386 J. P. Kimball on the Geology of Chihauhua. 
Between the city of Chihuahua and El] Paso del Norte the 
hysical and lithological features of the cantera still prevail. 
he road lies over the plain of Encenillas—one of the typical 
valleys above described, extending, together with its bordering 
mountains, within twenty miles of the Rio Grande where lime- 
stone hills are again met with, forming the bluffs of the valley. 
This limestone seems to bear the same relation to the cantera as 
the Cretaceous beds at Presidio del Norte below. Hence I must’ 
take exception to the determination of this limestone by Dr. 
Wislizenus as Silurian, especially as this was upon the inade- 
quate evidence of an “injured and imperfect” coral which he 
refers to Calamopore, and a bivalve shell of the genus Pterinea. 
I did not stop to collect fossils at this locality, no question hav- 
ing been present to my mind on the spot as to its equivalency 
with the Cretaceous as elsewhere observed, and as I was not then 
uainted with Dr. Wislizenus’s report.* The discrepancy be- 
tween the lithological terms employed by Dr. Wislizenus and 
myself proc m a difference of observation and reasoning. 
Cusihuiriachic), and overspreading the middle and northeastern 
art of Chihuahua, and probably also the western part, Dr. 
islizenus designated generally as porphyry, and sometimes as 
granitic, and again as trachytic. Phat what he thus terms 1s 
really the so-called cantera, under its various aspects, is clear 
from his specifying the building stone used in the city of Chi- 
huahua, from which I have taken its local name, as white por- 
phyry, as well as from the fact that he disti shed as granitic 
nd porphyritic its continuation on either side of the El Paso 
road. He similarly characterized the rock which forms the 
mountains of Cusihuiriachic, seventy-nine miles west of the city 
of Chihuahua. Thus it will be seen that in this respect our 
observations agree as to the extent and identity of the moun 
formation, but that we differ as to its origin and lithological char- 
cter. The orographic features of the country as phenomena of 
lenudation have not been noticed by this traveler. And the 
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* Memoir of a Tour to Northern Mexico, connected with Col. Doniphan’s Expe- 
in 1846-7. Senate Doe., Misc. 26, 30th Cong. 1st Ses, See also Report of 
Parry: Mex. Bound. Sury., vol.i, Pt. 2d, 8,9. On the Silurian in the Rocky Mts., 
see a notice by Prof. J. D. Whitney, Proc. Acad. Nat. Se., iii, 307. 
> 
