22 GULF TERTIARY OF TEXAS. 



and highly calcareous. They owe their black color to the combination of the 

 carbonate of lime with the vegetable matter on the surface. The soil is un- 

 derlaid by a subsoil of yellow and gray clays, with occasional thin seams of 

 sand, and many calcareous nodules, which latter supply an endless source of 

 lime. These soils are remarkably rich, and are well adapted for the cultiva- 

 tion of wheat, corn, oats, and other grain crops. Cotton is also most success- 

 fully cultivated here ; and in fact the soil can support almost any crop that can 

 be raised in the prairies of Central Texas. 



THE TIMBER BELT OR SABINE RIVER BEDS. 



The Basal Clays, everywhere from the northern part of the State to the Colo- 

 rado River, blend upwards into the sandy Timber Belt Beds. These form the 

 mass of the Tertiary formation in Texas, and underlie the great timber region 

 of the eastern part of the State. They are composed entirely of siliceous and 

 glauconitic sands, with white, brown, and black clays. The clays, however, 

 are greatly in the minority, and the siliceous sands compose by far the larger 

 part of the whole series. Lignite beds are of very frequent occurrence, vary- 

 ing from a few inches to ten and twelve feet thick; and the sands and clays 

 are often impregnated with vegetal matter to such an extent that numerous 

 traces of petroleum, asphalt, and natural gas have been found in the East 

 Texas region, sometimes in quantities of considerable economic importance. 

 Many of the black and brown clays and sands owe their coloring matter to 

 this ingredient of vegetable material, and burn white or buff color when ex- 

 posed to heat. These beds occupy an area over 125 miles wide in the north- 

 east part of the State, but thin down to less than 40 miles on the Colorado. 

 This greater development of the Tertiary strata to the northeast is probably 

 due to a greater deposition in the vicinity of the embayment which existed 

 in the lower Mississippi at the time they were laid down. The sands are gen- 

 erally much cross-bedded, gray to buff in color, and contain black specks, 

 which are often glauconite. This latter mineral is a common constituent in 

 many of the beds, and there are found all gradations, from a pure siliceous 

 sand to a pure greensand bed, such as are well developed in the iron ore re- 

 gions of Anderson, Cherokee, Rusk, and other counties. All the sand beds 

 are more or less impregnated with carbonate of lime, and often it is in such 

 quantities as to form beds of calcareous sandstone, where it acts as a cement, 

 and forms a soft friable rock. Sometimes even beds of limestone are found, 

 and calcareous nodules and concretions are of very frequent occurrence 

 throughout the whole of the Timber Belt Beds. One of the most character- 

 istic features of the region depends on this presence of carbonate of lime in 

 the sandy beds. It is the occurrence of great masses of sand, varying from 

 one to ten feet and more in diameter, and cemented into a hard rock by the 



