80 OULF TERTIARY OF TEXAS. 



the amount of exposure to the atmosphere. Eventually the whole mass de- 

 composes, and the whole of the carbonate of iron is converted into peroxide, 

 and as a result we have the brown hematite nodules and geodes of the pres- 

 ent iron ore beds. It doubtless occurred in some places, that as the clay- 

 ironstone gradually decomposed, a part of the resulting peroxide was taken 

 up by carbonated waters and redeposited again whenever the water was ex- 

 posed to a sufficient extent to lose its carbonic acid. This action would ce- 

 ment the nodules and geodes, and form the continuous stratified beds which 

 we often find in the iron ore region. Where such action has not gone on 

 we have the loose nodules, such as are seen at Leverett's Hill. An analysis 

 of this clay ironstone from Van Zandt County gives the following ingredients: 



Oxide of Iron (equals Metallic Iron 31. 11) 45.30 



Alumina 17.04 



Lime 15.68 



Silica 6.91 



Carbonic Acid 12.32 



97.25 



Here is a source of supply for all the constituents of the brown hematite. 

 In the chemical changes that have gone on some of the original constituents 

 have been partly lost and the percentage of others proportionally increased. 

 The carbonic acid was originally combined with the iron and lime. That 

 part of it associated with the iron has been entirely removed, and was prob- 

 ably the source of supply for the carbonated waters which dissolved and re- 

 deposited part of the peroxide of iron, as explained above. The lime, com- 

 bined with its portion of carbonic acid in the form of carbonate of lime, has 

 been carried away in solution, thus removing the last of the carbonic acid. 

 Consequently the percentage* of iron in the bed is increased directly as the 

 accompanying constituents decrease, and the result is that from a clay iron- 

 stone containing little more than thirty per cent of iron we now have the 

 brown hematite with a composition as shown in the analyses and an iron con- 

 tents of forty to fifty per cent. 



One of the strongest proofs of this mode of formation of the iron ores is 

 the fact that above the water level of the region, where the strata have been 

 exposed to all the oxidizing influences of the air, the iron ore is all, or most 

 all, in the form of brown hematite, while below the water level it is in the 

 form of clay ironstone, to whatever depth we go. Now, as the strata lie 

 almost horizontally, these two forms of ore, lying as they do one far above 

 the other, must occupy different horizons, and consequently it might be said 



*Of course in this process of breaking up of the clay ironstone, some of the iron is lost 

 also, but the amount carried away in this manner is very small. 



