minerals being present in about the same proportion 
as in granite. There is always, however, a parallel arrangement of the 
flakes of mica, which shows that they were deposited in water. In the 
slightly finer varieties there is a more distinct sedimentary structure, and a 
small amount of calcite is present, probably detrital. Zircon and magnet- 
ite are present in rounded grains, and crystals of specular iron and of red 
hematite, probably secondary, are common. ‘The mica is sometimes green, 
sometimes colorless; the green variety becomes colorless by a process of 
bleaching. The iron thus removed from the biotite has probably gone to 
form the crystals of hematite. 
From this rock to the finer dark-red sandstone there are many transi- 
tional stages. The red sandstones differ from the gray grits in being of 
finer grain, in containing more calcareous and less granitic material, and 
in the appearance of certain new minerals. Quartz and feldspar, chiefly 
microcline, with mica, are present in about the proportion in which they 
are found in granite. The quartz is always fractured and cracked, as in 
granite, and the feldspar is sometimes fresh, but usually more or less 
altered to muscovite and kaolin. The mica is either green and pleochroic 
or colorless and without pleochroism. Much of the colorless mica is 
undoubtedly muscovite, but the production of a colorless mica by the 
bleaching of biotite is a process which can be observed in all its stages. 
The iron which is separated out in this process at first concentrates along 
the cleavage of the biotite, and afterwards is leached out and disseminated 
in the rest of the rock as earthy hydrated oxide, giving the red color to 
the rock in the field. In some areas the beginning of concentration of this 
earthy oxide into the crystalline form is seen; this is essentially a bleach- 
ing process, so far as the resultant color of the rock is concerned. Among 
the minor detrital materials which are derived from the abrasion of granite 
are zircon, apatite, tourmaline, and magnetite. In many of the sections 
the magnetite is many times more abundant than in the granite, indicating 
a concentration of this mineral by wave action, such as at present produces 
the magnetic sands on our shores. here are also detrital grains of lime- 
stone or dolomite. A widely distributed mineral is glauconite, in small, 
irregular grains. This is generally fresh, or shows the beginning of oxida- 
