| OCR Text |
Show 10 INTEODUOTOEY. 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 distinguished 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 correspondence 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 have accomplished 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 around 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, marly, 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 2,000 feet without any interruption which can be specified. These Upper Cretaceous 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 100 to 250 feet from the base, contain also a brackish-water fauna, but which as we ascend gives places to molluscan |
