Lake Otero, N. Me.v. — Herrick. 177 
abut ag;ainst tb.c i;raniU' of the north ;Mid of the San Andres. 
The entire northern ridge of that rancre is bare of stratified 
rocks. To the south of the fault in licatcd the dip of the 
strata is uniformly to the west. The lava flow, part of which 
is represented, extends for about fifty miles uninterruptedly 
in the vallcv cast of the Oscuro mountains and ends abruj^tly 
on the plain formed by the basin of lake Otero, the upper 
part of which was covered by it. 
In the main, and south of the Little Burro block, the 
anticline is relatively simple and not unlike that illustrated as 
typical of "basin range" structure in Pussell's article on lake 
Lahontan (1. c. fig. 44) with which region the present locality 
is naturally compared. The dip on both sides, in the San 
Andres and Sacramcntos respectively, '.? moderate and subject 
to much local variation. 
It may be presumed that the orographic flexture creating 
this axis dates from a period earlier than that of the sedi- 
mentaries deposited on the granite which is everywhere their 
base, for it seems to be true that the 'owest sedimentaries on 
the east (Sacramento) side of the valley are of earlier age 
than those exposed in the San Andres, or, in other words, 
the formation was not homogeneous nor was it uniformly de- 
posited across the incipient uplift. This uplift probablv ex- 
posed the granite during the Carboniterous-Permian interval, 
if such there was, though the greater part of the granitic 
material characteristic of the lower P.^rmian may have come 
from special (focal) uplifts like that forming the foundation 
for Sierra Blanca north of the Sacramento range. 
If we were to assume that the entire Permo-Carbonifer- 
ous was uninterruptedly laid down before the arch began to 
be sprung this would mean that a tremendous elevation was 
at one tune reached and a still more enormous erosion would 
be required, for, granting a thickness of say 30CK) feet to the 
sedimentaries, even a very flat ^rch would have the effect of 
carrying the summit to a great hight in the fortv miles or so 
intervening between the buttresses left as the ranges men- 
tioned. But it is certain that the flexture was accompanied by 
faulting on a large scale. The small ridge forming Cerrito 
Tularosa east of the "sands," and others farther south but 
along the same axis, are of dark earthy limestone with fossils 
