COMPOUND REGRESSIVE AND TRANSGRESSIVE OVERLAP 627 



stones with beds of clay and glauconite and with several coal seams about 

 1,200 feet above the base. Marine fossils occur almost throughout the 

 series, but Laramie plants have been found in the neighborhood of the 

 coals. 



The interpretation of these sections in the light of the principles dis- 

 cussed shows us that the Dakota sandstone represents the deposits be- 

 tween the retreat and readvance of the sea. The retreat, as we have seen, 

 began in Washita time, almost at the beginning of that period. The Wash- 

 ita division itself is the depositional equivalent of the retreatal Dakota 

 sandstone, and hence the lower Dakota is actually of Washita 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 uncon- 

 formity 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 sandstone is 

 thus of Eagle Ford-Benton age, the return of the sea being finally accom- 

 plished in mid-Benton time. 



From this it appears that the Dakota sandstone can not be included as 

 a time element of the standard scale, since it represents different time 

 in different localities. This consideration also suggests that the Washita 

 be made the base of the Middle Cretacic, the classification being ap- 

 proximately the following: 



Marine. Non-marine. 



Upper Cretacic-Montanan •< <pavlor ^^S. Laramie. 



M ., P , • {Coloradoan | Eagle Ford. • 



M.d-Cretacc ^represented inferva.. < Dakota 



[Washitan. \ 



T /-( i • n u f Fredericksburg. 



Lower Cretacic-Comanehean < -^^j, fe 



If two systems are to be made from 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. 



Non-marine Progressive Overlap 



explanation of the term 



This term is applied to the large structure normally produced during 

 the formation of a great fan or subaerial delta from the wash carried by 



