48 GEOLOGY OF ASPEN MINING DISTRICT, COLORADO. 
The diorite of Castle Peak and White Rock Mountain differs only 
slightly from the rock which on the map of the Aspen district has been 
named a diorite-porphyry. The Elk Mountain diorite, of which Castle 
and White Rock peaks furnish examples, is, according to Cross, a fine and 
even grained rock, characterized by a nearly equal development of horn- 
blende and biotite, with some quartz and local augite. The Aspen diorite- 
porphyry differs from this granular rock chiefly in structure, being finer 
grained and possessing a weakly developed porphyritic structure. These 
peculiarities of crystallization, which in some of the sections examined show 
a tendency to disappear and to merge into the nonporphyritic or granular 
structure, are amply accounted for by the conditions under which it crys- 
tallized—in a thin sheet at some distance from any large mass of heated 
rock. Miners and others who have frequent occasion to mention the rock 
in everyday life will do well to call it ‘‘diorite,” and thus avoid confusion 
with the quartz-porphyry which occurs in the same district and which 
belongs to a quite distinct type. This rock has been intruded into the 
Aspen district from the south, the intrusive flow cutting down slightly 
across the bedding, and it is undoubtedly an offshoot of the great diorite 
mass of the Elk Mountains. 
QUARTZ-PORPHYRY. 
The quartz-porphyry of the Aspen district is easily distinguished from 
the diorite-porphyry in the field by its nearly white color, which may 
be changed to brown or yellow. It shows very few phenocrysts, and has 
a groundmass which is aphanitic to the naked eye. It has a close 
resemblance, both in the field and under the microscope, to the White 
Porphyry of Leadville. 
Habitus——The porphyry occurs chiefly as a sheet which is approxi- 
mately conformable to the bedding of the sedimentary rocks in which i 
occurs, but which locally cuts across the bedding. This sheet is always, 
in the Aspen district, at a higher horizon than that of the diorite-porphyry, 
being usually near the base of the Weber shales, so that the two rocks 
do not come in contact. A great number of faults have occurred since 
its intrusion, which, with the after-effects of erosion, have operated to 
remove portions of the sheet, so that its original distribution can not 
always be closely observed. Its thickness on Aspen Mountain, however, 
