GEOLOGY OF THE ASPEN MINING DISTRICT OF 
COLORADO, 
By J. E. SPURR. 
CH ART ER 1: 
ROCK FORMATIONS. 
sRANITE. 
Oldest of all the rocks in the Aspen region is granite. This rock, 
often changing into gneisses and schists, forms a permanent floor on which 
all the sedimentary rocks rest, and through which, so far as is known, all 
the other igneous rocks have forced themselves to reach the position in 
which they are now found. 
Description—This granite presents a considerably diversified appearance 
even over a limited area, but these variations are all in reality shght struc- 
tural modifications of one type, for chemical and microscopic work shows 
that the rock possesses a remarkable uniformity. The variations from an 
originally uniform rock seem to have been brought about by slight differ- 
ences in local conditions, and often by changes which have occurred since 
the original consolidation of the rock; and in many cases the nature of 
these changes can be discerned. Within the limited area shown on the 
special maps of the Aspen district (which was as far as careful study was 
carried, although frequent reconnaissances into the surrounding territory 
were made for confirmation of the results here obtained), the granite is 
mostly of a massive character. The most common variety is moderately 
coarse, of a general light-green color when fresh, and reddish-brown when 
weathered. It has a coarse granular texture, and the constituent minerals 
1 
MON XXxI——1 
