60 GULF TERTIARY OF TEXAS. 



Cretaceous fossils, yellow, gray, and white quartz, jasper, and chert. Here 

 also the limestone is from the Cretaceous area, and the other pebbles from 

 the Paleozoic area to the west of it. On the Rio Grande the pebbles are com- 

 posed of limestone, flint, quartz, chalcedony, agate, black obsidian, red pitch- 

 stone, jasper, and porphyry. Many of these doubtless came from the eruptive 

 rocks higher up the river, while the limestone and flint pebbles are from the 

 Cretaceous area. The gravel on the high bluffs of the Texas rivers is in 

 some places loose, and at others cemented in a ferruginous or calcareous 

 matrix. This cemented material forms a conglomerate of various degrees of 

 hardness. The ferruginous cement has its source in the older (Tertiary) 

 strata of the region which underlie the gravel. The source of the iron is not 

 necessarily in the beds immediately under the gravel, but it is invariably in the 

 neighborhood, as it is always the case that the pebble beds in the vicinity of 

 pyritiferous or glauconitic beds are more apt to be ferruginous than those 

 overlying beds destitute of iron-bearing minerals. For this reason the gravel 

 beds overlying the Tertiary strata are much more apt to be cemented than 

 those overlying the Cretaceous, the Tertiary strata as a rule containing much 

 more ferruginous material than the Cretaceous. 



A large bed of ferruginous conglomerate is seen at "Red Bluff," on the 

 Colorado, and in Burleson County, on the Brazos. 



Pebble beds with a calcareous cement are much more numerous on the 

 Brazos and Rio Grande than on the Colorado. This is doubtless due to the 

 fact that the Brazos and Rio Grande flow over a vastly greater area of calca- 

 reous rocks than the Colorado, and therefore carry down much more carbon- 

 ate of lime, not only in solution, but in a state of mechanical suspension. It 

 is doubtless the carbonate of lime held in suspension that supplies the greater 

 part of the cementing material. Such conglomerates vary from one to ten feet 

 in thickness, and differ greatly in hardness. At Roma, on the Rio Grande, 

 a rock of this kind is used as a building stone, and a hard compact form of 

 it is seen capping the bluff of the river three miles below Las Cuevas. On 

 the Brazos it is found near the mouth of Turtle Creek, McLennan County, 

 and in many of the bluffs in Falls County. 



Below these Upland Gravel beds is often found a series of lower gravel 

 and sand covered terraces, until we reach the river bottom silts. 



EIVEE SILT. 



The rivers of East Texas rise in various parts of the State, and hence the 

 sediments which the comparatively swift waters of their courses carry down 

 and deposit in the quieter basins in East Texas vary considerably in character. 

 The Red, Brazos, and Colorado rivers rise in the eastern slopes of the " Staked 

 Plains," in Northern Texas, pass through the red gypsiferous beds, the Pale- 



