680 TRANS-PECOS TEXAS. 



From this pass to the river the mountains extend in a southeast direction 

 about twenty-five miles, and are flanked and partly capped by stratified rocks ; 

 but the tilting, the rugged cliffs and ravines, and the hot springs at the south- 

 ern end show clearly the igneous character of this mountain range. In the 

 flat east of the Quitman Mountains Cretaceous hill groups rise and extend 

 toward the Eagle Mountains. 



The rocks of the Eagle Mountains are porphyries, porphyritic, vesicular, 

 and compact lavas, with occasional deposits of volcanic breccias, siliceous, 

 oolitic, and different metamorphic rocks, with numerous basaltic and retinite 

 dyke intrusions. The porphyries of these mountains are different in color, 

 fracture, lustre, and combination from those of the Quitman Mountains. 

 There is also a difference in the structure of the mountains. 



In the Quitman Mountains the porphyries are solid and massive, the various 

 colors blending and changing into each other, the fracture being uneven, in 

 places sub-conchoidal. They are hard, but more tough than brittle, and 

 though taking a fine polish, the fracture has a dull aspect. The colors of the 

 matrix show numerous shades of gray, greenish, reddish, brown, purple, to 

 black, and the included crystals, whatever the color of the matrix may be, 

 are of whitish color, nearly lustreless, and frequently look as if their sharp 

 edges and corners were fused and worn. 



The porphyry mountains of the Eagle group show in most places a kind of 

 stratification. A dark reddish and brown porphyry overlies a stratum of 

 yellowish porphyritic rock with sanidin and andesite(?) crystal inclusions. 

 This lighter porphyry has in many places the appearance of an excessively 

 metamorphosed sandstone, and were it not that in places where it appears 

 intrusive in the limestone the limestone itself shows also excessive metamorph- 

 ism, its peculiarities would justify the hypothesis that it is really a sand or 

 sandstone stratum altered by overflowing igneous eruptive rocks. The foot- 

 hills on the eastern side of the Eagle Mountains are Cretaceous sand and lime- 

 stone strata, dipping toward the mountains (west). 



The foothills on the north side near Eagle Spring are formed by Cretaceous 

 sand and limestones, intercepted by a small area of metamorphic Carbonifer- 

 ous limestone, and run out on the west side into the Cretaceous Devil's Ridge, 

 which extends into the Devil's Backbone (also a Cretaceous ridge), flanked 

 on the south side by a series of brecciate conglomerate hills, between which 

 and the Quitman Mountains a flat four miles wide extends south of the pass. 

 This flat, which connects with the large, level basins extending north and east 

 of Sierra Blanca Junction and the flats between Etholen and the northern 

 part of the Quitman range, is covered with fertile soil. 



In its southeastern part, which opens and falls towards the Rio Grande as 

 it approaches the river, the mostly dry water course? of the Green River and 



