40 GULF TERTIARY OF TEXAS. 



From here to Roma many other scattered cypress trees are seen, and are said 

 by Schott* to be colonies from other groves some distance up the Salado. 

 Below Carrizo, grass becomes more plentiful, and as we approach Rio Grande 

 City and Hidalgo, the low bluffs are often capped by rich grassy sod. The 

 willow here often entirely replaces the mesquite along the lower bluffs, and 

 the general appearance of the country is that of a better watered land than 

 up the river. This is especially true of the country about Brownsville, where 

 luxuriant corn fields are to be seen, as well as cotton and sugar cane planta- 

 tions and fine grass lands; all, however, assisted by irrigation. 



Though, as has been shown, the strata on the Rio Grande differ somewhat 

 in physical character from those of the country to the northeast, yet the com- 

 position, dip, and the general make-up of the beds are remarkably similar to 

 their stratigraphical counterparts in that region. In both places we see every- 

 where the signs of alternating marine littoral and brackish water lagoon for- 

 mation in the great banks of shells, the littoral character of the fauna, the 

 ripple-marked sands, the lignites, and the worn fragments of lignite even in 

 some of the shell-bearing beds. There is, however, one striking difference: 

 On the Rio Grande the marine character of the strata predominates, while in 

 Northeast Texas the lignitic or lagoon character occupies the vast bulk of the 

 strata. 



At Piedras Negras, directly across the river from Eagle Pass, are inter- 

 bedded siliceous sands, sometimes containing large quantities of glauconite 

 and thin seams of lignitic matter. The strata are undulating, and are capped 

 by gray river silt, some twenty feet thick. On the American side of the river, a 

 half mile northwest of Eagle Pass, is found a series of light brown or buff semi- 

 indurated calcareous sandstones, containing Cretaceous fossils. This forma- 

 tion composes an abrupt and flat-topped ridge, rising some seventy-five feet 

 above the town, and running off in a southeast direction towards the river, 

 with a gentle dip in the same direction. One mile below Piedras Negras, on 

 the Mexican side of the river, are found deposits similar to those at that town, 

 but here they contain trunks of silicified wood, and large indurated masses, as 

 seen on the Colorado. Just below Rio Escondido is found a low bluff of stiff 

 greenish clay, resembling very much the "Ponderosa Marls" of the Upper 

 Cretaceous. A quarter of a mile below, on the same side, are seen interbedded 

 sands hardened into flat slabs three to ten inches thick, containing many 

 shells, interbedded with gray sands, and dipping three degrees southeast. The 

 same formation is seen for a mile and three-quarters below this, and on both 

 sides of the river. At this point there are found, on the Texas side, numer- 

 ous Ammonites and other fossils in a hard gray limestone. The series consists 

 of the same sandstone as the last. Six miles below, on the Texas side, is seen 



* Mexican Boundary Survey, 185*7, Tol. II, Part II, Chap. II, p. 43. 



