INTRUSIVE ROCKS. AND 
important disturbances which are manifested in the rocks of this district it 
is necessary that this episode should be understood. 
INTRUSIVE ROCKS. 
There are two distinct varieties of intrusive rocks in the Aspen district, 
a quartz-porphyry and a diorite-porphyry, both usually much altered. 
DIORITE-PORPHYRY. 
Habitus —This porphyry is a dark-green, fine-grained rock, showing 
much decomposition, even to the naked eye. It occurs chiefly, so far as 
the limits of the Aspen region go, in the form of a single sheet, which has 
the usual characteristics of interbedded sheets in the Rocky Mountains. 
It is in a general way parallel to the bedding of the sedimentary rocks 
im which it has intruded itself, so that in any very limited area it appears 
as a simple interbedded sheet; when followed along the strike, however, it 
is found to cut across the beds at intervals, usually at a slight angle, so that 
it is only by its position relative to the various sedimentary beds that any 
change is noticeable. This single sheet of porphyry is found on the 
southern border of the Tourtelotte Park special area (which was the southern 
limit of the present examination), at about the horizon of the Parting Quartzite 
series. Here it has a maximum thickness cf about 150 feet; it frequently 
cuts across the Parting Quartzite, or surrounds it so as to conceal its out- 
crop. This sheet can be traced from here toward the north nearly contin- 
uously on the bare hilltop; it takes a position permanently below the 
Parting Quartzite series, and gradually cuts lower down into the Silurian 
dolomite. Just before reaching the area included in the special map of the 
Tourtelotte Park mining district, it cuts down across the formation a little 
more sharply, and near the southern end of this map it enters the Cambrian 
quartzite From here its outcrop shows that it still cuts downward toward 
the north, till on Aspen Mountain it lies at the very base of the Cambrian. 
On West Aspen Mountain, the most northerly point at which it has been 
found, there is only 5 or 10 feet of conglomeratic quartzite between the 
porphyry and the granite, and on Kast Aspen Mountain it is found in 
contact with the granite Ina lateral extent of 34 miles this sheet therefore 
cuts across the formation 500 or 600 feet, always downward to the north. 
There is also a gradual thinning of the sheet toward the north. Its 
maximum thickness of 150 feet at the southern limit becomes 30 to 40 
