GEOLOGICAL STRUCTURE OF THE ORGAN MOUNTAINS. 161 



GEOLOGY OF THE ORGAN MOUNTAINS. 



The Organ mountains lie about 14 miles east of the Mesilla valley, and have a granitoid 

 axis, the direction of which is north 12° west. 



The range may be about 100 miles long, running over two degrees of latitude ; it drops down 

 to the northward, where it forms the hills which lie on the east side of the Jornada del Muerto; 

 there the axis is not apparent, but on the slope of the Pass San Augustine quartzose felspathic 

 rock is found, and from there to the summit syenite, having slender crystals of hornblende and 

 reddish felspar. This syenitic rock crosses the range and appears on the east side of the moun- 

 tains, further south ; on the east foot slope felsjjathic rock again occurs. But the great mass 

 of the hills are made up of a felspar porphyry, somewhat resembling leptinite, the quartz being 

 white and sometimes crystalline, in a pale flesh-colored felspar ; the summits wear irregularly, 

 and present the lofty broad pointed spires which have given them their name, from the fancied 

 resemblance to a Spanish organ. From its structure it appears to be a very modern granitic rock. 



A large mass of augitic trap lies on the west side of this porphyritic felspar, and appears to 

 have come up through it. This dyke lies about three miles north of the Soldado canon ; 

 thence, southwards, flesh-colored jjorphyry is the chief axial rock. 



East of Fort Filmore the chain falls down into a range of much lesser height, the hills being 

 made up rather of the elevated sandstones and sedimentary rocks than of the igneous axis. 

 Such are the Chinaman's Cap and a few other peaks visible in travelling down the river. These 

 low hills continue to El Paso, where they cross over to the right bank. The lofty portions of 

 the range are found bordering the Mesilla valley. 



Immediately in contact with these igneous rocks is a dark blue limestone, similar in character 

 with that described as the carboniferous limestone of Chiricahui and further west. Near the 

 Pass San Augustine it flanks the axis for 600 feet of elevation, with a dip of 30° to the west a 

 few degrees south ; here is a large vein of argentiferous galena, which is worked by Mr. Steven- 

 son, and smelted in his furnace, near Las Cruces, on the river side. 



Further south of the pass, along the range, this limestone forms the small hills on the west 

 side, the dip gradually decreasing the further south these strata are examined. East of Las 

 Cruces is a low rounded hill, standing isolated in the plain, about 500 feet high ; it is of car- 

 boniferous limestone. Between this hill and the base of the Organ mountains no other eleva- 

 tion of rock occurs. 



At the western debouche of the Soldado caiion a yellow grit appears, dipping west ; further 

 up the caiion the limestone is seen underlying it. In this sandstone is a vein of galena, which 

 was worked some years back merely for its lead ; it was not reported as argentiferous. Smelt- 

 ing works were erected close by the mine by Signer Barilla, who opened the vein ; a small 

 stream of water close by furnished the means of washing the ore.* 



From the extreme red tint of the soil at the western base of the hills it would appear to be 

 underlaid by reddish sandstone. The rock itself, however, was not observed in situ at this place. 



On the east side of the Organ mountains the igneous axis is more exposed, the secondary 

 strata not reaching by several hundred feet as high as on the west flank, and have a slope not 

 exceeding fifteen degrees ; on this account the strata are not so well exposed as on the west 

 flank. The surface rock is a gypsiferous sandstone, contains veins and deposits of crystalline 



o Thuse works were of julobe, as are Mr. Stevenson's. I was uot able to visit the old miue at Soldado caBon, owing to 

 several lodges of Apaches being encamped there at the time of my visit. T. A. 



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