Page 21 - The Geology and Ore Deposits of Sierra County, New Mexico - Bulletin 10

20 GEOLOGY AND ORE DEPOSITS OF SIERRA CO., N. M.
effected by erosion in all these types have been extensive, resulting in the removal of vast deposits of the lavas from large areas. In general the volcanic activity seems to have been subsequent to the disturbances which primarily outlined the mountain, but in some places it evidently accompa- nied these disturbances.
To the first or faulted-block type belong several ranges, among them the * Cristobal, Caballos, and Cuchillo, though in all of these the present relief has been accentuated by vast outflows of lava which erosion has carved into a system of rugged peaks and deep canyons.
Of the second type are the Black and Mimbres (Black) ranges, in which the uplifted strata slope away on both sides from a central mass of granite. At present, however, the most prominent features of the ranges are due to the piling up of vast deposits of volcanic material. In this type the removal of either limb by erosion would produce a mountain apparently indistinguishable from a monocline, but the competency of erosion to bring about this result is questioned.
In the third type the mountain is constructed wholly of volcanic mate- rial. Of this type the most prominent examples are the San Mateo Moun- tains, which occupy a position intermediate between the Magdalena and the Black Ranges. These mountains consist almost entirely, so far as the exposed portions are concerned, of rhyolites and their associated volcanic products.
A number of small isolated hills which rise out of the bol- sons and mesas have not been named in the above classification. These are (1) remnants of partly eroded basaltic flews, (2) rem- nants of extensive flows of Tertiary lavas, or (3) the outcrops of small tilted blocks of sediments which have not yet been planed off by erosion. The low hills west of Hot Springs are an example of tilted fault blocks, and at several places in the Jornada and west of the Rio Grande, remnants of basaltic and of Tertiary lava flows make up a noticeable feature of the terrane. As to form, it may be said in general that those hills having conical or irregular outline are composed of remnants of basalt or other lava ; while those which are markedly lenticular and regular in outline, having one steep face and a more gently sloping opposite side, are of the faulted-block variety.
BOLSON PLAINS
Slightly more than half of Sierra County is floored with detrital material of Tertiary to Recent age in the form of talus, alluvial fans and lacustrine deposits. In general this valley fill may be divided into three main areas ; the Jornada del Muerto, the present Rio Grande valley, and the irregular area lying to the east of the Black Range and west of the Sierra 'Cuchillo and the Animas Hills. The last-named area is in reality three sep- arate small plains connected with one another by short, nar- row valleys. All these plains have been classed as bolson plains, although this classification has not gone undisputed. Quoting from Gordon:4
As defined by Hill5 and further elaborated by Tight,6 the bolson is
4Gordon, C. H., op. cit. (U. S. G. S. Prof. Paper 68), p. 221.
5 Hill, R. T., U. S. Geol. Survey Topographic Atlas, folio 3, p. 8, 1900.
6Tight, W. C., Am. Geologist, Vol. 36, pp. 271-284, 1905.