454 STUDIES FOR STUDENTS 



jungles whenever delta surfaces were exposed to the air without 

 either uplift sufficient to produce erosion or subsidence sufficient to 

 result in burial. With occasional exceptions, which may be due to 

 river driftage, it is conceded that the coal was formed in situ, and 

 thus each coal-bed becomes a determined land surface, although 

 once in a swamp condition. 



The analogy of the carboniferous swamps with those existing at 

 present upon delta surfaces or buried beneath the later river deposits 

 has been perceived and pointed out since the time of Lyell. This 

 analogy, together with the usual absence of marine fossils and the 

 occasional presence of land or fresh-water forms in the associated 

 strata, has led to the well-founded belief in the chiefly fresh-water 

 or brackish-water origin of the coal-measure shales and sandstones 

 of Nova Scotia, the Pennsylvania anthracite basins, and other regions. 

 But over the western portion of Pennsylvania and much of the con- 

 tinental interior, beds of hmestone with marine fossils occur at inter- 

 vals through, the coal-measures, indicating in those regions periodic 

 invasions of the sea. 



From these facts it is inferred that periodic subsidences took place, 

 allowing transgressions of the widespreading epicontinental sea across 

 the submerged delta surfaces. But the absence of the limestones 

 nearer the shore and in basins like that of Nova Scotia, where the 

 great thickness of shales, sandstones, and conglomerates testifies to 

 rapid erosion and sedimentation, indicates that subsidences did not 

 allow the sea to reach this far inland, but that it was kept out by the 

 rapidity of river aggradation, which, as the basin subsided, distributed 

 mud, sand, and gravel pari passu over the old forest swamp. Thus, 

 the same conclusion is derived inductively as that previously arrived 

 at by deduction concerning the phenomena of a subsiding delta 

 region, where it was concluded that the marine and continental por- 

 tions should be broadly interfingered, the sea cutting in from one 

 side, the rivers building out from the other, and the littoral being a 

 relatively unimportant transition zone resulting from the contest 

 between the two. 



Statements might be quoted from able writers in which it is 

 assumed that the subsidences were of an oscillatory nature, and that 

 the reclamation of the land surface from the sea was due largely to 



