•10 INTEODUCTORY, 



member (Dakota group) can probably be correlated very approximately, 

 although presenting a somewhat different fauna ; but the upper members 

 (2, 3, 4, and 5 of Meek and Hayden) cannot be so satisfactorily distin- 

 guished nor subdivided in the same way as elsewhere, though it seems 

 probable, in a high degree, that all these members are represented. The 

 lithological characters show the same agreement, though not an observed 

 coiTespondence of details. In one respect, however, there is a notable 

 distinction. The entire Cretaceous series of the Plateau Province abounds 

 in coal and carbonaceous shales, while in the more eastern exposures coal 

 appears to be confined to the higher members. 



CLOSE OF THE CRETACEOUS — UNCONFORMITIES. 



The closing period of the Cretaceous marks a change in the physical 

 condition of the region. The ocean gave place to brackish waters. What 

 orographic movements or what uplifts of broad areas may liave accom- 

 plished this change we do not know in detail, and it is at present impossible 

 to form any very definite idea of the geography of the region during that 

 period. We only know that the uppermost Cretaceous strata have hitherto 

 furnished only brackish- water fossils, and we naturally infer from them that 

 the Cretaceous ocean was subdived into a number of Baltics or Euxines by 

 the rearing of mountain chains and broad land areas aroimd their borders, 

 but leaving narrow straits communicating with the sea. The brackish- 

 water fossils either mean that or they are at present inexplicable. These 

 movements, however, involved no other changes in the physical condition 

 of the country, for the deposit of shaly, marl}-, and arenaceous strata with 

 seams of lignite went on as before, and continued through a long period 

 until the accumulations reached in many places a thickness of nearly 2,<)00 

 feet without any interruption which can be specified. These T'pper Creta- 

 ceous beds are without much doubt the equivalents of the Judith River 

 beds of Meek and Hayden and the Laramie beds of King. 



The continuity of deposition was at last broken. Resting upon these 

 Laramie beds is a series of calcareous shales alternating with sandstones, 

 which, through a thickness of 1 00 to 250 feet from the base, contain also a 

 brackish-water fauna, but which as we ascend gives places to molluscan 



