86 GULF TERTIARY OF TEXAS. 



1. There is generally found a cap of iron ore on the benches as well as on 

 the top of the hill, and of the same nature and the same thickness in both 

 places. Yet where a clean section of the deposit underlying the top ore bed 

 is seen, it is very rare that lower beds are found, and when they do occur 

 they are thin, discontinuous, and of a physically different ore than the main 

 bed. This would tend to show that the ore bed found on the benches be- 

 longed at the level of and had once been part of the main bed. 



2. The greensand underlying the ore bed varies from thirty to forty feet 

 thick, yet when there are several benches on the hill slope, and we measure 

 the vertical thickness of the greensand from the upper ore bed to the base of 

 the outcrop, it often appears almost a hundred feet thick. This can only be 

 explained by supposing the edge of the hill to have slipped. 



The alternation of hard and soft strata doubtless causes the formation of 

 many benches, but this generally occurs in the country north of the Sabine, 

 where almost all the benches are due to it. The soft strata over a harder 

 one are worn away, until the eroding agencies come to the hard floor, which 

 temporarily arrests the denudation, and hence arises a bench. The number 

 of benches that can be explained as sea beaches, or river and lake terraces, 

 is exceedingly doubtful, and the want of contour maps, as well as the con- 

 cealed condition of all the strata, makes it still more difficult to determine 

 the extent to which this cause has operated. No satisfactory work can be 

 done in the matter until good maps are obtainable. 



BUILDING STONES. 



Though the beds underlying the whole of East Texas are characterized by 

 their soft and a more or less incoherent nature, yet there are very often found 

 spots in these strata that have become hardened by local chemical action. 

 Such places, though limited in area, are very numerous, and supply a most 

 valuable source of structural materials for local use. They are at present 

 rarely used except for foundations, chimneys, and such purposes, but many 

 of them are capable of being applied to much more extensive structures. 

 These rocks may be divided into two classes: 



1. Sandstones and Claystones. 



2. Limestones. 



SANDSTONES AND CLAYSTONES. 



The most important of the sandstones are a series of local and limited de- 

 posits formed by the action of ferruginous solutions on the original loose 

 sands. This variety varies from a comparatively soft friable mass to a 



