568 A. W. GRABATJ TYPES OF SEDIMENTARY OVERLAP 



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Non-marine progressive overlap 627 



Explanation of the term 629 



Examples of non-marine progressive overlap 629 



Chemung-Catskill 629 



The Pocono 629 



The Mauch Chunk 632 



The Pottsville 634 



Other examples 636 



Introduction 



The sedimentary formations of the earth's crust fall readily into two 

 great stratigraphic groups, the marine and the non-marine, which in 

 their essential characteristics are strongly contrasted and which in the 

 analysis of sedimentary series must be carefully differentiated. In spite 

 of the practice to the contrary, stratigraphers will admit that only marine 

 deposits are suited to furnish the record for a complete time scale, and 

 that consequently the standard column of any region should be based 

 on marine deposits only. Where, as is often the case, the column 

 selected as a standard contains non-marine members, the column is im- 

 perfect as long as these are retained. Thus the standard Cretacic column 

 of North America is impaired by the retention in it of the non-marine 

 Dakota and Laramie formations, and until recently the standard Triassic 

 section of Germany was practically useless, as it contained only one 

 marine member. The substitution of an extensive series of marine 

 members for the Bunter Sandstein and Keuper has given us a perfect 

 standard of comparison, such as is hardly equaled by that of any other 

 of the geological systems. 



Non-marine sediments, however, while not serviceable as members of 

 a standard time scale, are still of great stratigraphic importance, since 

 they furnish us with records of physical changes not determinable from 

 the deposits of the marine series; but as long as non-marine sediments 

 were regarded as lake deposits only, their true significance was over- 

 looked. Now that stratigraphers recognize that non-marine deposits are 

 oftener than not of fluviatile or seolian origin, their real meaning becomes 

 more and more apparent. 



The two types of sediment are distinguished from each other not only 

 by their fossil content, but also, and almost as easily, by their physical 

 characters, especially the larger ones. The most striking difference of 

 all lies in the manner in which the successive members of either series 

 are related to each other. In the following discussion the distinguishing 



