Geological Papers. 67 



pies discussed, shows us that the Dakota sandstone represents the 

 deposits between tlie retreat and readvance of the sea. The retreat, 

 as we have seen, began in Washita time, almost at the beginning of 

 that period. The Washita division itself is the depositional equiva- 

 lent of the retreatal Dakota sandstone, and hence the Dakota is 

 actually of W^ashita age — of lowest Washita in the northern and 

 of highest Washita in the southern area. The retreat of the sea 

 was considerable, as shown by the unconformity between the Buda 

 and the Eagle Ford and by the thinness of the latter, With the 

 readvance of the sea a new fauna, an immigrant from Europe, came 

 in; and as the sea continued to advance the continental sands of 

 the Dakota -Woodbine -Graneros were reworked and incorporated 

 as basal deposits of later Cretacic age. The Upper Dakota sand- 

 stone is thus of Eagle Ford-Benton age, the return of the sea being 

 finally accomplished in mid-Benton time. 



"From this it appears that the Dakota sandstone cannot be in- 

 cluded as a time element of the standard scale, since it represents 

 difPerent time in different localities, This consideration also sug- 

 gests that the Washita be made the base of the Middle Cretacic, 

 the classification being approximately the following : 



Marine. Nonmarine. 



Upper Cretacic-Montanan \ ^3^v3.rro [ Laramie. 



I Taylor \ 



I Coloradoan.... i ^*^«^'"i^ ^ 



Mid-Cretacic ' , , ( Eagle Ford. 



I Unrepresented interval [ jjnkota 



[ Washitan \ 



Lower Cretacic-Comanchean \ Fredericksburg. 



} Trinity. 



"If two systems are to be made of the present Cretacic, the 

 Comanchic system would end with the Washitan, and the Cretacic 

 begin with the Coloradoan, the unknown base of which must be 

 looked for in southern Texas or in Mexico.^" 



Dr. W. B. Scott, in his "Introduction to Geology" (2d edition, 

 1907), in his tabular presentation of Cretaceous formations, follows 

 Grabau, and represents the Dakota as belonging to the Lower Cre- 

 taceous. In the discussion in the text he says: 



"Whenever the marine Upper Cretaceous is in contact with the 

 Comanche limestones north of Mexico, the two are unconformable, 

 and no species of animal is known to pass from one to the other. 

 In Mexico the Lower Cretaceous passes into the Upper without a 

 break, the disturbances there taking place at a later date. 



"The Upper Cretaceous rocks have a far wider distribution over 

 North America than have those of the lower division, which is due 



6. Bull. Geol. Soc. Am, (1905), vol. 17. p. 627. 



