AGE OF STRATA IN MESILLA YALLEY. 
167 
up. Small masses of coal have been observed in the arroyo beds leading from the range, and 
have been tried in fires at Fort Fillmore, and are said to burn well, although ashy. This has led 
to the belief that a coal seam exists in the range. I have examined the shale beds without 
being able to find any seam or any specimen of plant impressions; from which failure, as well 
as the thinness of the shale, it is scarcely likely that a productive coal seam exists there. 
The proximity of the Mesilla Picacho (although on the opposite side of the river) to this 
range, which is chiefly a trachyte and porphyritic upheaval, leads to the belief that it is con¬ 
nected with it, and is the southern continuation of the range ; however this may be, there is at 
the river bed a great dislocation of the strata, raising the red and white sandstones of the 
right bank of the river two or three hundred feet above the level of those on the left bank. 
The river appears to have taken this angle of depression as the most suitable for its bed, and 
follows this line for thirty-five miles south, until the Organ mountain range touches at the El 
Paso, and constitutes the pass through which the river drops, forming rapids having a descent 
of twelve feet in one-half mile, advantage of which Mr. S. Hart has taken to establish a grist 
mill, whose prime mover is a turbine. The rock over which the rapids flow is the limestone, 
which towers on each side of the river in hills nearly 1,500 feet high.—(See Plate XIV, fig. 3.) 
It is in this limestone that the vein of argentiferous galena already alluded to occurs ; the 
gangue stone investing it is quartz, with sulphate of barytes and the associated minerals, 
phosphate and carbonate of lead, with some sulphuret of copper. The ore is described and its 
value estimated in the chapter on Chemical Analysis. 
OF THE STRUCTURE AND AGE OF THE IGNEOUS ROCKS IN THE VICINITY OF 
THE MESILLA VALLEY. 
Within a breadth of twenty-five miles there are three distinct species of igneous rocks, viz : 
1st. The granitoid rocks of the Organ mountains. 
2d. The porphyritic rocks of the Dona Ana, the Picacho, and the Monument Hill. 
3d. The trappean and basaltic lavas of the belt west of the Monument Hill. 
The granitoid rocks are the syenite and leptinite, which compose three-fourths of the chain. 
This must be looked upon as the most ancient of the whole, although not an ancient rock. 
The porphyritic rocks are mostly trachytes, are destitute of hornblende or mica, and are 
mostly of reddish felspar, with minute glassy crystals of quartz. This variety of rock, very 
commonly found from the eastern edge of the Cordilleras, I have not seen east of the Sacra¬ 
mento mountains, Texas, though evidently in a much more viscous state when upraised than 
the granitic rock, yet they, with the exception of the Peloncillo hill trachytes, can scarcely 
be accounted a lava. 
The trappean rocks are distinct lavas ; much variety in the augitic basalts is met with, in 
respect to their physical condition, being found as compact trap, passing by various shades into 
cellular lava. Olivine is a constant constituent of all these trappean rocks in the central 
plateaus and their mountains. The wide vein of trap, in the Organ mountains, appears to 
have cut its way through the leptinite. The basalt of the plains west appears merely to have 
bent up, cracked, and forced through the strata, but not to have upheaved them. The upheaval 
appears to have been already affected by the porphyritic range of Dona Ana, which uplifted 
the Picacho. If this be correct, there are, probably, then, three distinct periods of disturbance 
in this district: 
1st. The elevation of the Organ mountains. 
