168 UKDETERMINED POSITION OF RED SANDSTONE. 



2d. The elevation of the Dona Ana range. 



3d. The trajjpean disturbance. 



The first and most ancient of these, the upheaval of the granites of the Organ mountains, 

 and other chains, must have occurred posterior to the Triassic or Permian (or Serai, of Rogers,) 

 period. Perhaps most of it occurred between then and the cretaceous period, although, from 

 the slight western slope of all cretaceous beds in this region, there was some further and final 

 elevation since the latter deposit ; which final uplift may be, as stated further on, attributable to 

 the trap overflows. 



The second, or the outpouring of the trachytes, occurred anterior to the third, as the latter, 

 in many places, cuts through the red felspathic amygdaloid and trachyte. 



The third, the trappean elevation, occurred posterior to the cretaceous period, and anterior to 

 the general deposit of the tertiary conglomerates and sandstones, which contain many pebbles 

 of these volcanic rocks. 



AGE OF THE RED SANDSTONE WEST OF THE RIO GRANDE. 



I had at first classed the reddish sandstone found in the Mesilla valley, and west to the Rio 

 Mimbres, as an upper member of the carboniferous formation. I did this with some hesitation, 

 as, in lithological character, it differs considerably from the texture common to the sandstones 

 of the coal measures. It is of a bright brick-red color, fiae grain, and, to the careless eye, 

 homogeneous in texture. Under the lens, earthy white spots are observed, intermingled, 

 composed of rounded grains of vitreous and opaque quartz, with minute spots of a black mineral. 

 The rock is porous, and, though not friable, easily comminuted. 



"When pulverized and elutriated, the coarse parts examined by the lens exhibit the reddish 

 fragments as angular, rough, felspathic particles ; the black minute specks are small portions of 

 amphibole which are disseminated through the red felspar. The quartz fragments are rolled 

 grains of quartz. 



The red particles, treated with hyrdocloric acid, partially dissolved, have the color removed, 

 (produced by peroxide of iron,) leaving the insoluble residue chiefly consisting of amorphous, 

 fine clay, (silicate of alumina.) 



There is no trace of carbonate of lime or magnesia in the sandstone. 



From its texture and chemical composition, it is a sandstone derived from the decomposition 

 of the red, felspathic igneous rocks, which constitute the axis of the upheaved districts west of 

 the Rio Grande ; a granitic porphyry rock, which is found extensively difi'used in the canons of 

 the Gila, at the mouths of the San Pedro, and along the sides of the Sierra Calitro and Mount 

 Graham ranges, and to which allusion has already been made. 



On account of its conformability with the whitish grit, and the bed of shale at the Horse 

 mountains, northeast of Dona Ana, both of which it overlies conformably, it might be classed 

 among the coal measures, looking upon the bed of shale as one of that series. The shale is 

 dark colored and semi-bituminous, but nowhere could I observe any fragments of plant impres- 

 sion to indicate their age. The bed of shale, which is here thin, I have not been able to trace 

 to the west. Although the limestone can be found in the valley to the west of Chiricahui moun- 

 tains, and at the various exposures of it and of the white and red sandstones, at the Pichaco de 

 los Mimbres, the Rio Mimbres, and at Ojo de la Vacca, I have nowhere seen the underlying 

 shale ; it may be that the elevation of the grits is not sufficient to bring the shale into view. 



From its dissimilarity to the carboniferous grits, I incline to separate the red sandstone from 



