Page 127 - The Geology and Ore Deposits of Sierra County, New Mexico - Bulletin 10

126 GEOLOGY AND ORE DEPOSITS OF SIERRA CO., N. M.
West of the Animas Hills, a small bolson-like valley stretches for 15 miles in a north-south direction between the Animas Hills and the foothills of the Black Range. The Rio Percha heads in the Black Range and flows easterly through a box canyon in the Black Range and through similar canyons carved across the axis of the Animas Hills. Where the river traverses the bolson plain it meanders over a flood plain that reaches a maximum width of half a mile.
GEOLOGY
SEDIMENTARY ROCKS
Montoya Limestone.—The oldest sedimentary rock in the
Hillsboro district is the Montoya limestone of Ordovician age, which outcrops in a small area in the southeast part of the dis- trict. Apparently it has been faulted into view at this place by minor longitudinal and transverse faults, and nothing is known about its thickness.
Fusselman Limestone.—Above the Montoya is the Fussel- man limestone of Silurian age, which crops out along a high ridge east of the highway leading to Hillsboro from Hot Springs. Here it forms a prominent cliff or fault scarp of dark to light gray massive limestone. Much of the limestone has been silici- fied, and vein quartz deposited from hot solutions is prominent. In many places the escarpment is colored brick red and brown, and resembles the outcrop of an enormous vein of quartz. The thickness of this limestone is about 200 feet, which is more than the Fusselman measures at any other place in the county, and it is quite possible that the lower portion of this exposure belongs to the Montoya limestone of Ordovician age, although the
writer was unable to make the separation in the field. The lime- stone east of the main fault scarp has been silicified, and the top- most member now exposed on the dip slope side of the hill has been altered to silica over such a large area that the exposure is locally known as a "quartzite' or a "limestone quartzite." These upper beds of the Fusselman limestone are very irregular and are much brecciated in places, and near the escarpment parallel faulting and brecciation have occurred over a width of several hundred feet. The surface of the dip slope and the face of the escarpment are strewn with huge boulders of iron-stained vuggy to dense quartz. North of the district, along a northward-trending fault contact, Lower Paleozoic beds, probably Silurian, are in contact with the volcanic rocks of the district, as shown in the northeast corner of the map, Plate VI, but no attempt was made to study them, as they are outside the mineralized area.
Percha Shale.—Overlying the Fusselman limestone and in unconformable contact with it is the Percha shale of Devonian age, which is 200 to 250 feet thick. It consists of a lower green- ish-black part barren of fossils, and an upper grayish or light greenish-gray part. The outcrops of the Percha shale are re-
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