LOCATION, CHARACTER. AND FAUNA OF THE SECTIONS 269 



probable that they belong to post- Washita time. Whatever their relations 

 with the Washita may be, it is reasonably certain that they are the same 

 as the lowest marine Cretaceous shale in the Horse Creek section (num- 

 ber 11) and the shale beneath the sandstone bearing a Dakota flora in the 

 Lander, or central Wyoming, section (number 12), and most probably 

 the shale beneath the Newcastle sandstone of the Black Hills section is 

 also of the same age. 



It will be remembered that the overlying sandstone in all these sections 

 is not a continuous bed, but varies in thickness from to 60 feet. In 

 regions farther north and west, as around the Bighorn and Little Kocky 

 Mountains, where no such sandstone is identifiable, it seems a reasonable 

 interpretation to correlate an indeterminate lower part of the Ther- 

 mopolis shale with the lowest marine Cretaceous shale of the northern 

 Colorado and eastern Wyoming sections, since the shales above and below 

 the sandstone are of the same lithologic character and would not be dis- 

 tinguished if they were not separated by the sandstone. If these marine 

 beds are of Washita age, it necessarily follows that all of the Dakota as 

 typically developed in eastern Nebraska and Kansas are also of Washita 

 age, as Twenhofel and others have held; but it has already been shown 

 that the arguments for referring the Dakota to the Washita, so far as 

 they are derived from the distribution of the invertebrate faunas, are 

 based on false premises. The typical Dakota is most probably represented 

 on the west side of the Great Plains not only by the uppermost sandstone 

 of the "Dakota hogback" in northern Colorado and its probable equiva- 

 lent, the Newcastle sandstone of the Black Hills, but also by the under- 

 lying marine shale and probably part of the sandstones beneath it. 



Section number 9 of the chart, in the Apishapa quadrangle, southern 

 Colorado, and number 8, at Garrett, Oklahoma, show the same kind of 

 progressive change toward the conditions of the Denison section that is 

 seen in going from central Kansas through southern Kansas to Denison 

 (sections 3, 2, and 1). 



NEW MEXICO SECTIONS 



Another line of sections running west from Denison, near Tucumcari, 

 New Mexico (number 5), at Canyon Largo, southeast of Las Vegas 

 (number 6), and near Casa Salazar, on Puerco Kiver, northwest of Albu- 

 querque, New Mexico (number 7), show the rapid thinning and disap- 

 pearance of the Washita in that direction. Its fauna is not found many 

 miles beyond Tucumcari, and at Casa Salazar there are no rocks to repre- 

 sent it. The thin Dakota sandstone, which there rests on the Morrison, 

 is followed by marine shales and sandstones in which a number of faunal 



