pines and shrubs. On one side, the rocks1
tower into a precipice, and so overhang as to form a cave, at another
place the side is low, and formed by the broken rocks that were removed
from the top of the cliff. The excavations, which appear to be about
200 feet in depth and 300 or more in width, were made in the solid
rock, and thousands of tons of rock have been broken out. The lower
part of the working is funnel-shaped, and is formed by the sloping
banks of the debris or fragments of the side walls. On the debris, at
the bottom of the pit, and on the bank of the refuse rock, pine trees
are now growing. There are several other pits in the vicinity more
limited in extent, and some of them, apparently, more recently
excavated. Prof. Benjamin Silliman, Jr., who visited this locality in
1880, states : " The age of eruption of these volcanic rocks is
probably tertiary. The rocks which form Mount Chalchihuitl are at once
distinguished from those of the surrounding and associated ranges of
the Cerrillos by their white color and decomposed appearance, closely
resembling tufa and kaolin, and giving evidence of extensive
alteration, due probably to the escape through them, at this point, of
heated vapors 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 turquoise is the most conspicuous
and important. In this yellowish-white and kaolin-like tufaceous rock
the turquoise is found in thin veinlets and little balls or concretions
called nuggets, covered with a crust of the nearly white tuff, which
within consists generally, as shown on a cross fracture, of the less
valued varieties of this gem, but occasionally affords fine sky-blue
stones of higher value for ornamental purposes. Blue-green stains are
seen in every direction among the decomposed rocks, but the turquoise
in mass is extremely rare, and many tons of the rocks may be broken
without finding a single stone that a jeweler or collector would value
as a gem. The waste or debris excavated in the former workings covers
an area which extends over twenty acres at least. On the slopes and
sides of these great piles are large cedars and pines,
1
The Chalchihuitl of the Ancient Mexicans : Its Locality and
Association, and Its Identity with Turquoise. Am. J. Sci. II., Vol. 25,
p. 227, March, 1858.
