THE EOCENE PLATEAU. 
17 
the arroyos with rushing torrents, and displaying a peculiar character of 
this marl when wet. It became slippery, resembling soap in consistence, 
so that the hills were climbed with difficulty, and on the levels the horses’ 
feet sank at every step.' The material is so easily transported that the 
drainage-channels are cut to a great depth, and the Puerco Eiver becomes 
the receptacle of great quantities of slimy-looking mud. Its unctuous 
appearance resembles strongly soft-soap, hence the name Puerco, grease. 
These soft marls cover a belt of some miles in width, and continue at the 
foot of another line of sandstone bluffs, which bound the immediate valley 
of the Puerco to a point eighteen miles below Nacimiento. Here the sand¬ 
stone again turns to the westward, presenting a southern escarpment of 500 
to 1,000 feet elevation. This forms the southern boundary of the Eocene 
basin. I could not be sure whether this sandstone is identical with that of 
the escarpment twelve miles north, but suspected it to be such. Immedi¬ 
ately south of it, low hills of Cretaceous No. 3 extend across the Puerco 
and continue south of the Eocene bluffs at a distance of a mile or two with 
a western strike. They were as elsewhere of a soft yellowish sand and 
clay, including shale beds, and contained abundance of Inoceramm, like 
those found on the Gallinas. 
Ten miles to the southward, the underlying Cretaceous beds are capped 
by a horizontal table of basalt, thus forming a mesa, through which the 
Puerco passes in a cation. I supposed this to be the forerunner of the 
great basaltic plateau, which, according to Lieutenant Wheeler, constitutes 
the country south of the Rio Chaco for a great distance one of little 
promise to the agriculturist.' These tracts are known as the Mesa Fachada 
and Mesa de los Lobos. The season being well advanced (October 22), I 
thought best to commence the return march, which we accordingly did. 
The soapy marls, or, as they may be called, the Puerco marls, have 
their principal development at this locality. I examined them throughout the 
forty miles of outcrop which I observed for fossil remains, but succeeded 
in finding nothing but petrified wood. This is abundant in the region of 
the Gallinas, and includes silicified fragments of dicotyledonous and palm 
trees. On the Puerco, portions of trunks and limbs are strewn on the hills 
and ravines; in some localities the mass of fragments indicating the place 
2 G K 
