LOCATION, CHARACTER, AND FAUNA OE THE SECTIONS 263 



GENERAL DISCUSSION OF THE ORIGIN OF THE SECTIONS 



The sections so far considered are all near the eastern margin of the 

 great sea which in later Cretaceous time covered the Great Plains and a 

 large part of the Rocky Mountains. During a large part of Comanche 

 time the northern shore of the Mexican sea was in northern Texas, or not 

 far beyond it, as evidenced by the nature of the Trinity and Paluxy sands 

 and the northward thinning of all the limestones. In later Comanche 

 time, however, about the beginning of the Washita, a somewhat rapid 

 northward transgression of the sea began, which soon reached southern 

 Kansas and by the close of the Washita had extended as far north as 

 Salina, in central Kansas. The advance was probably of a halting nature, 

 with possibly slight reversals of movement, so that with shallow waters, 

 low-lying lands, shifting currents, and varying amounts of sediment re- 

 ceived, the shoreline sometimes rapidly shifted many miles both north 

 and south and east and west, thus giving alternations of purely marine 

 beds with those that represent swamp or terrestrial conditions. 80 far 

 as can be determined from the rocks now exposed along the eastern shore, 

 the sea did not reach northeastern Nebraska until after the close of 

 Washita and Comanche time. 



How far north the Comanche sea extended in the western plains and 

 Rocky Mountains is one of the questions that need more evidence than is 

 now available before a satisfactory answer can be given, but the few data 

 that have accumulated may be considered. The evidence must be sought 

 toward the western margin of the Great Plains. The nearest exposures 

 west of the typical Dakota area of eastern Nebraska are around the mar- 

 gin of the Black Hills uplift. The details of the section vary from place 

 to place, but I have taken as representative of the general conditions the 

 section in the Newcastle quadrangle on the west side of the Black Hills 

 (number 13 of the chart). The diagram represents the section from the 

 Niobrara down to the marine Jurassic Sundance. The Dakota as now 

 generally interpreted is the upper sandstone of the "hogback" which sur- 

 rounds the Hills, and between it and the Sundance are the Fuson shale, 

 Dakota sandstone, and Morrison shale. The Fuson and the Dakota are 

 both plant-bearing, and the evidence of the plants apparently shows that 

 both these formations are older than the Washita, their floras being more 

 primitive than that of the Cheyenne and comparable with the Potomac 

 floras of Maryland and Virginia. This fact should be kept in mind in 

 considering the relations of the Purgatoire formation with the Morrison 

 in southern Colorado and in interpreting the meaning of the dicotyled- 



