MOUNTAINS NEAR EL PASO. 



mistaken at a distant side view for an igneous formation. Connected with this upper crest, we 

 also notice outweathering masses of siliceous rock, (No. 38 ;) these frequently assume grotesque 

 forms and positions, representing various tower-shaped prominences. 



M0DNTAIN3 EAfST OF RIO BRATO SEEN FROM KL PASO. 



A. Cretaceous rocks resting on granite, and diping at an angle of 10-15°. 



B. Granite. 



C. Carboniferous limestone. 



D. Porphyry peak. 



E. Drift. 



At another point this mountain range is interrupted in its usual stratified character hy the 

 presence of a porphyritic exposure, (specimen rock No. 41,) forming a dyke, passing through 

 the entire ridge from east to west, and constituting the highest point in this range. 



This igneous mass is variously associated with the adjoining limestone strata, lying either 

 ahove or below, without showing any local variation of the ordinary dip, or exhibiting meta- 

 morphic changes at the point of junction. 



On its eastern aspect, this range exhibits a precipitous slope, revealing the thickness of the 

 formation in the regular succession of the uplifted strata, as thus exposed, from summit to base; 

 there is developed at several points a thickness of not less than 1,500 feet. No very marked 

 change is observable in the character of the rock from above downwards, or any local evidence 

 of a change of formation. 



We have, however, evidence from erratic fossils of the presence of a lower order of rocks, 

 belonging to the Silurian period, in this vicinity. Such a formation has been assigned to a 

 corresponding location west of El Paso, by Wislizenus, and there is little doubt but a careful 

 examination along the lower line of these uplifted strata would bring to light this lower class 

 of rocks. 



This range continues to the north, forming the Organ mountains, at which point these strati- 

 fied rocks give place to various forms of igneous products, as indicated in rock specimens Nos. 

 42 to 46, inclusive. 



About seven miles north of El Paso, the mountains adjoining the river give place to the more 

 usual character of gravelly table-land and alluvial bottoms, as noticed below. The table-land 

 is here seen swelling to its broadest dimensions, encircling the distant mountains in every 

 direction, and stretches northward to form the dreaded "Jornada del Muerto;" thence sweeping 

 round the northern point of the Organ mountains, it constitutes the extensive desert tract be- 

 tween the Rio Grande and the Sacramento mountains. To the west the same formation is seen, 

 variously interrupted by mountain ranges and isolated points of igneous rock, extending to the 

 base of the Sierra Madre. 



In further continuation of our general sketch of external and geological features of country, 

 2M 



