220 E. T. DUMBLE — GEOLOGY OF THE RIO GRANDE. 



points has a direction S. 27° E., and a length of 81.7 miles. The distance 

 hy the river is probably half as much more, or one hundred and twenty 

 miles. The general direction of the dip is about 8. 45° E., which in this 

 region of very slightly inclined strata makes the section practically 

 follow the clip . 



Topography. 



For the distance given, the Rio Grande flows in a valley eroded in 

 Quaternary (or later Tertiary) and upper Cretaceous sediments, and its 

 hanks vary in character with the different materials of which they are 

 composed. When the strata are of sufficient hardness, bluffs of from 50 

 to 100 feet in height are found stretching along on one side or the other 

 of the river, while opposite there is generally only a gentle slope from 

 the water. In places the line of hills drops back some three or four 

 miles from the river, leaving broad fertile valleys. The general nature 

 of the topography, while resembling that of the same formations in cen- 

 tral Texas, is characterized by somewhat more angular and stronger lines, 

 due, no doubt, to the difference in climatic conditions. The tributaries 

 which empty into the Rio Grande on the Texas side are mostly small 

 and carry comparatively little water. The principal creeks are San 

 Felipe, Sacatosa, Sycamore or San Pedro, Pedro Pinto, Cow, Texaquito, 

 Las Moras, Elm, Rosita, Willow and Cuero. On the Mexican side, how- 

 ever, there are bold streams which add considerably to the volume of 

 water in the river. Among them are the San Diego, Escondido and San 

 Domingo rivers. 



The elevation of Del Rio, according to the Southern Pacific railroad 

 engineers, is 073 feet; that of Eagle Pass, by the same authority, is 762 

 feet. According to a line of levels run for an irrigating canal, the bed of 

 the river is 120 feet lower at Eagle Pass than at the falls some forty miles 

 above, giving about three feet fall per mile in direct line, or about two 

 feet per mile of river. All of the falls and rapids, which are numerous, 

 are caused by the edges of the harder strata as they are carried under by 

 the dip. 



Geologic Structure. 



lower cretaceous. 



The town of Del Rio is situated on the Arietina clays, which have here 

 a great development. Just southeast of the town there is a conical hill 

 or "mountain " 100 feet, or possibly more, in height, composed of clays 

 and shales and containing great numbers of shells of Exogyra arietina. 

 Roemer, Nodosaria texana, Conrad, and various other fossils. The hill 



