Page 199 - The Geology and Ore Deposits of Sierra County, New Mexico - Bulletin 10

I96 GEOLOGY AND ORE DEPOSITS OF SIERRA CO., N. M.
the flatter beds of the old bolson plain through which the river has since cut its channel at the base of the range. On the east side of the range, similar fans were built up where canyons cut- ting back into the dip slopes of the range discharge their loads upon the valley floor. Only the tops of these fans are now visible above the detrital material of the bolson of the Jornada del Muerto, which rises gently toward the range and is slowly bury- ing the outlying cuestas at many points. At the southern end of the range the drainage system of the Rio Grande has worked back into the "Jornada" and is destroying the bolson topography.
Near the south end of the range and west of the main fault scarp, there is a fault striking S. 3° W. and intersecting the main fault. In the angle between these faults, Cretaceous and Tertiary rocks have been faulted down between pre-Cambrian rocks on the west and Paleozoic sediments on the east. A small mass of mon- zonite porphyry, either a stock or dike, has been intruded along this fault.
Granite is the chief rock of the pre-Cambrian, but schist is locally prominent, and near the north end of the range diorite containing abundant hornblende is exposed in the bottoms of the arroyos. Pegmatite dikes occur in places, and much vein quartz is present in the fan debris.
The oldest rocks exposed to the east of the main fault be- long to the Magdalena formation, and the thickness exposed is approximately 1,000 feet. Lying disconformably upon these lime- stones is typical red Abo sandstone 400 feet thick, and above it is the Chupadera formation. The Chupadera is divided into 385 feet of Yeso beds consisting of shale, gypsum, sandstone and interbedded thin layers of limestone, and 650 feet of San Andres limestone and sandstone. Cretaceous beds overlie the San Andres limestone.
ORE DEPOSITS
No detailed descriptions of the mineralized outcrops or of the old prospects of the Fra Cristobal Range can be given. Weak mineralization occurs in places, and it is reasonable to believe that in some parts of the area outcrops could be found that would be worth prospecting. Between the Elephant Butte reservoir and the exposures of pre-Cambrian granite, there is a slight pos- sibility of finding placer gold in workable quantities in the dry creek beds or alluvial fans.
High up on the east side of the range several large solution caves in the limestone have been found. Millions of bats inhabit these caves, and from them shipments of guano have been made from time to time. The largest of these caves is visible from the road leading northward out of Engle.