POST-TERTIARY DEPOSITS. 63 



beds of the same material as those capping the high hills and from one to ten 

 feet thick. These beds are not seen in the lower part of the river.. 



This alluvium is seen extending all the way to the coast at Brazos Santiago 

 and Point Isabel. Here it has been eroded, and forms low hills five to twelve 

 feet above the surrounding tide water marshes, and frequently capped with cac- 

 tus and mesquite. The sides of these hills are often very abrupt and free from 

 vegetation, clearly showing the character of the material. The alluvium now 

 forming is likewise highly calcareous. It varies from a fine sand to an im- 

 palpable silt, so fine that clouds of dust often rise in the wind from the dry 

 river bars. 



Reynosa Limestone. — The Mexican town of Reynosa, in the State of 

 Tamaulipas, on the Rio Grande, is situated on a hill rising some fifty feet 

 above the level of the river and composed of a hard white limestone. This 

 is made up of very hard calcareous nodules, one-half inch to over three 

 inches in diameter, white to creamy-brown in color, and in places showing 

 a concretionary structure imbedded in soft white material of the same com- 

 position. This deposit lies on the sands of the Fayette series, which are 

 exposed at the water's edge. Though usually hard, yet in places on the 

 surface it is soft and crumbly, and in this softer material were found many 

 specimens of Bulumulus alternatus, Say., a shell found now in great quanti- 

 ties on the Rio Grande. These prove the late origin of at least the material 

 in which they were imbedded, and it apparently blends into the harder rock. 

 But unfortunately time did not permit a thorough investigation of the 

 region, and consequently, though it is probably true that the whole of the 

 limestone is of the same late origin, its relations and extent have not been 

 studied. It undoubtedly overlies the Fayette Beds, as this is proved by 

 the small outcrop of the latter at the water edge at Reynosa. Similar lime- 

 stones are said to be found in various parts of Hidalgo County, and that in 

 digging wells they are passed through for about thirty feet, when they find 

 water in an underlying sand or sandstone (Fayette Beds?). 



COAST CLAYS. 



Along the coast of Texas there extends a belt of country underlaid by 

 clays and sandy clays of a gray, yellow, or mottled color, and often black on 

 the surface from the combination of their calcareous contents with vegetable 

 matter. These have been seen by the writer only in a few places along the 

 Brazos, and therefore can not be treated in full at present. They represent 

 the Port Hudson group of Hilgard, and the country underlaid by them is a 

 flat, open prairie, with an exceedingly rich soil. They extend inland to a 

 distance of fifty to one hundred miles, and in some places probably more, 

 until the Fayette Beds rise up to the surface and cut them out. The line 



