2 GEOLOGY OF ASPEN MINING DISTRICT, COLORADO. 
of granite can be easily recognized megascopically—bluish quartz, yellow- 
ish or pink feldspar, and mica. The mica is brown when fresh, but most 
of the granite has been slightly altered, and this alteration changes the 
color of the mica to a dark green, which gives the characteristic color to 
the rock where it has not been actually exposed to the atmosphere. Where 
such is the case the active oxidation changes the condition of the iron in 
the mica and its alteration products, so that the prevailing color of the 
rock changes from green to the red of iron oxide. The beginning of this 
alteration is first seen megascopically in the feldspars, for, as is shown by 
the microscope, the first solutions of iron derived from the mica and other 
ferruginous minerals penetrate into the colorless feldspars along their 
cleavages and there deposit their iron as oxide. It is in this manner that 
the characteristic pink color of the feldspars is produced 
Variations of the granite occur in places not far from Aspen, in the 
form of considerable areas of gneisses and schists, but in the Aspen district 
such rocks are not found to any extent. In some places, however, there is 
a slight gneissic structure developed, and these beginnings of parallel 
arrangement are interesting as having a bearing on the history of the more 
completely altered rock. A common type of such gneissic rock is coarser 
in grain than the ordinary variety, is of a dark-green color, and is made 
conspicuous by its brick-red feldspars, which have developed a porphyritic 
habit. There are irregular but in a general way parallel parting planes 
running through this rock, which are often polished and striated; these 
show that the parting is due to deformation of the granite at some time 
later than its consolidation, and is not an original characteristic. This 
conclusion is also reached by microscopic study. Another type of gneissic 
granite is considerably finer grained than the ordinary rock, so that it has 
almost a schistose appearance. This rock seems peculiarly liable to oxida- 
tion, and is therefore often of a reddish color. It contains many small 
flakes of dark mica, which have a parallel arrangement, and in many places 
these mica plates, while still preserving their perfect parallelism, are concen- 
trated into spherical or lenticular bunches, which do not have crystalline 
boundaries. The bunches are similar to the ‘‘eyes” of the so-called 
augen-eneiss, so that the rock is transitional between this rock and 
granite. Whether the large porphyritic crystals of the one variety of 
eneissic granite and the lenticular aggregations of mica in the other have 
