LARGER RELATIONS OF DELTAS 395 



nental origin as a whole he regards as demonstrated from the fact that 

 the higher formations overlap northwestward.** But that this is not 

 proved is indicated by the existence of a brachiopod fauna in the Middle 

 Pottsville (Horsepen) as far north as Sewell, on New River, and less 

 conclusively by the occurrence of Naiadites and Spii-orhis in the Lower 

 Lykens group of the Anthracite region.'' David White also reports other 

 localities and horizons which contain marine invertebrates. The present 

 writer holds that the beds of coal and the heavy conglomerate horizons 

 are clearly continental, and this implies that some part of the remainder 

 is also fiuviatile. In the north and east this is thouglit to apply to prac- 

 tically the whole of the formation; but the mai-ine faunas, more abun- 

 dant in the south and west, prove at least a considerable proportion of 

 marine beds in that region. The great thicknesses which the Pottsville 

 attains in the southern Appalachians, from 2,000 to 6,000 feet, clearly 

 indicates that the level of the npper surface was as much controlled by 

 subsidence, which tended repeatedly to bring in the sea, as by river up- 

 building, tending to raise the surface above sealevel. 



From these examples it is seen that overlap away from the source of 

 supply can not be used as a criterion of continental or marine origin any 

 more than transgressive or regressive overlap, but may be due to regional 

 subsidence or tilting or a climatic change which shifts clastic material 

 of a certain kind progressively farther from the source of supply.^*' 



THE DELTA CYCLE AND ITS USE .4,9 A CRITERION OF ORIdlN 



Comparison of the erosional and depositional tlieories. — Previous to 

 the recognition of the principle of baseleveling and the key which it 

 offered to the erosional history of the lands, the processes of river erosion 

 as elaborated in geologic texts were essentially detailed descriptions, 

 unrelated to a law which should connect the sequential stages into a 

 cycle of erosion. The recognition of that principle has raised the subject 

 from the descriptive and qualitative to the quantitative and predictive 

 plane and permits the erosion history to be read in terms of time and 

 crustal movement, modified by the factors of climate and rock structure. 



But a graded river consists ideally of three parts : its upper waters are 

 erosive agents, its middle waters are fully engaged in transportation of 

 the rock detritus, and its lower waters deposit part of the burden before 



" Types of sedimentary overlap, pp. 6.34, 636. 



^ David White : Deposition of the Appalachian rottsvillo. Bull Geol. Soc. America, 

 vol. 15, 1904. pp. 277, 280. 



1" The relations of climate to overlap have been more fully di.scussed by the writer in 

 another paper, "The relations between climate and terrestrial deposits," part III. Jour- 

 nal of Geology, vol. xvl, 1908, pp. 363-384. 



