THE TIMBER BELT OR SABINE RIVER BEDS. 25 



Iron pyrites is an almost inseparable accompaniment of the Timber Belt Beds, 

 and is also the source of many of the iron ores south of the Sabine, as will be ex- 

 plained later. One of the most striking appearances in these beds is the mot- 

 tled red, yellow, and white character of many of the strata. This is due to 

 weathering, and though it is of very common occurrence throughout the 

 Tertiary, it is also seen in deposits of Quaternary age. The phenomenon is 

 usually brought about in one of three ways: 



1. By the oxidation of iron pyrites in the bed, making red or yellow spots, 



according to the amount of decomposition which that mineral has under- 

 gone. 



2. By the infiltration of iron-bearing solutions into a white or light colored 



bed. 

 When the solution of a soluble salt of iron percolates into a sand bed, it is 

 quickly decomposed by the oxidizing action of the air and precipitated as a 

 red or yellow oxide. 



3. By the extraction of iron from a red ferruginous bed through the dissolv- 



ing action of carbonic acid. 



Frequently a ferruginous bed is pierced by the roots of trees, grass, or 

 shrubs, and when these die and begin to decompose they generate carbonic 

 acid. This is taken into solution by the surface waters and dissolves the iron 

 in the enclosing bed, frequently leaving it perfectly white in the neighbor- 

 hood of the roots. Often the sand or clay immediately around the dead root 

 is white from complete solution or yellow from partial solution of oxide of 

 iron, while the rest of the oed is of a bright red color. When there are many 

 such roots, the sand is often mottled in a most striking manner, and the dis- 

 tance that the light colored sands extend from the root depends on the thick- 

 ness of the latter. A root an inch thick will often bleach the sand for over 

 an inch on each side of it. This same bleaching action is often brought 

 about by the carbonic acid solutions from the decomposition of leaves and 

 other vegetable matter on the surface of the ground.* 



The strata of this series are well exposed on the Brazos and Colorado 

 rivers. These streams run across the strike of the strata in a southeasterly 

 direction, and therefore as we descend them we pass successively from the 

 oldest to the newest beds. The sections seen on them are given below : 



BRAZOS RIVER SECTION. 



Descending the Brazos River from Waco, we pass over strata of Upper 

 Cretaceous epoch until we reach the northeast corner of Milam County, where 



* It might appear that causes 2 and 3 were directly opposed to each other in their 

 action, but such phenomena are influenced in their effect largely by the physical conditions 

 under which they operate, and frequently show directly opposite results. 



