32 GEOLOGY. 



impregnated and sprinkled over with oxide of magnetic iron. Some of these strata contain 

 pieces of the latter formed into every shape, but most commonly give to the matrix the real 

 habitus of volitic texture. 



This imbedded formation increases as you approach its lower edge, (Las Moras,) leading to 

 another change in the lithological features of the country along the Rio Bravo. 



Before considering this change, however, another fact of much geological importance is not 

 to be overlooked. 



THE DYKE. 



About twenty miles below Las Moras is Elm creek, (Arroyo de Los Olmos,) the next tributary 

 of the Rio Bravo on the Texan side. Its valley belongs to still another geological belt ; none 

 of the more solid metamorphic limestones before referred to are now to be seen. The whole 

 country from Las Moras to the mouth of the Rio San Juan, and even as far down as the vicinity 

 of the Mexican town of Reynosa, forms another link of the cretaceous system; a more soft and 

 brittle sandstone, (partly chloritic?) varying in grain, color, and cohesion, constitutes the main 

 part of this formation. This resembles very much, if it is not in reality, the green sandstone 

 or chloritic chalk itself. 



Its northern limit, where it joins the more recent metamorphic limestone some distance below 

 the Las Moras, is distinctly marked by a line which shows on the surface or in the soil signs of 

 a geological disturbance. This limit is the valley of Elm creek, two miles and a half above 

 Eagle Pass. It is wide and flat; the ridges of hills bordering it are often overthrown and 

 washed down, whilst the horizontal strata in many places are brought into synclinal or anti- 

 clinal positions. The creek itself, not the one of that name on the El Paso road, is sluggish, 

 and carries only a dirty, greenish, and brackish water, which often disappears in its bed, leav- 

 ing only here and there small ponds and muddy pools. Out-crop of pretty extensive beds of 

 lignite coal occur on both sides of the mouth of this creek well worthy of examination, and may 

 prove to be of commercial value. 



These coal-layers probably gave the name " Piedras Negras" to the Mexican military colony 

 in the vicinity. The aspect of the valley of Elm creek, and the character of the country to 

 the right and left of the Rio Bravo at this point, justify the idea that a subterraneous volcanic 

 dike crosses the basin of the Rio Bravo. By turning for a moment from our course along the 

 Rio Bravo, and proceeding from the mouth of Elm creek in a direction southwest by south for 

 about seventy miles, we reach the foot of a high and bold mountain range formed of metalliferous 

 limestone, {zechstein,) the precious contents of which once made Santa Rosa famous as a silver- 

 mining town. 



On the line between the Rio Bravo and the Santa Rosa mountains, the face of the country 

 shows many signs of a geological disturbance ; the usually undulating region becomes more 

 broken, whilst the flat^ong-stretching ranges of hills are frequently overthrown. The slope 

 of the Santa Rosa mountain is rocky, wildly broken, and steep, and large portions of the strata 

 are entirely dislodged and most anomalously placed. The stratification here is not only seldom 

 horizontal, but frequently thrown up vertically. There is some regularity, however, in this 

 apparent disorder, particularly with respect to the parallelism — the characteristic of all the 

 Cordilleras of the American continent. 



