10 GEOLOGY OF ASPEN MINING DISTRICT, COLORADO. 
carbonate is not in any way affected by their presence, so that often a 
quartz grain is entirely inclosed in a single crystal of dolomite. In certain 
areas, however, these quartz grains become more numerous and cluster 
together, and irregular portions may be made up mainly of quartz, while 
closely adjoining portions are chiefly of dolomite. Where the silica is thus 
concentrated into an area of considerable size a chert nodule is the result. 
The silica then becomes cryptocrystalline and sometimes chalcedonic, and 
it incloses some carbonate in the form of rhombohedra, sometimes scattered 
sparingly through the chert, sometimes becoming very numerous. Besides 
the small rhombohedra, there are larger irregular areas which have the 
typical structure of dolomite, consisting of numerous small interlocking 
erystals; these are evidently residual, while the isolated rhombohedra, 
which have a marked zonal structure attesting gradual growth by succes- 
sive additions, are evidently concentrations of similar smaller residual areas. 
When the rock is strained so that oxidizing agents have obtained 
entrance along cracks, there is often formed uniformly throughout the rock 
a small amount of iron oxide, which occurs along the cracks themselves, 
between the crystals of dolomite, and along the cleavage, especially where 
this is strongly developed by the strain. he oxide seems from its arrange- 
ment to be derived from material already in the rock, rather than that 
brought in along the crevices; and analyses of the fresh rock usually dis- 
close a small but constant percentage of iron. This iron is probably in the 
form of carbonate, and is crystallized with the dolomite. | . 
Thickness of beds— This dolomite varies in thickness in the same way as do 
the Cambrian beds below it, thickening gradually toward the south and 
thinning toward the north of the area mapped; the maximum thickness at 
the southern margin of the Tourtelotte Park special area being about 400 
feet, while at Lenado the average is probably 250 feet. 
age—The age of these beds is fixed as Silurian by fossils which have 
been found in the same formation in various parts of Colorado, but no 
further subdivision can well be made. This horizon was assigned by 
Peale! to the Caleiferous epoch, at the base of the Silurian series, while 
Mr. Emmons’ reported not only fossils of the Calciferous epoch but also 
some which resembled forms of the Niagara and the Trenton. What 
1A. C. Peale, Ann. Rept. Hayden Survey for 1874, p. 112. 
2Mon. U.S. Geol. Survey, Vol. XII, 1886, p. 61. 
