686 TRANS-PECOS TEXAS. 



miles, make it advisable to classify both mountain ranges with the mineral 

 districts; still more so as according to a Mexican tradition quicksilver was 

 found there. 



The country east of the Bofecillos is broken and very rough. Vesicular 

 lava and trachyte cover the surface, which is cut by the basalt intrusions and 

 decomposing retinite. Nearer to the Alamo Cesario Springs bowlders of very 

 light vesicular lava and pumice stone fragments demonstrate the volcanic char- 

 acter of the mountains in this vicinity. Evidently the eruptions broke out 

 through fissures, and the lava extravasation and overflow seems to extend 

 over the greater part of the Bofecillos and Refugio into the Limpia range 

 and the intervening mountains. 



Similar extrusive and extravasated rocks appear also east of the Sierra St. 

 Jago and the Pena Colorado, which last is a range south of Marathon about 

 twenty-five miles and extends about fifteen miles in a northeast direction from 

 this railroad station in gently sloping hills of quartz and quartzite, strongly 

 metamorphosed limestones, and semi-fused, small-grained, siliceous conglom- 

 erations. The rock of the Pena Colorado Mountains is a ferruginous quartz- 

 ite, with occasional basalt intrusions frequently faced by metamorphosed 

 limestone. And since east of the Comanche Mountains some of the hills are 

 composed of slightly indurated marine sand, similar to that found by boring 

 at Marfa twelve hundred feet below the surface, I think it justifiable to sup- 

 pose that the quartzites of the Pena Colorado range are the same sands, fused 

 and thrown up by the same protrusive volcanic rocks which farther west and 

 north appear as extrusions and extravasations. 



True, there is four miles north of Marathon an isolated granitic upheaval 

 covering about four square miles, but its appearance indicates that it was con- 

 siderably disturbed by later eruptions, probably the forces which rent ob- 

 liquely the mountains of West Texas and Mexico, opening the channel by 

 which the Rio Grande emptied the Cretaceous basins of West Texas and 

 Mexico through a canyon sixteen to eighteen hundred feet deep and about 

 twenty miles long. Besides, the Carbonic strata of the Comanche Mountains, 

 even in close proximity to the granitic upheavals, are nearly horizontal, and 

 exclude the thought of disturbance by the granite, and it can not be doubted 

 that the metamorphosis of the Pena Colorado rocks was effected in post-Car- 

 boniferous times when the Rio Grande broke through the Canyon of San 

 Vincente. 



Desirable as it is to examine this canyon, its exploration can not be effected 

 without preparations, the expenses of which go beyond the means of our ap- 

 propriation. 



