CARBONIFEROUS FORMATIONS. 35 
Aspen to Lenado. Its thickness was measured in the Bimetallic tunnel 
at Lenado, where it is not complicated by faults, as in the Cowenhoven 
tunnel, and was there found to be approximately 200 feet. It is, however, 
variable, for the gray color sometimes extends up into the overlying 
calcareous standstones, so that these are included in the formation. This 
color is not always, perhaps not usually, original, but is the result of 
bleaching in rocks which have once been colored. Cases of this were 
noticed in the exposures in Hunter Park, where the brown calcareous 
sandstones which immediately overlie the basal gray limestone were 
bleached for a short distance on each side of fracture planes to a gray 
color. Microscopically this rock is made up of cryptocrystalline calcite, 
with considerable crystalline quartz in irregular grains, and crystals of 
eypsum and of pyrite. The texture is porous and the pores are irregular 
in shape. 
Above the basal gray bed the rocks are practically the same 
throughout the whole thickness of the formation, except that toward the 
bottom there are rather more thin limestone beds than at the top. The 
prevailing rock is a dark reddish-brown, impure, micaceous sandstone, 
thin-bedded, and often shaly; in the more massive portions cross-bedding 
may ordinarily be observed. This sandstone passes by easy transitions, 
vertically and also laterally, into various allied but distinct rocks, which 
form beds of slight thickness. Most important among these rocks is a 
gray grit, which is made up mainly of quartz, mica, and feldspar, all 
being evidently derived from the disruption of granite. On the one hand 
this passes into a fine conglomerate and on the other into a light-gray, 
often reddish, micaceous sandstone or fine grit. Generally associated with 
the grits, but sometimes occurring in isolated beds in the brown sand- 
stones, are other rocks—shales, generally red, sometimes green, which are 
transitional from the sandstones, and various types of green and blue 
limestone. These shales and limestones, however, make up a very small 
part of the rocks. Probably nine-tenths of the formation consists of thin- 
bedded brown sandstones, while four-fifths of the remainder is made up 
of gray grits and fine conglomerates. There is no massive light-red 
sandstone in the series, so far as observed. 
Microscopic structure—Jnder the microscope the gray grits are found to 
consist almost wholly of granitic material—quartz, feldspar, and mica, 
