THE PENNSYLVANIAN PERIOD. 581 



while the other members of the system were being laid down. Dis- 

 solved by the land waters from the soil and rocks, it was brought to 

 the marshes in some soluble form. In the marshes, it was precipitated 

 either in the form of the carbonate or ferric oxide. Subsequent oxi- 

 dation has changed some of the original carbonate into ferric oxide. 

 The principal iron ores of the Pennsylvanian system occur in Penn- 

 sylvania (Allegheny and Monongahela series) and eastern Ohio (Alle- 

 gheny series). 1 



Geneeal Geographic Conditions in the Eastern Interior. 



Returning for a moment to the system of which the coal-beds form 

 an inconsiderable part, it is to be noted that many of the clastic beds 

 associated with the coal are known by their fossils to have been laid 

 down in fresh water. Some of the occasional limestone-beds, on the 

 other hand, as well as some of the clastic beds, were deposited where 

 marine conditions prevailed. It follows that marine, lacustrine, and 

 marsh conditions may also have alternated with the others, but such 

 conditions, especially if the land be too low to suffer notable erosion, 

 leave but indistinct records. 



The succession of Pennsylvanian beds in southwestern Pennsyl- 

 vania (Fig. 242) illustrates the great series of changes which took 

 place in the sedimentation in the course of the period. These changes 

 were probably the result of geographic changes, such as variations in 

 the height of the land, the depth of the water, or in the areal relations 

 of land and water, due to gradation. It is not to be inferred that 

 changes were more frequent at this time than during other periods. 

 Their record is conspicuous because of the coal; or, in other terms, 

 because the land was near sea-level, so that extensive submergence 

 and emergence resulted from slight changes of relative level of land 

 and sea. It should be remembered that a series of equally frequent 

 and equally extensive movements, or that equivalent degradation and 

 aggradation, would leave no such record of themselves, if the surfaces 

 concerned were well above or well below sea-level. It was the oscil- 

 lation just above and just below water-level which allowed the record 

 to be so clearly preserved. How far the oscillations were due to warp- 



1 For statistics concerning production of iron, see Census Reports (Vol. XV, 10th 

 Census) and Mineral Statistics, U. S. Geol. Surv. 



