ORE DEPOSITION. 229 
They often form isolated crosses, and are most frequent in the center of the 
grain, from which they diminish toward the edges. Fig. 10 is a drawing of 
such a grain magnified 100 diameters. 
Besides the gangue minerals which have already been mentioned cal- 
cite in its crystallized form is common in many ores. In every case where 
this has been studied microscopically it is found to be later than any of the 
ordinary gangue materials, and also later than the accompanying sulphides. 
Fra. 10.—Microscopic fractures in granitic quartz. 
Where it occurs in connection with barite, ferriferous dolomite, and quartz, 
it follows irregular spaces which are inclosed by the idiomorphic crystals ot 
these minerals, and where, as in the Late Acquisition mine, galena occurs 
as a crust on the fragments of a breccia, this crust is covered with coarsely 
crystalline calcite which forms the immediate wall of the cavity. 
LOCUS OF ORE DEPOSITION. 
With scarcely a single exception the ore in the Aspen district is found 
along faults or faulted and fractured zones. Thus, for example, on Aspen 
