THE IRON ORES OF EAST TEXAS. 75 



iferous strata. It is only natural that iron pyrites, dependent as it is on the 

 combined decomposition of organic matter and soluble salts of iron, should 

 be somewhat uncertain in its presence, and it is a most remarkable fact that 

 it should be so regularly distributed as it is found to be in the iron ore re- 

 gion. The McBee School section also explains one of the most perplexing 

 occurrences in connection with the iron ore region, and that is the capping 

 of dark brown hard, flinty, and siliceous sandstone, directly overlying the ore 

 and from one to six inches thick. This rock is sharply defined from the ore, 

 and often contains fragments of ferruginized lignite.* Such a deposit could 

 easily be formed from the constituents of beds 2 and 3 by the decomposi- 

 tion of the pyrites, first into sulphate of iron and then into the hydrous per- 

 oxide, and the subsequent cementing of the sand by this latter material. The 

 McBee School section is the only one of the kind that has been seen by the 

 writer. But this rarity of such occurrences is very easily explained, as they 

 can exist only when protected by either a heavy covering or an impervious 

 one, and hence are usually obscured. The exposal to view of the case in 

 question is an accidental occurrence, due to the cutting of a steep ravine, and 

 the fact that the overlying sandy clay was so dense and impervious that a 

 thin cover of it protected the underlying strata. A fact, however, that tends 

 to prove its existence elsewhere is tte almost universal occurrence at the con- 

 tact of the giauconite and overlying sandy clay of highly ferruginous springs, 

 especially in places where no brown hematite is seen. The exact contact is 

 invisible, as the sandy clay has invariably drifted over and obscured it. The 

 shape of the ore bed is also strong proof that these chemical and physical 

 actions have gone on; the upper surface of the bed is usually flat, but the 

 base of it is very uneven and shows a bulging mammillary form and large 

 concentric bulbs at the bottom. Frequently the bulging parts of the bed are 

 in separate masses, though they are closely compacted together like a paved 

 road. A yellow clay lies directly under the ore, often resulting from the de- 

 composition of the giauconite. The upper surface of some of these masses is 

 distinctly concave, while the lower is still more convex. This evidently tends 

 to show that it was formed somewhat in the manner of a stalactite, by a 

 downward movement of the iron bearing solutions. The interior of the ore 

 contains inclusions of a very plastic gray clay, giving it a buff color. Some- 

 times the base of the ore bed is very uneven, and shows the very irregular 

 shape of the bed deposited by the percolation of soluble salts of iron. 



Frequently minor seams, beds, and concretionary masses of iron ore are 



* The chemical reactions by which this lignite became ferruginized are somewhat uncer- 

 tain, yet the fact that the material was once lignite or wood is proved by its structure. Hil- 

 gard speaks of similar occurrences in the Quaternary deposits of Mississippi. (Agriculture 

 and Geology of Mississippi, 1860.) 



