636 



A. W. GRABATJ— TYPES OP SEDIMENTARY OVERLAP 



amount of river wear. Of course, the accumulation of these pebbles is 

 the result of continued selection by the rivers of the most rounded peb- 

 bles. Since such rounding implies, as above stated, a very long period 

 of river wear, it is not surprising that only quartz pebbles survived the 

 ordeal. Pebbles of more destructible material were ground to sand or 

 decomposed during the process. Thus the remarkable purity and round- 

 ness of the quartz pebbles of these conglomerates, so entirely inexplicable 

 on the hypothesis of a transgressing sea, is exactly what might be ex- 

 pected in the case of extensively transported river material. 



Since the latest Pottsville beds are overlapped by the Kanawha in 

 northern Ohio, it is more than probable that the Pottsville is entirely 

 absent in Michigan and other western areas, except where represented by 

 equivalent marine strata. Thus the Saginaw formation of Michigan and 

 the Mansfield of Indiana and the Mississippi valley is most certainly post- 



N.W 



^*0 C O 71 o — ?77 cc 



C h u n /( /T ros Son /V<s 



Figure 17. — Diagram illustrating Relationships of Members of Pottsville Formation. 

 Diagram is based on the theory of non-marine progressive overlap (fluviatile). 



Pottsville, limiting that formation according to the standard of the 

 typical Pottsville area. The Saginaw formation of Michigan is indeed 

 now recognized as probably of the age of the Mercer group, which in turn 

 is correlated by White with the later Kanawha. This and the later coal- 

 bearing beds of eastern United States also show evidence of non-marine 

 progressive overlap. They appear to be the remnants of one or more 

 series of fans built out from the Appalachian highlands by streams flow- 

 ing northwestward, the later members of each fan overlapping the earlier 

 ones of the same series. 



Other examples. — Non-marine series, overlapping away from the source 

 of supply, and therefore of fluviatile origin, are to be found in other 

 formations of North America. Among them may be mentioned the 

 Upper Silurics of the Appalachians, the Laramie, and the non-marine 

 Tertiaries of the Great Plains region. The Potomac and Newark forma- 

 tions will probably also show this type of structure when they are studied 

 in greater detail. Wherever it is found, the fluviatile origin of the strata 

 involved is established by it beyond contravention. 



