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Show 1298 REPOBT OF THE CHIEF OF ENGINEERS. No. 394. Dolerite, from Costilla Canon, N. Mex., consisting of pyroxene and plagto* clase. I have the honor to be, very respectfully, yonr obedient servant, ALFRED R. CONK LING. Lieut. GBORGR M. WHEELER, Corps of Engineers, in charge. WASHINGTON, D. C, June 30,1877. The following manuscript, prepared by Mr. A. R. Coukling, and received too late* to be forwarded with the annual report of 1876, is herewith submitted. GEO. M. WHEELER, Lieutenant of Engineers. APPENDIX H2. REPORT ON THE FOOT- niLLS FACING THE PLAIN8 FROM LATITUDE 35° 30' TO 38° APPROXIMATELY, BY MR. A. R. CONKLING. Beginning at Las Vegas, Now Mexico, which is just east of the foot hills, we have the rolling prairie extending as far as Fort Union, 28 miles northeast, without any marked elevation. The foot- hills on the eastern side of the Las Vegas range consist of grayish sandstone, horizontally stratified. Upon approaching Fort Union broad mesas of moderate elevation lie in front of the foot- hills. These mesas are also composed of sandstone. At the head of the Rio Mora, a grayish mass of ernpti ve granite has broken through the sandstone. In the vicinity of Fort Union extensive dikes of basalt occur. The western limit of the basalt is, according to my observation*, at_ Torquillo, which lies about 17 miles west of Fort Union. Basalt occurs on the plain lying north of the fort. It covers the surface as far as Ocato Crater, which is just 13 miles north. With the exception of the basaltic lava of Ocate Crater, the basalt is invariably of a dark blue oolor and scoriaceous texture. Ocate Crater or Mountain is 8,902 feet above the sea- lovel. It is longor from north to south than from east to west. The summit is bowl- shaped, with the opening on the west side. The slopes, and even the interior of the crater,. are covered with grass, while on the northwestern side there is soil enough to support a small growth of pifions and cedars. The northern slopes of the mountain are impassable for animals, but a mule can be ridden to the summit by way of the south side. The land slopes very gradually south of the crater. There is a low bntte of basalt on the southwest. The major part of the basaltic lava on the plain of Fort Union has probably come from the Ocate Mountain. There is much variety in the lava of Ocate. The color of the lava varies from bright red to brownish black, and the texture is generally vesicular, though also compact aud scoriaceous in some cases. The reddish lava has so many crystals and grains of white lencite disseminated through it as to present a porphyritio structure. Between Ocate Mouutain and Fort Union there is a broad and low mesa of basalt, which is bordered on the west'by a mesa of gray sandstone. On the southern part of the Fort Union reservation blue limestone outcrops, dipping vtry slightly to the southwest. Fossil shells, of the genus Inoccramus, are found in the limestone, which, according to Professor White, are of cretaceous age. On the ordnance reservation, one mile west of the post, a well has been sunk to the depth of 78 feet. The following section will illustrate the geological structure at this place: feet. Clay* 17 Soft yellow sandstone 6 Decomposed lava 6 Basalt 37 Red volcanic scoria 4 Sandstone and gravel 8 The well adjoins Captain Shumaker's quarters. ORE DEPOSITS NEAR FORT UNION. There are a few deposits of ore near Fort Union, but none of them are workable. In the Turkey Mountains, about 9 miles from the fort, ground has been broken, where some malachite, or rather rock stained by green carbonate of copper, has been found. There is a single shaft at present, about 30 feet deep, which is partially tilled with water. The cupriferous rock occurs in a vein 2£ feet wide at the surface and 6 feet wide at the bottom of the shaft. The lode runs north and south, and occurs in red sandstone. At several other localities ground has been broken in the hope of finding valuable min- |