24 
THE LOUP FOEK BEDS. 
me to form the northern boundary of the basin a few miles north of the 
town of El Kito, east of the Eio Chama. These beds crop out in high 
bluffs, and doubtless formed the precipitous western shore of the fresh lake 
which, during the Loup Fork epoch, filled the valley of the Rio Grande 
from its upper waters to an unknown distance toward Mexico. 
These red and variegated beds cover the stratigraphical axis of the 
Sierra Madre at this point, although not the water-shed between the waters 
of the Rio Grande and Rio Colorado. The geology west of this point has 
been considered in the divisions devoted to the Mesozoic formations of the 
Sierra Madre and the Eocene area west of it. 
The earliest information which we possess respecting the existence of 
Vertebrate remains in the lacustrine deposits of the Rio Grande Valley is 
due to the interest displayed by Hon. William F. M. Arny, then governor of 
New Mexico. He obtained, from the region northwest of Santa Fd, the 
fragments of a lower jaw of a Mastodon productus, Cope, and sent them to 
the Smithsonian Institution. This specimen formed the subject of a descrip¬ 
tion by Dr. Leidy, who referred the species to his Mastodon ohscums. The 
next observations of Vertebrate fossils were made by the members of the 
Wheeler survey of 1873. Francis Klett and Dr. O. Loew obtained a number 
of specimens from near San Ildefonso. Following the directions of these 
gentlemen, I made the examination during the season of 1874 which resulted 
in the discovery of thirty-one species of Vertebrata, of which all but four are 
determinable. Some of these have been already described in my report, pub¬ 
lished in the Annual Report of the Chief of Engineers for 1874, page 603. 
The Placita marls, alluded to in the beginning of this chapter, were 
examined by me during an exploration of the eastern base of the Sandia 
Mountains. Facilities for the accomplishment of this purpose were kindly 
placed at my disposal by General Gregg, at that time commanding the 
district of New Mexico. 
A section carried across from Algodones on the Rio Grande to the 
Sandia Mountains, through the village and creek of Placita, gave the follow¬ 
ing results: The road winds among and ascends for several miles, mesas of 
coarse Tertiary gravel and cobble-stones until it reaches a wide plateau, 
from which the mountains rise on the east. This tract is traversed by 
