54 



GEOLOGY. 



of bottom-land, and occasionally small islands. This character continues for ten or twelve 

 miles, when we enter on a more extended basin, through which passes the Comanche trail, 

 leading from Upper Texas into Mexico, by the adjoining Mexican settlement of San Carlos. 



SECTIONAL SKETCH AT COMANCHE CBOSSING, ON THE RIO BRAVO, BELOW PRESIDIO DEL NOKTB. 



A. Cretaceaus limestone marked with large ossils of Inoceramus, inclining N. N E., at an angle of 80°. 



B. IVIesa, or gravelly table-land formation, resting unconformably upon A. 



At this point the rock exposure exhibits outcrops of limestone belonging to the cretaceous 

 period, being quite abundantly marked by fossil impressions of Inoceramus, often of large size. 

 The rock exposure exhibits a very variable dip, mostly inclined towards the west, occasionally 

 at a very sharp angle. It rises at various points in the adjoining table-land, forming ochreous 

 colored rocky bluffs, where at several points the gravelly table-land is seen to rest unconform- 

 ably on the sharply-tilted strata. 



Further down the river, in an eastern direction, this cretaceous formation assumes a nearly 

 horizontal position and a closer texture. It is here seen overlaid by a variable sheet of dark 

 colored lava rock, corresponding in character to that noticed above in connexion with the Bofe- 

 cilla mountains. This sheet of igneous rock is seen to conform closely to all the inequalities 



SECTIONAL VIEW ON A RAVINE LEADING TOWARD THE RIO BRAVO DEL NORTE, NEAR SAN CARLOS, SHOWING IGNEOnS ROCK DIRECTLY 



ASSOCIATED WITH CRETACEOUS LIMESTONE. 



A. Cretaceous limestone, having an earthy texture, containing fossils of Inoceramus, ]50 feet. 



B. Dark-colored igneous rock, 80 feet in thickness. 



C. Cretaceous limestone of closer texture than that above, 15 feet thick. 



D. Debris. 



of the underlying limestone, exhibiting in the walls of the caiion below a distinct line of sepa- 

 ration, traceable for a long distance. The westerly dip of the cretaceous formation underneath 

 gradually thins out this upper igneous capping, which finally disappears, and solid limestone 

 walls continue along the line of the river. 



