SOUTHERN PACIFIC LINES 93 
About midway along the south edge of the Glass Mountains is the 
bald reddish knob of Iron Mountain, a stock of intrusive syenite which 
cuts across the Permian strata. 
Four miles west of Marathon the railroad crosses low ledges of the 
Gaptank formation, which contains marine fossils of upper Penn- 
sylvanian age. Several railroad cuts expose steeply dipping shale, 
sandstone, and conglomeratic limestone. About a mile to the south 
of the tracks are several low hills of novaculite and chert. In other 
novaculite hills, visible farther southwest, the novaculite has been 
overthrust onto the Gaptank formation by a fault, with the older 
strata in nearly flat contact. This displacement, called the Dugout 
Creek overthrust, had a horizontal movement of nearly 6 miles in a 
northwestward direction. Some of its relations are shown in Figure 
12. North of the novaculite hills above mentioned and west of Lenox 
siding, the basal Permian beds are exposed in Dugout Mountain, 
resting on the steeply inclined and contorted Pennsylvanian rocks; 
as the Permian beds contain coarse conglomerates derived from the 
erosion of older rocks, the Dugout Creek overthrust is pre-Permian 
and probably later Pennsylvanian 
Near Lenox siding the railroad Hasies between low hills of the lower 
Permian rocks, of which Dugout Mountain is one, and leaves the 
Marathon Basin. The high escarpment of the Glass Mountains 
comes into view to the northeast, behind lower foothills. The slopes 
of the escarpment are shale and sandstone of the Word formation, 
which are surmounted by cliffs of Capitan limestone. The most 
imposing exposure is on Cathedral Mountain, several miles northeast 
of the railroad. The limestone of the cliffs was probably built up in 
reefs on the Permian sea floor by calcareous algae and other organisms. 
These beds thin to the west, and on the scarp near the railroad near 
Altuda they are of negligible thickness. 
Altuda siding is in the pass or low, wide gap between the north end of 
the Del Norte Mountains and the southwestern extension of the 
Glass Mountains. For several miles in this vicinity 
ae there are excellent views of both ranges, which are 
Elevation 4,642 feet. ‘ 
ean bet nies, VeLY rugged and bare. Altuda Mountain, to the west, 
and Mount Ord (named for General Ord, one time 
commandant of the military forces in Texas), to the south, are 
spines and minute internal structures 
are of the type known as the Gua- 
are preserved. h America i 
dalupian, which in North 
lent condition. Eroded surfaces of | is found in the typical Permian area. 
these limestones are usually a tangled | In the higher parts of the Glass Moun- 
tains section most of the limestones 
silicified mat of shells which accumu- 
se 
appear to have been built up of reef- 
making 
