240 GEOLOGY OF ASPEN MINING DISTRICT, COLORADO. 
which run into the hill in this vicinity show the Weber rocks to be much 
broken up between the glacial drift and the Pride fault. Ordinarily the 
brecciation is greatest close to the fault and at the other end close to the 
surface, while in the middle the rock is not so much broken. This seems to 
show that the brecciation was due partly to the fault movement and partly 
to the overriding of the glacier. Throughout the whole rock the main 
feature is the extraordinary alteration to eypsum. Vein gypsum (selenite) 
forms in every crevice and chink, and many of the limestone bowlders are 
also completely altered to eypsum while still preserving the structure of 
limestone. Such bowlders show ordinarily under the microscope that the 
gypsum has actually resulted from the alteration of calcite. Thin sections 
show mainly gypsum in coarse, interlocking crystals, while scattered 
throughout is considerable calcite in coarsely granular aggregates or 
interlocking crystals. The calcite forms small, irregular masses with well- 
defined boundaries, or occurs in small grains more or less thickly dissemi- 
nated in the gypsum; or, not infrequently, it is concentrated into isolated 
rhombohedra, recalling the phenomena attendant upon the silicification of 
limestone. The alteration of the carbonate to sulphate is thus shown in 
its various stages. The Castle Creek fault, from the point where it crosses 
Woody Creek after traversing Red Mountain, is continuously marked for 
several miles down the valley by a zone composed largely of gypsum. 
From a thick deposit which outcrops near Woody Creek along this fault a 
sample was taken, which was a fine, white powder consisting almost 
entirely of gypsum, as seen by the following analysis: 
Analysis of sample from deposit along Castle Oreek fault, near Woody Creek. 
Per cent. 
Silica (SiOy yarns nek eee ee eee LO OSS oe Soe nae eee a net 1.46 
Carbonidioxiden(COs)i ese) seo eee eens jo See eee ee anes eee 2.29 
Siallalanwee twenopat6ls) (SO))) asee sadsaseaosneesenas ossceceacuass Saad Seas 40.74 
Timoe\(Ca@) eo asseee ee wee ae tae eee Hee Le Eels eto stars 31.86 
sMiaonesiay (Mie @)) ie ene we ae Berry ae eee etl eens eee ee 44 
The gypsum on Aspen Mountain is evidently post-Glacial, since it has 
altered and cemented a glacial breccia. Along Woody Creek its distribu- 
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