376 E. T. DUMBLE — CRETACEOUS OF TEXAS AND MEXICO. 



Introduction. 



While in its broader features the Cretaceous of western Texas and of 

 the northern portion of the Mexican state of Coahuila corresponds closely 

 with that of the Colorado River section east of it, there are, nevertheless, 

 many important differences in the stratigraphy and faunal relations well 

 worth more detailed study than they have yet received. A few of these 

 differences, which have come under my personal observation during trips 

 made through various parts of the region, are presented as indicating 

 the general character of the formation. 



Localities of Occurrence, Character and Relations of the Rock. 



Only a few remnants of areas are found north of the Texas and Pa- 

 cific railroad, and that line may well be taken as marking the northern 

 boundary of the Cretaceous deposits of western Texas, since, as a body, 

 they pass north of it only (if at all) under that portion of the road which 

 crosses the Llano Estacado. 



In Trans-Pecos Texas the basal rocks of this Cretaceous system are 

 best exposed in the vicinity of the railroad, and, as the Rio Grande river 

 is neared in going south from it, beds higher and higher in the section 

 are found. The country, as a whole, slopes rapidly from the north toward 

 the river, and, while in the southern portion of the area the Cretaceous 

 rocks are found at some of the highest altitudes, along the railroad they 

 occur as the foot-hills of the mountain-blocks, whose cliffs of Silurian 

 and Carboniferous limestones tower from 1,000 to 2,000 feet above them, 

 or as detached mesas, buttes or ridges in the wide spread flats which 

 separate these mountains. 



Much of the limestone of the Lower Cretaceous is metamorphosed as 

 highly as is that of the Carboniferous, and is consequently as well adapted 

 to withstand erosion, yet no trace of it has been observed upon the tops 

 of the ranges north of the railway. Even where it now occupies higher 

 altitudes in this region, as on Sierra Blanca and in the vicinity of Gomez 

 peak, in the northern portion of the Davis or Apache mountains, its posi- 

 tion is evidently due to the orographic action which formed the moun- 

 tains. 



So far as I have observed, while the Carboniferous rests indiscrimi- 

 nately on various horizons of Algonkian and Cambrian, the Cretaceous 

 has only been found in contact with the Carboniferous, Permian and 



Triassie. 



San Lorenzo Section. 



its location and the character of the country. 



This section was made in northern Coahuila, beginning at the head of 

 the San Diego river, in that portion of the Burras range known as the 



