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Show 1 2 9 6 REPORT OP THE CHIEF OF ENGINEERS. In some places this rock is in crusted with calcite; and in the amygdaloidal varieties the cavities are occasionally filled with zeolites. Near the head of Uraca Creek, New Mexico, a dark- brown species of scoriaceous basalt occurs having the cavities elongated and v « ry narrow. A gray variety of this rock is found abundantly near Fort Union, New Mexico. The diorite has in general a compact texture, but considerable variety of color. The granite presents great variety in both color and texture. Tbe various colors of this rock observed are gray, red, pink, light and dark shades of blae, and white; reddish granite is the most common, on account of the feldspar of the same color being the predominating constituent. The varieties in texture observed are compact, granular, and porphyri ic, and the latter being the most frequent. The granite is generally poor in mica, although a micaceous variety of this rock occurs just west of the Moreno Valley, New Mexico. The feldspar porphyry usually presents great irregularity in the size of the crystals imbedded in the compact matrix. The color of this rock is commonly gray, occasionally incliuing to white. The granuMre is in general of a reddish color, and granular iu texture. Grannlite and granite constitute the predominating rocks, in the several mountain ranges from La Yeta Pass to Santa F6. The trachyte presents a compact texture as a rule, and a cdor varying from light gray to pink. The sandstone is generally of a yellowish color, and has a iine- graincd structure. In several localities the rock is ferruginous when the color becomes bright red. Near Costilla peak the sandstone passes into a conglomerate. Tbe fossiliferous sandstones have been described in the geological report. The limestone is usually light blue in color, and of a compact texture. Thin rock covers a large tract of country in the eastern portion of the area explored in 1875. Beside* these rocks may be mentioned clay- slate, and hornblende porphyry, both of which occur iu several localities. The paucity of crystaMine schists is remarkable, the only localities being Aear Uraca Mountain, on Elizabeth Baldy, and on the eastern side of Autelope Creek, iu the Wet Mountain Valley. A hard siliceous variety of conglomerate occurs near the top of the sandstone mesa bounding the Vermejo Valley on the west. The matrix of this rock is dark browu, and contains black and white fragments of quartz. Iu order to determine with accuracy those rocks having a texture so compact that the constituent minerals could not be recognized with the naked eye, I h* ve made sections of such rocks that were deemed desirable to prepare for microscopic examination. On account of the heterogeneous texture and opacity of some of the specimens of rock, much time and labor have been expended in preparing them. In some casus the biittleuess of the rock rendered it impossible to make a section sufficiently « htii for microscopic analysis. Such was the case with some specimens of trachyte that were interrupted by fissures. The rock could be groand on the wheel to a certain degree of thinness, after which it iuvariably crumbled, thus destroying the section. Fifty sections of rock have been prepared by me, and mounted on glass with Canada balsam. The results of my microscopical investigations may be coudeused in the following description: No. 139. Diorite, from Rio Hondo, N. Mex., consisting of white plagioclase, and a few crystals of hornblende. Much olivine, and specks of magnetite occur as accessories. No. 144. Basalt, from Rio Colorado, N. Mex., containing much disseminated greenish nepheline, pyroxene, and specks of magnetite. The rock is slightly a'nygdaloidal. No. 171. Dolerite from Huerfano Butte, Colo., consisting of plagioclase, augite, and much olivine. No. 173. Nepheline- doleritefrom west side of Huerfano Butte, Colo., containing augite, nepheline, and particles of magnetite. The texture of the rock is very compact. No. 174. Diorite from Cucharas River, Colorado, composed of hornblende and plagioclase. Olivine and magnetite occur as accessories. No. 190. Syenitic granite from Ute Creek, New Mexico, containing quartz, triclinic feldspar, and a few crystals of mica; black hornblende is abundantly disseminated. No. 205. Basalt, from near Laughliu's Peak, N. Mex., consisting of nepbeliue, augite, and some olivine. • No. 206. Dolerite from East Spanish Peak, Colo., made up of plagioclase aud pyroxene. No. 207. Granite from head of Cimarron Creek, New Mexico, composed of reddish orthoclase, grains of quartz, muscovite, and specks of magnetite. A few crystals of a black mineral occur * hich may be melanite. No. 208. Syenite, from Cieueguilla Valley, N. Mex., containing plagioclase, hornblende and many graius of quartz. No. 210. Dolerite, froin Rider's CaQon, Colo., composed of plagioclase, many crystals of augite, and specks of magnetite. No. 211. Diorite, from near Taos Peak, N. Mex., consisting of hornblende, plagioclase, and a few grains of quartz. No. 216. Dolerite, from San Luis Valley, Cal., containing plagioclase, brownish pyroxene, and a few particles of mica. No. 217. Dolerite, from Colorado Cafion, N. Mex., consisting of crystals of augite, plagioclase, and specks of magnetite. The rock has a porpbyritio structure. |