Geological Papers. 69 



rational and most convenient place of division would be above the 

 Dakota. 



2. The invertebrate remains of the Dakota are closely akin to 

 those counted Lower Cretaceous, and are quite distinct from those 

 of the Upper Cretaceous. The plant life, also, though less decisive, 

 is in part at least coordinate with that of the beds below rather 

 than with that of those above. The paleontological evidence, there- 

 fore, favors the same division as the stratigraphical. 



3. Lithologically, also, the most natural classification will be to 

 put the whole of the Dakota, as originally limited, in the Lower 

 Cretaceous. To divide it in most cases brings greater confusion- 

 The division between the Dakota and Benton is not very sharply 

 defined, for it takes a few scores of feet to change from a decidedly 

 sandy formation to one decidedly clayey, several variable thin 

 strata of sand and shale being intermingled between. As it marks 

 the advent of the sea, however, the occurrence of marine fossils as- 

 sists in the demarkation. 



Though such a division cannot be looked upon as marking a point 

 of time, but as progressively constructed from north to south over the 

 Great Plains, yet it is better marked and probably records a shorter 

 time than most similar divisions in geologic history. 



