68 B. Sitliman—Turquors of New Mexico. 
earry chiefly argentiferous galena, with some gray copper rich 
in silver, giving promise of mines of value when opened in 
depth. I have elsewhere spoken more particularly of these 
veins and of the rocks that contain them. ese rocks are all 
eruptive rocks of the family of the augite trachytes, the kind 
which, the world over, carries the richest and most permanent 
ores of silver, with some gold. In the center of this district, 
which is not more than about six miles by four in extent, 
rises the dome of Mt. Chalchuitl (whose name the old Mexi- 
-In the other direction this mountain has its drainage into the 
valley of the Galisteo, which forms the southern boundary of 
the Cerillos district. The age of eruption of these volcanic 
rocks is probably Tertiary. The rocks which form Mt. Chal- 
chuit] are at once distinguished from those of the surround- 
ing and associated ranges of the Cerillos by their white color 
and decomposed appearance, closely resembling tuff and kaolin, 
and giving evidence to the observer familiar with such phe- 
nomena of extensive and profound alteration; due, probably, 
to the escape through them, at this point, of heated vapor 
of water and perhaps of other vapors or gases, by the action 
of which the original crystalline structure of the mass has been 
completely decomposed or metamorphosed, with the production 
of new chemical compounds. Among these the turquois is the 
most conspicuous and important. In this yellowish-white and 
kaolin-like tufaceous rock the turquois is found in thin veinlets 
and little balls or concretions called “ nuggets,” covered with 
a crust of the nearly white tuft, which within consist generally, 
as seen on a cross fracture, of the less valued varieties of this 
gem. = 
The observer is deeply impressed on inspecting this locality 
with the enormous amount of labor which in ancient times has 
been expended here. The waste or debris excavated in the 
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acres. On the slopes and sides of the great piles of rubbish 
are growing large cedars and pines, the age of which—judging 
from their size and slowness of growth in this very dry regioD 
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