PLATEAU STRUCTURE FROM PENASQUITAS TO THE MIMBRES. 155 



On the north side a hill, 100 feet high, yielded a similar section, excepting the exposure of 

 the lower conglomerate. In some places the sandstone is hardened into enamel. The trachyte 

 and porphyry protrusions run N. 60° W. (mag.) The trachyte in places has a ribboned struc- 

 ture, and resembles fossilized or silicified wood, but the appearance is communicated to it by its 

 cooling while in motion, and being thus drawn out in an uneven plane; occasionally, chalcedony 

 is found occupying cavities in it. — (Plate XIII, iSg. 1, exhibits the strata as at Penasquitas.) 



From Peiiasquitas to the Rio Mimbres the country is an elevated, broad plain, interrupted 

 here and there by a few erupted masses of porphyry and quartz rock, forming conical hills. 

 The same disturbance which upraised the Burro hills, acting here to a lesser extent, but in the 

 same plane, and extending its influence in every direction around, even as far as the Sierra 

 Florida, which presents a volcanic rugged surface similar to those in this neighborhood. 



The only rock visible between Peiiasquitas and the Ojo de la Vacca is a coarse red meta- 

 morphic sandstone, which crosses the trail obliquely, or from N.W. to S.E. The spring or 

 " Ojo" is a deep well in the centre of a plain, depressed somewhat below the general level. 

 Several holes have been dug about five feet around the natural spring to obtain a readier supply, 

 the edge of the Ojo being boggy and full of rushes. The water is good and slightly sulphurous, 

 but full of vegetable matter and animalcules. The evaporation of the surface water appears to 

 keep pace with its bubbling up in the spring, since there is no stream rolling off from it, a 

 slight marshy condition of the ground being the only effect. One-fourth of a mile east of the 

 Ojo is one of the conical hills alluded to ; it is quartzose and porphyritic trachyte. Perhaps its 

 protrusion may have dislocated the red sandstone, producing a fault, and giving an opportunity 

 for the subterranean water to ooze up from beneath the fractured edges of a stratum. 



Between Ojo de la Vacca and the Mimbres river the red sandstone strata are again crossed, 

 which are better exposed here than west of the Ojo. It is a brick-red, homogeneous rock, with 

 whity felspathic clay and nodules, and cavities sparingly scattered through it. The direction 

 of the strata E.S.E., and the dip 26° N.N.E. The actual exposure was only 20 feet in thick- 

 ness, though it is reasonable to think it reaches to next exposure of rock, making 190 or 200 

 feet in all. This forms the road bottom which crosses it, and in the angles between the edges 

 of the strata water holes exist, which were well supplied with water at this time. 



Lying immediately over this sandstone was a thin bed of whitish conglomerate or grit. The 

 paste was aluminous, and the pebble porphyritic felspar. This bed was 15 feet thick. 



Beyond this and overlying it was a thin bed of grayish clay rock, about two feet thick. 

 Further east a thick bed of white felspathic conglomerate, 210 feet across. The included 

 pebble is a dark, silicious stone. The conglomerate beds have the same trend as the sandstone, 

 but the dip is somewhat less^ being 22°. 



The local disposition is thus : 



Feet. 



1. White felspar conglomerate 210 



2. Grayish clay porphyry rock, stratified 2 



3. White sandstone grit 15 



4. Red fossiliferous sandstone 188 



Total , 415 



This last rock is in proximity to the amygdaloid porphyry. The geological relations of these 

 sandstone beds will be considered further on, when describing the geology of the Mesilla valley 

 and the Organ mountains. 



