22 
THE LOUP FORK BEDS. 
The wide valley between the Jemez and the Sangi’e de Cristo ranges 
is almost entirely filled with the Santa Fd marls. Their sandy character is 
not favorable to agriculture, being scarcely preferable to the basalt, so that 
cultivation is confined to the narrow valleys of the tributaries of the Rio 
Grande. The intervening country is either absolutely naked or covered 
with cedars. Occasionally, as near San Ildefonso and near San Felipe, a 
fragment of the lava remains, protecting the underlying Pliocene beds, form¬ 
ing a flat-topped butte, generally termed a huerfano. The beds of the Santa 
F^ marls are alternately softer and harder calcareous sandstones and con¬ 
glomerates, varying from white to greenish-gray and light rufous. They 
dip generally 10° to 15° toward the east, and away from the basaltic mass 
of the Jemez range. They contain the remains of extinct Vertehfata, mostly 
Mammalia, which have enabled me to correlate them with the Loup Fork 
Tertiary of Colorado and Dakota. The species discovered by our party 
number thirty-one, of which twenty-six are Mammals, three Birds, and 
two Reptiles. An enumeration of them is given in the last chapter of this 
report. 
Twenty-five miles west of the Rio Grande, at San Ildefonso, the east¬ 
ern masses of the Jemez Mountains rise. The greater part of this interval 
is occupied by a plateau which is traversed by more or less parallel ravines, 
which issue in the trough of the Rio Grande. The mesas which separate 
the ravines terminate abruptly, like the wharves of a city front. Their 
material consists of sandstone, conglomerate, and arenaceous marl, of 
whitish, gray, and drab colors, having a gentle dip to the northwest. Many 
of their upper beds contain numerous pieces of pumice, which readily dis¬ 
integrate, and the resulting siliceous dust, under the influence of wind, 
excavates the surrounding sandstones into caverns and pigeon-holes of many 
sizes and shapes. Nearer the mountains, the northwest dip of the beds is 
distinct, and they accordingly present escarpments to the southeast and 
gentle pine-covered slopes to the northwest. The ravines have a northeast 
and southwest direction, and extend to the base of the mountain. The 
escarpments are composed of orange-colored and reddish rock of uniform 
constitution, which breaks into prism-like masses as it falls, forming taluses 
below. It is entirely distinct in character from that of the bluffs nearer the 
