110 THE PLIOCENE AND PLEISTOCENE DEPOSITS OE MARYLAND 



usually considered pertinent to a discussion, these " multiple hypotheses 

 will not be again referred to. 



CONTINUITY OE DEPOSITS. 



The " continuity of deposits " was one of the most helpful methods 

 employed in tracing out the extent of any given formation. In certain 

 deposits which have undergone a complicated history, the mere continua- 

 tion of a deposit over a large area may not in itself be a proof of its unity 

 in age throughout, for one part may have been deposited so much earlier 

 than another as to actually belong to a different age, or the formation 

 may contain diagonal faunas. In the problem presented in the Coastal 

 Plain, however, no such complications are present. There the geologist 

 is dealing with formations but a few feet in thickness, each one of which 

 is terminated upward by a level surface which forms the top of a terrace. 

 It is then legitimate to conclude that so long as the surface is continuous, 

 the formation is also. There is, however, an exception to this usage. It 

 was pointed out a little above that occasionally the present surface of the 

 land slopes down and merges with the modern beach without a topo- 

 graphic break. This has been found to be occasionally true of the Talbot 

 terrace, although usually it is separated from the beach by a distinct 

 scarp. Such conditions must have held in past time as truly as in the 

 present and, therefore, caution must be used in applying the method of 

 " continuity of surface " in order to avoid passing from one surface to 

 another of different age. This method must be constantly checked by 

 the legitimate use of all the others. 



SIMILARITY OF TOPOGRAPHIC FORM. 



In applying the method of " similarity of tomographic form " to the 

 surficial deposits, the author regarded the "plain" as the type surface 

 Any topographic feature which deviated from this "type plain" called 

 for an explanation. Before taking up these topographic variations, it 

 will be well to consider the plain itself. In a previous chapter, the author 

 described in a general way the physiography of the region and pointed 

 out -nine of the most striking characteristics of the Coastal Plain deposits. 



