210 GEOLOGY OF ASPEN MINING DISTRICT, COLORADO. 
from a locality on the top of Aspen Mountain, on the divide between Keno 
Gulch and the broad depression on the north side of the mountain. Across 
this divide the Saddle Rock fault runs, having blue limestone on the east 
side and granite on the west. Four samples were selected trom this vicinity 
for analysis, being taken out of tunnels which cross the fault and penetrate 
the blue limestone below. Nos. 1 and 2 are of samples taken along the 
main fault zone. Nos. 3 and 4 are from different points about 100 feet 
east of the fault, where the limestone was only partially altered. 
Analyses of rock from dolomized zones of Aspen Mountaan. 
No. SiOz. Cao. MgO. Fe.0,. FeO. | 
1 1.02 33.74 16.76 2.10 .06 | 
2 13.68 35.98 8.25 1.88 64 | 
3 31.12 37.28 54 .36 .19 | 
4 7.78 38.85 9.97 .88 22 
Study of these analyses shows that they represent four distinct transi- 
tional stages in the passage of limestone to dolomite, and that the dolomi- 
zation is also accompanied by a corresponding ferration, while the amount 
of silica varies. Thus No. 3 is nearly pure siliceous limestone, while No. 1 
is not far from the composition of the average dolomite. 
This local dolomization almost invariably accompanies the ore. Even 
when the latter is in blue limestone there is usually a sort of envelope of 
dolomite around it, which in turn is surrounded by the limestone. In rare 
cases, when the ore is directly inclosed in blue limestone without such an 
envelope, its analysis shows the presence of magnesia, while the limestone 
is almost entirely pure. 
The microscope affords means to trace the alteration of the blue 
foraminiferal limestone into dolomite in its various stages. In this process 
the coarse calcite becomes broken up into smaller crystals, which assume 
the rhombohedral form and the yellowish tinge distinctive of dolomite. 
The characteristic structure of the blue limestone persists for a time during 
this alteration, but becomes fainter as the crystallization proceeds. The 
coarsely crystalline calcite and the cryptocrystalline variety which form 
the two distinct phases of the blue limestone both become finely crystallme 
dolomite, but the texture of the two varieties continues different for some 
