220 GEOLOGY OF ASPEN MINING DISTRICT, COLORADO. 
dolomite. This jasperoid: may be white or various shades of red, gray, 
brown, or black, the colors resulting from different forms of iron in varying 
proportions. This term covers in part what is known among the Western 
miners as flint, chert, quartz, etc., and also includes the so-called “jasper” 
of the Lake Superior regions. 
So great has been the silicification along certain fault zones that these 
faults may be traced continuously by the blocks of jasperoid which outcrop 
or lie on the surface, for the jasperoid forms a dikelike body in the lime- 
stone. These zones are especially well developed in the southern part of 
Tourtelotte Park. Pl. XXXII shows in the center such an outcrop of jas- 
peroid which has been left above the surrounding surface by differential 
erosion. 
Silicification is always an accompaniment of ore deposition, both 
occurring under the same conditions. The former process, however, is 
more widespread, and hence has occurred not only in places favorable to 
ore deposition but along zones where there has been no mineralization of 
importance. Quartz formed by the replacement of the limestone is always 
present in the ores as a gangue. Some ores are especially high in silica, 
that of the Dubuque in Queens Gulch on the Castle Creek fault containing 
40 or 50 per cent. This quartz occurs in the ore itself and extends from it 
out into the surrounding limestone, showing its great abundance and the 
easy conditions under which it is deposited. The phenomena accompany- 
ing the development of quartz in ore bodies are similar to those along frac- 
tures where there is no great mineralization. Where, as is often the case, 
it occurs in company with other gangue minerals, among the most promi- 
nent of which are barite and dolomite, the relation of all these shows that 
they have crystallized at about the same time and under the same condi- 
tions. All have evidently replaced the limestone, in which they generally 
form idiomorphic crystals which are often quite isolated. 
From the association of jasperoid with dolomite belonging to the 
second period, as well as from the evidence which microscopic study 
affords, it is clear that the quartz was deposited at the same time and 
under the same conditions as the magnesia. In the case of the dolomite 
the conclusion has been arrived at that the change from limestone has been 
brought about by ascending magnesia-bearing waters, and it has been 
shown that such a change is now going on at Glenwood Springs. The 
