THE PENNSYLVANIAN PERIOD 639 



principal iron ores of the Pennsylvanian system occur in Penn- 

 sylvania and eastern Ohio. 



The Pennsylvanian system yields oil and gas in some places, as 

 in Oklahoma, Kansas, and Illinois. 



General Considerations 



Geographic conditions in the eastern interior. Returning for 

 a moment to the system of which the coal-beds form an inconsid- 

 erable part, it is to be recalled that many of the clastic beds asso- 

 ciated with the coal were laid down in fresh water, while other parts 

 of the system were deposited in the sea. It follows that marine, 

 lacustrine, and marsh conditions alternated with one another, and 

 perhaps with land conditions. 



The succession of Pennsylvanian beds in southwestern Penn- 

 sylvania (Fig. 439) illustrates the great series of changes which 

 took place in sedimentation in the course of the period. The cause 

 of the variation was probably geographic, but it is not to be inferred 

 that geographic changes were more frequent at this time than during 

 other periods. Their record is conspicuous because of the coal; or, 

 in other words, 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 equally 

 frequent and equally extensive movements, or that equivalent 

 degradation and aggradation, would leave no such record of them- 

 selves, if the surfaces concerned were far above or far below sea-level. 

 It was oscillation just above and just below water-level which 

 allowed the record to be so clearly preserved. How far the oscilla- 

 tions were due to warpings of the land, and how far to changes in 

 the level of the sea, cannot now be determined; but when we recall 

 that the ocean-level must respond to every deformation which 

 affects its bottom (unless compensated by an equivalent opposite 

 movement) , and to every stage of filling, l it does not seem strange 

 that its level is in a nearly perpetual state of change. 



In general, it may be said that the movements of the crust 

 which have been of most importance, from the point of view of 

 continental or biological evolution, are not those which have 



1 Salisbury, Jour. Geol., Vol. XIII, p. 469. 



