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Show 100 primitive rocks, intersected hore and there by rhyolitic dikes. The top of the highest peak shows a vein of serpentine and of a porous ferruginous quartz, in which particles of malachite can readily be detected, and contains most likely a small proportion of gold. This quartz- vein is G inches thick. The peaks rise more than 2,000 feet above the timber- line, mnd, with the exception of a solitary specimen of a thistle or a graminee, are perfectly barren. The crests are rugged and sides steep ; and the slopes being covered with sharp fragments of siliceous rocks, the ascent of these peaks is laborious and difficult. Descending the southern slopes of the Cerro Blanco we reach the valley of the Ute Creek ami Fort Garland, where the Carboniferous again becomes prominent, covering the rim of San Luis Valley to a great extent. The southern portion of this valley is covered by extensive sheets of bu& ilt reaching from Culebra and the vicinity of Costilla as far west as the base of the San Juan Mountains. The Rio Grande and San Antonio Creek, ( or Rito de los Pinos,) one of iti tributaries, have cut deep channels through the volcanic material. The Ute Mountain and Mountain San Antonio are two isolated basalt cones rising from the volcanic sheets. The basalt from the vicinity of Culebra has a crystalline structure and in accompanied by dolerite, while at the Ute Mountain it is amygdaloid, aud contains in these amygdaloid spaces reniform carbonate of lime. TIIE SAN JUAN MOUNTAINS. This important and extensive range of mountains was crossed by division No. 2, to which I was attached, in the most southern portion, and I had no opportunity to examine its more northern parts. The chief mass of these mountains is undoubtedly composed of rocks of Azoic age, but so frequently and exteusively are they intersected by rhyolitic and trachytic dikes that the latter appears almost as the prevailing material. Sandstone and limestone of Paleozoic age'skirt the foot- hills aud spurs of the higher ridges. The grauite of these mountains resembles that of the Cerro Blanco exactly in structure and in lithological character; the feldspar is white and forms a fine granular mixture with the particles of quartz, while the biotiteis not uniformly distributed through the mass, but concentrates in larger masses, producing a spotted appearance. On the southern slopes of the mountains, in the vicinity of the Brazos Creek, qnarrzite becomes massive, and a cafion of 800 feet in depth has b en cut in the rock by this river. This quartzite is joined toward the eastward by volcanic scoria and conglomerate, while just north of it rhyolite occurs. This rock is there of a uniform purplish matrix, containing sparingly embedded crystals of sanidine and hornblende. Descending farther to the southward we pass through the Carboniferous strata into the extensive Eocene beds that have been so minutely examined by Professor Cope, paleontologist of the expedition. Near Abiquiu, forty- seven miles south of Tierra Amanita, these beds present the same barren forms and grotesque architecture as the analogous deposits of the '• mauvaises terr& s" of Dakota. The extreme southeastern portions of the San Juan range give rise to the Ei Rito Creek and Calionte Cre k, tributaries of the Chama, the latter flowing into the Rio Grande a little above Santa Clara. The Ojo Caliente Creek cute its way through quartzite aud trachyte, then through gneiss and granite, aud finally through basalt and the Eocene beds. " About fourteen miles above its junction with the Chama are the famous hot springs described in Vol. Ill of the Survey Reports. The gray, fine- grained gueiss in which these springs take their rise, is intersected in a direction northeast to southwest by a dike, from 3 to 8 feet wide, of granite, perhaps the coarsest ever observed; the reddish feldspar forms masses of 4 to 5 cubic feet, the quartz of from 1 to 6 cubic feet, and the mnseovite large plates several inches in thickness. About three miles south of the hot springs is a basaltic mesa, 160 feet high, which forces the river into a westerly course. A close examination of this mesa revealed the fact that its interior is sandstone, while the basaltic sheet on the top and the vast masses of basaltic bowlders covering the sides create the opinion on first sight that it is exclusively volcanic. Doubtless this mesa represents a sandstone island that resisted the erosive force under the protection of the basaltic cover, while the adjacent strata formerly in existence have been carried away by the waters. Cases quite analogous occur in the vicinity of Abiquiu. There the Carboniferous sandstone formed the shores of the Tertiary sea, and represents a very tine- grained, hard rock, made up entirely of particles of quartz cemented together by silicic a id. Hydrochloric acid has not the slightest action on this cementing material, showing that no carbonate of lime is present. In connection with this sandstone, it should be mentioned that the snrfaces for a considerable distance are covered with a black crust about VK ° f a n » ncn thick, a peculiarity not often met with. This black crust is soluble in hydrochloric acid with disengagement of chlorine gas, and the solution thus formed gives all the specific tests for manganese. The black crust is therefore black oxide of manganese, and is probably the deposit of water that contained considerable quantities of carbonate of |
