THE TIMBER BELT OR SABINE RIVER BEDS. 23 



calcareous matter. These rocks vary much in shape and hardness. Some- 

 times they have a concretionary shape and weather in concentric layers; at 

 others they show the horizontal stratification of the beds in which they occur, 

 and gradually blend into the soft enclosing sand. The presence of this car- 

 bonate of lime is due to two sources: 



1. The calcareous matter of shells in the strata. 



2. The carbonate of lime derived by solution from the old Cretaceous shore 



line. 

 This latter source is probably the principal one, as all the waters flowing 

 from Texas into the Tertiary Sea had to pass over hundreds of miles of 

 calcareous strata, and could not help being strongly impregnated with car- 

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

 Prestwich* estimates that 290,905 tons of carbonate of lime from calcareous 

 strata resembling those of Central Texas are yearly dissolved in the basin of 

 the River Thames and carried to the ocean in solution alone. From this we 

 can get an idea of the immense amount of the same material that was, and is 

 still, carried down by the Texas rivers. In fact the amount dissolved by a 

 given quantity of water in a Texas river, other things being equal, must be 

 much greater than by the Thames, as the water of the southern streams is 

 much warmer than that of the Thames, and therefore capable of taking up 

 into solution a much larger per cent of carbonate of lime. Doubtless, how- 

 ever, the calcareous matter held by the river waters in suspension has given 

 to the Tertiary strata more carbonate of lime than that in solution, as a large 

 part of the latter would be carried out to sea. This presence of carbonate of 

 lime is of the greatest importance, from an agricultural point of view, to the 

 welfare of East Texas, as it renders soils underlaid by such strata of great 

 fertility and durability ; whereas without it, many of them would be perfectly 

 barren. Many of the sands are also intimately mixed with a fine impalpable 

 white clay, which renders the beds soft and highly plastic when wet, but 

 when dry it forms a hard, solid mass, often -occurring as a friable sandstone. 

 When such beds are exposed to erosion by creeks and in gullies they break 

 up into lumps, which become rolled and rounded, and form putty-like peb- 

 bles. This is a very characteristic kind of erosion in some of the Lower Ter- 

 tiary strata, and such beds are well developed in central Van Zandt County. 

 The sand beds are generally also variable in composition. They blend by 

 insensible gradations, both vertically and laterally, into clay or sandy clay 

 beds, so that minute correlations, even in beds very close to each other, are 

 difficult to make. This extreme variability in composition is simply one of 

 the many proofs of a near shore deposit. The sand beds often contain consider- 



*" Geology: Chemical, Physical, and Stratigraphical," Yol. I, p. 107, by Joseph Prestwich, 

 M. A., F. R. S., F. G-. S., London, 1886. 



