22 GEOLOGY OF ASPEN MINING DISTRICT, COLORADO. 
sandstones. Thus the entire sequence of deposition is seen to have been 
remarkably like that at Aspen. Mr. Walcott observed evidence of a slight 
unconformity by erosion between the Devonian beds and the overlying 
limestones. Evidence of such erosion interval was not obtained at Aspen, 
but it has been observed by Mr. Emmons?* between the Parting Quartzite 
and the overlying Carboniferous limestones on the East Fork of the 
Arkansas. 
These grounds, therefore, are thought sufficient for placing the age of 
the Parting Quartzite series as probable Devonian.’ 
CARBONIFEROUS FORMATIONS. 
LEADVILLE LIMESTONE. 
Above the Parting Quartzite series there comes a heavy dolomite sim- 
ilar to the Silurian, which is in turn overlain by a massive blue limestone 
of quite different structure. This blue limestone is distinctly separated from 
the argillaceous and carbonaceous limestones and shales above, which 
belong to the Weber formation. The dolomite and the blue limestone are 
classified together as the Leadville limestone, the name being taken from 
the corresponding dolomitic beds at Leadville. Locally the dolomite and 
the limestone are known by the names of ‘‘blue and brown lime,” from the 
circumstance that the dolomite contains a small quantity of iron, which, 
when oxidized, as in most of the rock near the surface, gives a general 
brown tinge, while the pure limestone retains its blue color. The thickness 
of the Leadville formation is comparatively uniform throughout the district, 
so far as observed, having an average of about 350 feet, of which from 200 
to 250 feet are dolomite, and 100 to 150 feet are of blue limestone. 
THE DOLOMITE. 
origin. —It has been a hotly contested question in Aspen, and one which 
has had an important economic bearing, whether the Carboniterous dolomite 
was originally deposited as such or became dolomized by subsequent action. 
Without going into details, it will simply be stated here that there is abun- 
dant evidence of two periods of dolomization, one oe which oceunred before 
\Geology of inemyatvailts: Mon. U.S. Geol. Survey, Vol. XX, p. 61. 
»Since the above was written, additional fossils have been collected from the bed which has 
been mentioned above, by Mr. Tower, in the summer of 1896. These fossils were submitted to 
Mr. Charles D. Walcott, who pronounces them to be scattered and broken plates of placoganoid 
fishes and to be undoubtedly of Devonian age. 
