PARTING QUARTZITE SERIES. 17 
The alteration of the maroon color to banded light and dark green, 
by a process of evident reduction, was observed in a specimen kindly 
given by Mr. D. W. Brunton, of Aspen. This specimen was from the 
Free Silver shaft, about 700 feet below the surface. The color of the rock 
is the typical dark red of the oxidized Parting Quartzite series. In the 
vicinity of a set of fine, almost microscopic fractures, which, however, have 
been brought into promimence by the concentration of the reduced iron 
along them, the rock is altered to a light-green color, with bands of darker 
Fig. 1.—Oxidized fine-grained dolomite, with residual areas. 
green following each of the individual fractures. The width of the largest 
band thus altered is about an inch and a half, while smaller bands of 
varying width ramity from the main one (see fig. 2). Within these altered 
zones there are discernible numerous small grains of pyrite, while in the red 
surrounding rock none can be found. The conclusion is that the altera- 
tion has been produced by the reduction of the iron oxide in the rock to 
sulphide, and that this change has been effected by the percolation of 
waters along the fracture crevices. In the locality where this specimen 
MON XXXI——2 ; 
