SOUTHERN PACIFIC LINES 163 
is overlain by white massive limestone with many large Exogyras of 
Upper Cretaceous age. 
Not far beyond this place the tracks approach those of the north 
line, which they closely parallel to Anapra siding. Here begins the 
long climb to the top of the high terrace plain that borders the Rio 
Grande Valley and extends far westward across southwestern New 
Mexico. (See p. 133.) On the ascent there are fine views to the north 
up the Rio Grande Valley, to the east to the long west slope of the 
Franklin Mountains with its succession of westward-dipping strata, 
and to the south to high ridges in Mexico. On this upgrade there 
are many cuts which afford excellent exposures of the sand and gravel 
making up the desert plain, the top of which is reached near Mastodon 
siding, so named because the remains of a mastodon were excavated in 
the slope on the northeast. This great elephant was formerly abun- 
dant over a large part of the present United States, and his remains, 
NW. 
Cerro de Muleros 
% 
WSS PRESS aE so 
Da 
,, 
Vertical scale 
! Mile ° so0Feet 
i i i. 
FIGURE 40.—Section through Bowen siding, New Mexico 
sa 
as also those of the mammoth, a somewhat similar animal, are found 
at many places. 
From Mastodon west for 17 miles there is a tangent to the foot of 
the Potrillo (po-tree’yo) grade where there is a rise to a slightly higher 
bench on the general plateau. To the north are the steep-sided 
East Potrillo Mountains, which consist of limestone of Comanche 
(Lower Cretaceous) age.” 
From Potrillo siding west there is a down grade to Mount Riley 
siding, which lies south of Mount Riley. This very prominent vol- 
canic mass, just west of the north end of the East 
Potrillo Mountains, was apparently a vent from which 
40." lavas were ejected in Tertiary time. To the west are 
New Orleans broad recent lava fields of the West Potrillo Moun- 
miles. ; : 
tains, with many craters and cinder cones, some of 
large size. Lava flows from these vents extend in various directions, 
some of them being traversed by the railroad from a point 2 miles 
west of Mount Riley siding to their western margin, about 15 miles 
70 This limestone contains Caprina | nella dolium, a Fredericksburg fauna _ 
occidentalis, Trigonia sp., and Actaeo- | determined by T. W. Stanton. 
