16 
THE EOCENE PLATEAU. 
]iog-backs, while tlio corresponding part of the second approaches nearer, 
forming a line of bluffs of considerable height running north and south par¬ 
allel with, and half a mile from, the hog-backs. Beyond the Puerco divide, 
hills of this formation rise*on both sides of the trail, and near the Ojo de San 
Josd the Eocene beds repose on the foot of the Nacimiento Mountain s.everal 
miles to the east. 
Below the sandstones which form the portals of the Canoncito de las 
Yeguas, another stratum of marls shows itself in hills of 100 feet and 
higher, in the sage-brush plain that separates them from the Cretaceous 
hog-backs. They are soft and of mixed black and dark-green colors near 
the locality in question, and capped by light and yellowish sandstones. 
These conform to the beds of the Eocene, and I traced them for forty miles 
to the south along the belt of country intervening between Cretaceous No. 
4 and the reddish sandstone. At the locality just mentioned, they conform 
to the sandstones above, having a dip of 10° southwest, while they do not 
conform to the hog-back of Cretaceous No. 3, the nearest available outcrop, 
which dips at 25° west. Farther south, this marl is represented by low 
hills of generally lighter color. Near Nacimiento, it has an increased im¬ 
portance, as it rises both to the east and south. The valley of the Upper 
Puerco is excavated in it for some distance, and its blackish, greenish, and 
gray hills are seen on both sides of the river. At a point on the river 
about six miles below the village of Nacimiento, the lower sandstone of the 
Eocene forms a perpendicular bluff, which terminates in an escarpment of 
500 feet elevation facing the south. The red-striped marls, having acquired 
a gentle northern dip, disappear from view some miles to the north, and the 
termination of the underlying sandstones warned us that we were approach¬ 
ing the southern border of the basin. 
The border of the sandstone turned to the west at this point, the line 
of bluffs continuing as far as vision extended. Below and south of it, the 
varied green and gray marls formed the material of the country, forming 
bad-land tracts of considerable extent and utter barrenness. They formed 
conical hills and flat meadows, intersected by deep arroyos, whose perpen¬ 
dicular walls constituted a great impediment to our progress. During the 
days of my examination of the region, heavy showers of rain fell, filling 
