16 GEOLOGY OF ASPEN MINING DISTRICT, COLORADO. 
ground waters, there is a secondary growth upon these grains, so that they 
often become perfect crystals. Crystals of pyrite are also often found. 
Such specimens are usually traversed by a system of fractures, indicating 
the nature of the altering agents, and these fractures are filled with fresh 
crystalline calcite. The coloring of the rock on oxidation is due to the 
formation of very small but uniformly disseminated amounts of iron oxide. 
In those rocks which contain pyrite some of this oxide is seen to come 
from the alteration of the sulphide; but as the change takes place also in 
rocks which have no trace of pyrite, it is probable that there exists a small 
amount of iron in the fresh reck, in the form of carbonate. Analyses of 
the rock show it to be strongly magnesian, although the amount of magne- 
sium is not always quite equal to that in normal dolomite. 
The maroon member—Structurally this is simply a variety of the sandy 
lithographic dolomite, which is thin bedded and sometimes shaly, and so 
has been especially easy of access to altermg agents, which have given it 
its brilliant coloring. The more compact dolomite in the vicinity of these 
shaly beds often partakes of their peculiarities of tint, showing that the 
color does not arise from any original difference. 
Three different processes of coloring have been noted—one of reduc- 
tion, one of oxidation, and one of leaching by surface waters. The process 
of oxidation gives the dark-red or maroon color; that of reduction a dark- 
gray, dark-green, or nearly black color; that of leaching a light-yellow 
to nearly white color. The coloring matter is probably chiefly iron, 
although the brilliancy suggests the presence of some of the other metals. 
The various stages of the oxidizing process are well seen in the specimens 
taken from mines. The fresh rock is a vivid light green for the most part; 
and while this may not have been the original color, it is the earliest 
stage which has been observed. This becomes red in places, there being a 
variety of transitional tints, which are often symmetrically concentrated into 
concentric rings; in others there is a very sharp line between the red and the 
green portions of the rock, and as these portions are irregular in shape and 
intimately mingled, there results a pronounced mottling. The oxidation is 
accompanied by a shrinking of volume, so that while the green rock may be 
hard and compact, the red is closely jointed and brittle. When the rock is 
mostly altered to the red it still contains small rounded or lenticular resid- 
ual areas of the green, often no larger than peas or shot. (See fig. 1.) 
