CANNONBALL MARINE MEMBER OF LANCE FORMATION 543 



above the Laramie. Numerous localities in Wyoming, Montana, 

 and North and South Dakota are described by Knowlton where 

 this unconformity is supposed to be shown, (i) by erosion channels 

 in the Fox Hills sandstone; (2) by discordance of dips between 

 the two formations; or (3) by the great variation in thickness of 

 the Fox Hills sandstone. In each case, however, the facts can be 

 explained on a theory of practically continuous and unequal 

 deposition.^ Furthermore, the presence in the Dakotas of a 

 marine fauna very similar to that of the Fox Hills sandstone, over- 

 lying the fresh-water sediments of the Lance, renders less tenable 

 the theory of an unconformity of any importance at the base of 

 the Lance, since the open sea must have persisted throughout 

 Lance time in a region not very remote from western North Dakota. 

 Two other explanations have been suggested, the first that the 

 fresh-water Lance strata of the Dakotas are the equivalents of the 

 Laramie, Arapahoe, and Denver formations of the Denver Basin 

 and of both the "Upper Laramie" and "Lower Laramie" of Carbon 

 County, Wyoming. In this case the unconformity at the base 

 of the Arapahoe is not of widespread general importance. The 

 other and, to the mind of the writers, the more plausible hypothesis 

 is that the Fox Hills sandstone of the Denver Basin is not the time 

 equivalent of that in the type locality in South Dakota but is 

 considerably older. The Fox Hills sandstone is,, under this sup- 

 position, merely a near-shore phase of the sedimentation of the 

 Montana sea. This sea persisted to much later time in the Dakotas 

 than in Colorado, and the sedimentation was practically continuous 

 until the close of the Fort Union. The possibihty of finding a 

 region where sedimentation was continuous throughout Laramie, 

 Arapahoe, and Denver time was pointed out by Cross in 1909.^ 

 This deposition, it would seem, continued in the Dakotas in the 

 open sea until near the close of Laramie and probably also through- 

 out the period of erosion following the Laramie. Then the sea 



^ T. W. Stanton, "Fox Hills Sandstone and Lance Formation ('Ceratops Beds') 

 in South Dakota and Eastern Wyoming," Am. Jour. Sci., 4th ser., XXX (1910), 

 172-88. 



^Whitman Cross, "The Laramie Formation and the Shoshone Group," Wash- 

 ington Acad. Sci., Proc, XI (1909), 35. 



