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of inassivf bliu'-gmy luul blue- black 
stratified limestone of great hard- 
ness and duraJ)iIity. tilted at slight 
angles in various directions, and 
void of any marked folds. No 
granitic or volcanic rock is an}-- 
where exposed in these, and the 
rocks which underlie this limestone 
series are concealed. At least 
4.00i) feet in thickness of these 
rocks are exposed, and from the 
occurrence of characteristic fossils, 
they are seen to belong to the great 
system of rocks in Mexico and 
Texas, known as the Comanche 
series, and are of Lower Cretaceous 
and probable Jurassic age, i. c, 
older and lielow the Meek and 
Ha^'den section of the North Amer- 
ican Cretaceous. This is the same 
system of limestone which com- 
poses many of the mountains of the 
Trans- Pecos region of Texas, and 
the silver-producing mountains of 
Mexico and New Mexico, and 
which occurs in Texas as less un- 
durated chalky strata. The.se rocks 
were elevated into their present po- 
sition at the close of the T'ljper 
Cretaceous period, and have un- 
dergone much denudation and 
erosion by subsequent events. 
i l> ) Tilt Si'irrii iliinnitd or lnuf 
hacks. This system of small moun- 
tains, skirting the north flank of 
the main mountain mass, and in 
which the mines are located are a 
part of and composed of the same 
rock as the Sierra Grande or main 
mountain mass, but differ from 
