20 GEOLOGY OF ASPEN MINING DISTRICT, COLORADO. 
the same time. From what little chemical examination has been made, it 
seems that the dolomization has not been so complete in the aphanitic 
dolomite as in the crystalline variety, a fact which may be earls by 
the closeness of texture of the former. 
While the evidence shows that in many parts of this region there 
must have been an erosion interval between the deposition of the lower 
dolomite and that of the Parting Quartzite series, it is not clear whether 
such was actually the case in the Aspen district, although such an interval 
is suspected. . 
' correlation—At Leadville there occurs at this same horizon a bed of 
quartzite separating the dolomite of the Silurian from that of the Carbon- 
iferous. Mr. Emmons! states that its thickness averages 40 feet, with a 
maximum of 70 feet, and gives 
it the name ‘Parting Quartz- 
ite.” Since this quartzite, 
although differing somewhat 
lithologically from the inter- 
calated quartzites and litho- 
graphic dolomites which are 
found at Aspen, is yet evi- 
dently their  stratigraphical 
equivalent, occurring at the 
same horizon, being of the 
same thickness, and bearing 
aillewee of dopositien under closely similar conditions, the term ‘‘ Parting 
Quartzite series” has been adopted for the formation at Aspen. Mr. 
Eldridge? has described what is also evidently the equivalent of these beds 
in the Crested Butte area, thus: 
Fic. 3.—Nodule of lithographic dolomite in dolomitic sandstone. 
The upper division, 60 to 90 feet thick, consists mainly of green, yellow, red, 
and white shales, with more or less arenaceous and calcareous layers, the latter 
passing into thin limestones. The persistence of its general lithologic character 
renders this horizon easily recognizable. 
Age of Parting Quartzite beds—In the absence of fossils this series has been 
generally included in the Silurian beds, and the Devonian has been sup- 
1 Geology of Leadville: Mon. U. 8. Geol. Survey. Vol. XIT, 1886, p. 61. 
2Geologic Atlas U. S., Anthracite-Crested Butte, folio 9, 1894, p. 6. 
