30 GEOLOGY OF ASPEN MINING DISTRICT, COLORADO. 
The Nodosinella is near N. priscilla Dawson; the Lagena forms are 
near L. parkerina Brady; Textularia is similar in section to T. gibbosa 
D’Orb.; but none of these types admits of positive specific identification 
im cross sections alone. 
age—In Leadville fossils are reported by Mr. Emmons which place this 
horizon in the Lower Carboniferous. At Aspen no further attempt to fix 
the age was made, on account of lack of time and because the determina- 
tion already made is quite sufficiently established. Various Carboniterous 
fossils—brachiopods, crinoids, ete.—as well as the Foraminifera which give | 
the peculiar character to the ‘rock, are abundant in the blue limestone. 
WEBER FORMATION. 
Description—Above the blue limestone, and separated from it by a dis- 
tinct plane, comes a series of thin-bedded carbonaceous limestones and 
caleareous shales. The typical rock of this series is a black limestone, ” 
thin-bedded and aphanitic in texture. It has two mineralogical features 
which are secondary in origin, but which are peculiarly characteristic of 
this horizon—the occurrence of scattered or segregated pyrite and the pres- 
ence of many small irregular veins of white crystalline calcite. The rock 
is usually somewhat dolomitic and locally becomes a true dolomite. 
Near the lower part of the series the rock is slightly more massive, 
becoming often dark blue in color or gray on oxidation. It then somewhat 
resembles certain varieties of the altered blue limestone, with which it may 
sometimes be confounded in the field. This variety of the Weber limestone 
is found throughout a large part of the Hunter Creek and Lenado districts, 
where it occurs in contact with the Leadville dolomite. The contact, how- 
ever, is along a fault which has removed the blue limestone throughout 
this whole area. Above this lower division the limestone becomes black, 
shaly, and carbonaceous, with local thin beds of impure coal. These shales 
change above to the micaceous, thin-bedded gray limestone which has been 
taken as the base of the Maroon formation. 
The Weber limestones are easily attacked by altering agents. ‘Thus they 
are altered by underground waters along faults and watercourses, becom- 
ing silicified and dolomized and changing in color to various shades of red, 
brown, and yellow. Where the Weber limestone is completely dolomized, 
as along fault planes and often in the vicinity of ore bodies, there may 
