RED BEDS OF RIO GRANDE REGION 53 
and limestone, as is the case in San Andreas Mountains east of 
Engle, where the gypsiferous division is nearly 1,000 feet thick. 
The upper division, consisting of alternating layers of yellow, pink, 
and white sandstones and shales, has an observed thickness of many 
hundreds of feet, but is not always clearly differentiated from the 
middle or gypsiferous division. 
The Red beds of the Rio Grande region are similar in general 
appearance and lithologic character to the red sandstone formation 
occurring along the eastern base of the Rocky Mountains, and 
extending thence eastward across southern Colorado and New Mexico. 
In other words the red sandstones and shales of the Rio Grande 
region form part of the complex which is frequently called ‘“‘the Red 
beds” of the Rocky Mountain region. 
In the Rio Grande region the Red beds rest unconformably upon 
an extensive series of Upper Carboniferous limestones. At the base. 
of the red strata occurs a limestone conglomerate, the pebbles of 
which contain fossils identical with those of the limestones upon 
which the conglomerate rests. ‘This was observed in the hills east 
of Socorro, in Abo canyon east of Belen, and at the northern end of 
Sandia Mountains. ‘Twenty-five to seventy-five feet above this basal 
conglomerate occurs a persistent limestone member containing a 
rich fauna. Collections were made from this limestone in several 
localities. 
Many of the limestones of the middle or gypsiferous division are 
also very fossiliferous, and large collections were obtained from them 
in widely separated localities. ‘The upper division, so far as observed, 
is sparingly fossiliferous, and not faunally different from the under- 
lying or middle division. But overlying it in the southern half of 
the region occurs a limestone several hundred feet thick, which 
yielded an abundant fauna. This overlying limestone is absent in 
the northern part of the region, but occurs in the central part and 
apparently thickens toward the south. It is well-developed in the 
mountains east of Socorro, in the San Andreas Mountains east of | 
Engle, in the Caballos-Fra Cristobal Range, and elsewhere. On 
account of its important bearing on the age of the underlying Red 
beds, fossils were collected from it in about thirty separate localities. 
The fossil collections from the various horizons are too voluminous 
