162 AXIAL ROCKS AND STRATA IN RIO GRANDE VALLEY, 



gypsum, which in some places had been formerly worked by the Spanish settlers. This rock, 

 probably of the upper trias, rests conformably upon the carboniferous limestone and forms low 

 mesas, at the foot of which the trail to Salt lake travels. 



Huge boulders and drift cover up the mesas, and deep ravines worn by the arroyo show 

 the occasional intensity of moving water. The first mesa covered with these rocks derived from 

 the range, imbedded in a reddish soil, extends about two miles from the base of the mountain. 

 Then a second slope descends to the level of the wide plain, which reaches to the Sacramento 

 mountains. — (Plate XIV, fig. 2, exhibits this section.) 



A vein of white magnesian (talcose) rock is visible, cutting the porphyritic felspar rock 

 through from the summit of the mountain to the eastern base, at an angle of 55°, and about 

 20 feet in thickness. In the vicinity of this is an old gypsum quarry, said to have been worked 

 by the Spanish early settlers. 



In the axial granitic rock mica is very sparingly distributed, and over large areas wholly 

 wanting, only two ingredients, the quartz and felspar, being present; occasionally a few plates of 

 talc occur, the mass in some places resembling protogine, and in others leptinite. It is scarcely 

 a granitic rock, of which there is no true example in the Organ mountain range. The term 

 felspar porphyry has, therefore, been used to express this species of rock ; brownish grey masses, 

 resembling gneissose fragments, are found included, and veins of argentiferous galena cut 

 through this rock ; one of these, on the east side of the mountain, about three miles south of 

 Pass San Augustine, has been explored to the depth of nine feet, the vein winding from a half 

 inch, at the surface, to four inches; its direction is nearly S.E. and N.W., almost vertical, 

 with an inclination of 10° eastward.* 



The galena vein is, in all probability, the same which is found on the west side of the pass, 

 further north, in which Mr. Stevenson's mine is, and which has thinned out in this igneous rock. 



The Jornada del Muerto is a valley of elevation ; examining it at Dona Ana, its southern 

 outlet, it is found to form a synclinal axis of sedimentary rocks, lying west of the syenitic and 

 felspathic basin, constituting the axis of the Organ mountain range. 



The hills north of Doiia Ana, which form the eastern boundary of the Jornada, lie close to 

 the Kio Bravo, and have the axis upon the east side of the range ; approaching the summit, 

 the carboniferous limestone is met with, not fully exposed; then, overlying the limestone, there 

 is a thin bed of black shale, not more than 50 feet thick ; then whitish grit, and above all, 

 forming the summits of the range, reddish sandstone, at least 1,200 feet high. These dip at an 

 angle approaching 20° westward, and, after sinking under the plain of the Jornada, rise on 

 the opposite sides to form a short chain which skirts the Rio Grande northward, the summits 

 of which are 2,400 feet, and appear to be made up almost wholly of carboniferous limestone. 

 The sandstones which form the summits of the western range do not appear to rise up on the 

 Doiia Ana range, but, like as in the Organ mountains, foim the substratum at the base of the 

 hills. A section of the Jornada is given on Plate XIV, fig. 2. 



The thickness of the whitish grit on the western range was 130 feet, and the red sandstone 

 might have been 300 feet ; it was much disintegrated. 



The carboniferous bed could be better estimated on the eastern side of the Jornada, where it 

 appeared to be from 1,000 to 1,200 feet on the Organ mountains. It showed a greater thick- 

 ness north of San Augustine Pass than at any point where I have seen it. 



A review of the structure of this range is given on page 166. 



" Those gneissoid fragments are vrry different from true gneiss, and from the masses found included in tlie Cordilleras 

 near Vallecitas ; they are, perhaps, segregated masses of granitic mineral nucleating the hornblende or micaceous elements 

 collecting in isolated patche ■ 



