ae Et RO Te ee 
J. P. Kimball on the Geology of Chihuahua. 381 
more or less perfect, fluting their whole surface which is of 
a dark ochreous color. <A similar effect is to be seen in the 
bluffs back of Fort Davis. The lithological characteristics above 
noted here likewise prevail, but the rock is still softer than in 
i its sedimentary character more positive. 
A quarry opened by Maj. Gen. Merritt in the base of a high cliff 
of this rock, exhibits it to great advantage. Here the interior 
proves an earthy, or soft, fine-grained, alumino-silicious aggre- 
gate, containing scattered grains of quartz and minute crystals 
of pyrites. It is of light reddish and gray tints, quarries soft, 
and hardens on exposure. Of this material the officers’ and 
other quarters are built. Thus, within a short interval, we find 
the same formation presenting the two lithological extremes, 
between which in its wider development it affords numerous 
varieties. The softer variety has all the characteristics of the 
cantera, of which the better structures of Chihuahua are built, 
and which is found in all parts of the state under conditions simi- 
lar to its occurrence at Fort Davis. Hence I have adopted the 
Mexican term, and, as common, applied it to the whole forma- 
tion. In this connection I may venture to mention, without 
attempting at present to dot ite the impression which these and 
correlative facts have forced upon me, namely, that the meta- 
morphism of this formation consists of chemical and molecular 
changes under the influence of the atmosphere and ercolating 
waters, and that the range of alteration will be found to be lim- 
ited to the exposed portions. : 
The relation of the cantera at Fort Davis to the underlying 
Cretaceous limestone is rendered clear by the fact that a quarry 
for the supply of a lime-kiln has been opened by Gen. Merritt 
. 
Just below the level of the plain. 
a the skirting highlands in heavy rains, the plain issud- 
oe eats | 
* See a cut in Mex. Bound. Sury., i, p. 88. 
. 
