32 GEOLOGY OF ASPEN MINING DISTRICT, COLORADO. 
The beginning of the process of oxidation is seen in these sections to 
be marked by the alteration of the pyrite to iron oxide. This oxide stains 
the rock, concealing the black color given by the carbonaceous material; 
and thus the red, yellow, and brown varieties of the Weber limestone, which 
are found outcropping and in mine workings near the surface, are formed. 
The more complete alteration, which produces from the firm, hard; thin- 
bedded limestone the soft black “‘shale,” seems, judging from both micro- 
scopic and chemical examination, to be essentially a process of leaching. 
Much of the iron and of the calcite is removed by percolating waters, which, 
moreover, generally bring. about complete dolomization of the remaining 
carbonate. The withdrawal of this material destroys the cohesion of the 
rock and causes it to assume a soft and plastic form; and analysis shows 
that the argillaceous materials are greatly concentrated by this removal of 
the more soluble constituents. 
Conditions of deposition —Hvidence in certain parts of the Rocky Mountain 
region’ shows that there was an important break between the deposition of 
the lower and the upper Carboniferous, or between the formation repre- 
sented in the Aspen district by the Leadville blue limestone and the Weber 
carbonaceous shales. The Leadville dolomite was probably deposited in 
the waters of a shallowing sea, in accordance with the conclusions already 
stated. At the close of its deposition a subsidence took place, so that the 
water became purer and ceased to contain any excess of magnesia. The 
conditions under which the blue limestone was deposited were uniform. 
The locality was sufticiently remote from shore to be entirely unaffected by 
land sediments, and the water was comparatively deep. The deposits were 
made up mainly of the shells of marme organisms, chiefly Foraminifera. 
At the close of this period of quiet deposition a great upheaval took place, 
so that what had been sea became dry land. When this land became again 
submerged, sediments were evidently deposited in shallow seas near to 
land, and at a rapid rate. The carbonaceous material which is character- 
istic of the Weber formation, and which sometimes becomes so important 
as to form local seams of impure coal, is the remains of plant material which 
was brought down from the land and buried in the rapidly accumulating 
mud at that period. The first sediments consisted chiefly of materials worn 
from the preexisting sedimentary beds, chiefly limestones and dolomites. 
1S. F. Emmons, Geologic Atlas U. 8., folio 9, Anthracite-Crested Butte, Colorado, 1894, p. 1. 
