60 Cretaceous. 



variously tinted sandstone shoals. The solidity of the strata varies 

 very much. They are sometimes formed into very solid rocks, well 

 suited for mechanical or architectural operations; again, the}^ consist 

 of loose and coarse!}' grained sandstone slate, which rapidl}' crumbles 

 on exposure to the air. The general characteristic of this belt and its 

 subdivisions is the strict horizontality of its strata. It is only here 

 and there that some slight local disturbance has taken place, as for 

 instance, near Laredo, and again, some 40 or 50 miles above, where a 

 dip of about 8° W., S. E. and E. is exposed. 



From Las Moras to the vicinity of Arroyo Sombreretillo, which is 

 about 10 miles above Laredo, lignite coal occurs quite frequently. 

 On both sides of the mouth of Elm creek, near Eagle pass, particularly 

 on the north bank of this water course, laj'^ers are exposed from 3 to 4 

 feet thick. On the slope of Lizard Hills, below the deserted Rancho 

 Palafox, coal occurs from 4 to 5 feet thick. 



Septarise abound in the lower belt, especially below the mouth of 

 Arroyo Sombreretillo; on the oyster-terraces, some 40 miles below 

 Laredo, and near the Rancho San Ignacio; on the slope of Red Ridge, 

 of Shady Bluffs, and Septarige Hills. Their most common shape 

 resembles very much a small flat loaf of bread. Both on the outside 

 and inside large irregularlj^- shaped divisions, like a net work of veins 

 occur, which are composed of crystals of g3q3sum, a mineral very 

 abundant in these rocks. 



C. C. Parry found the Cretaceous in the basin of the Rio Grande, where 

 the Comanche trail crosses from Upper Texas into Mexico, near the 

 Mexican settlement of San Carlos. The rock exposure exhibits a 

 very variable dip, mostly inclined toward 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 unconformabl}^ on the sharply- 

 tilted strata. Further down the river, in an eastern direction, the 

 Cretaceous assumes a nearly horizontal position, and a closer texture. 

 It is here seen overlaid by a variable sheet of dark-colored lava rock. 

 This sheet of igneous rock conforms closely to all the inequalities of 

 the underlying limestone, exhibiting, in the walls of the Canon below, 

 a distinct line of separation traceable for a longdistance. The wester- 

 ly dip of the Cretaceous underneath gradually thins out this upper 

 igneous capping, which finally disappears, and solid limestone walls 

 continue along the line of the river. 



At one point on the line of the trail leading round the broken ranges 

 of the mountain ledges, directly bordering the river, to reach its bed 



