18 GEOLOGY OF ASPEN MINING DISTRICT, COLORADO. 
was found there are waters which carry at the present time sulphu- 
reted instead of carbonated waters, as the occurrence of strong sulphur 
springs in the Molly Gibson mine shows; other watercourses close by 
carry carbonated surface waters. In this case a rock which had been 
exposed to surface waters and had become considerably oxidized was by 
some slight change of currents made the channel of sulphureted waters, 
with the change which has been described. 
The bleaching under immediate surface influences is best observed 
in actual outcrops, where it is most actively going on. It is especially 
well shown on a road on the mountain side to the south of Lenado. 
The weathered surfaces are altered to a light greenish yellow; and this 
Fic. 2.—Zones of reduction in oxidized fine-grained dolomite. 
alteration is also well marked along the joint planes, the alteration having 
taken place for an inch or so on both sides of the crack. The process 
is one which has been observed in the Maroon formation, also in the 
Triassic and in the other red-colored beds of the region; the microscope 
shows that it is essentially a withdrawal of the iron from the bleached 
rock. Carbonated surface waters, which take the iron into solution, are 
the probable agents. 
Conditions of depositionThe character of the basal bed of the Parting 
Quartzite series indicates deposition in water much shallower than that 
in which the dolomite sediments below were laid down, and in a position 
nearer the shore. Most of the coarse materials—quartz, feldspar, occasional 
mica, and tourmaline—are evidently the product of the erosion of granite, 
