CARBONIFEROUS FORMATIONS. 23 
the deposition of the blue limestone, while the other was much later, and 
was closely connected with the ore deposition. There are numberless proots 
of the latter process all through the Aspen district, especially in the more 
highly mineralized localities, where the blue limestone has been altered into 
dolomite in zones following faults or fractures which cut across the bedding 
of the limestone, or in zones following the bedding planes. It is also proved, 
however, that the great body of dolomite which forms the lower part of the 
Leadville formation was formed previous to the fractures along which the 
waters which effected the later dolomization ascended; for the contact of 
the dolomite and the limestone is faulted and broken by these fractures, 
exactly as the other sedimentary formations are. 
Throughout the district the dolomite and the limestone maintain about 
the same relations and have about the same thickness and the same well- 
marked plane of separation, although in places this uniformity is obscured 
by faults, as is the case on the whole southern part of the district, from the 
Roaring Fork to Lenado, over which area the blue limestone is cut off by a 
fault which runs nearly parallel to the bedding. In Tourtelotte Park, near 
Castle Butte, is a locality where it was at first supposed that the dolomite 
was missing and that the blue limestone rested directly upon the Parting 
Quartzite series; but subsequently this appearance was found to be due to a 
fault. At Leadville the corresponding formation is entirely of dolomite, 
but has a thickness of only about 200 feet. In the Crested Butte district 
Mr. Eldridge gives a thickness of 400 to 525 feet, of which the upper 75 
to 150 feet is a massive bluish limestone, while the rest is grayer and 
dolomitic. The lateral extent of this dolomite bed is therefore very great, 
and the stratigraphical distinction of the dolomite from the overlying 
limestone is persistent, at least throughout the Aspen, Crested Butte, and 
Anthracite districts. Such widespread lithological peculiarities can not be 
ascribed to any local metamorphism, but to some uniform widespread cause 
which acted before the deposition of the blue limestone, since this horizon 
shows none of its effects. The microscopic peculiarities of the Carbon- 
iferous dolomite are identical with those of the Silurian, and for the same 
reasons which have been enumerated in considering the origin of the 
Silurian it is probable that the lower 250 or 200 feet of the Leadville 
formation was deposited originally as a calcareous sediment, and that these 
sediments became dolomized subsequently, but before the deposition of the 
