120 The American Geologist. August, 1905 
and probably of early Tertiary age. The same is true of the 
neighboring Los Cerrillos hills, the Cerro Pelon, the Tuertos 
group, and the San Ysidro. 
Thirty miles west of the Rio Grande, and about the 
same distance north of the city of Albuquerque are the Na- 
cimiento and Jemez mountains. The first mentioned of 
these is a block mountain 20 miles long. Along the great 
fault scarp, and under the Carboniferous limestones form- 
ing the backslope, the basal crystallines are well exposed. 
These appear to be chiefly granites, so far as observation 
goes. Their age is as yet undetermined. They are for the 
present referred to the Azoic. 
Near the continental divide west of Albuquerque is lo- 
cated the Zuni dome, its top eroded off down to the crystal- 
line basement. The age of the pre-Carboniferous crystal- 
lines is presumably Azoic. As early as 1856 Marcou* men- 
tions a belt of crystallines in the heart of the Zuni range 12 
miles wide, consisting of reddish granite, gneiss and schist. 
Blaket also calls attention to the gneisses and granites of 
this district, and corroborates Marcou's observations. In 
Dutton's* account of the Zuni plateau the presence of 
gneisses or schists is riot mentioned. The granites are call- 
ed Archaean. If, however, the observations recorded are 
correctly interpreted some of the granites are certainly of 
much later intrusion. This author states that they have 
metamorphosed the overlying Carboniferous limestones, and 
calls particular attention to this phenomenon as it is well 
displayed in Mt. Sedgwick, the most prominent feature in 
the field. 
The remarkable mountain blocks known as Sierras Os- 
cura and San Andreas are over 100 miles long and extend 
northward from the Organ mountains north of El Paso. 
The fault-scarps of the two ridges face each other at their 
proximate extremities, a flat valley lying between the two. 
Herrick* mentions the granitic character of the crystallines 
beneath. the the Carboniferous limestones, which dip in op- 
posite directions in the two ranges. The age and lithologic 
* Pac. R. R. Sur., vol. iii, p. 170, 1856. 
t Pac. R. R. Sur., vol. iii, p. 38, 1856. 
± U. S. Geol. Sur., 6th. Ann. Rept., p. 158, 1886. 
* Bull. N. M. Univ., vol. ii. Fasicle No. 3. v. 5, 1900. 
