228 GEOLOGY OF ASPEN MINING DISTRICT, COLORADO. 
mineral. Along these crevices it is apparent that the metallic sulphides 
have been introduced by circulating waters and have there been deposited. 
The analysis on page 224 of tennantite from the Mollie Gibson mine shows 
that it contains a large amount of silver sulphide, and explains why this 
crisscross spar invariably forms pay ore. The slight greenish stain which 
arises from the alteration of the sulphide is taken by miners as indicating 
the richness of ore throughout the district. Fig. 9 is a photograph of a 
particularly good specimen of } this crisscross spar, kindly made by Mr. 
D. W. Brunton, of Aspen. 
It seems evident that these crosses were ‘formed along fractures made 
Fig. 9.—Crisscross spar. 
in the barite since its formation. But as they are typically nonpersistent, 
the question arises as to whether the fractures themselves were nonpersist- 
ent or whether the sulphides have been deposited only at the intersection 
of two or more fractures. The writer at first was inclined toward the latter 
belief, but some phenomena of fracturing which point to the former alterna- 
tive have been observed in the granites of Smuggler Mountain, not far from 
the place where the crisscross spar occurred. Certain zones of this granite 
have developed a gneissoid or slightly banded structure. Microscopic study 
shows that this change is due to shearing, and one effect of this shearing 
has been to bring about the formation of tiny fractures in the quartz grains. 
These fractures are intermittent, like the sulphide seams in crisscross spar. 
