Mesozoic and Ccsnozoic Geology and Paleontology. 89 



In 1872, Prof. E. W. Hilgard* showed that the Cretaceous of Alabama 

 and Mississippi lias a dip sensibly at right angles to the trend (i.e., 

 between W. and S.) at the rate of 20 to 25 feet per mile. That the 

 lower division, called the Coffee Group, or the Eutaw Group is from 

 300 to 400 feet thick, and consists of noncalcareous sands, and blue or 

 reddish laminated clays, with occasional beds of lignite, and rarely 

 marine fossils, silicified, as at Finch's Ferry in Alabama. This group 

 corresponds with Hayden's Dakota Group, and in its upper part, as at 

 Finch's Ferry, probably with the Fort Benton Group. 



The Middle or Rotten Limestone Group is not less than 1,200 feet in 

 maximum thickness, consisting of soft, mostly somewhat clayey, whit- 

 ish, micro-crystalline limestones, and calcareous cla^rs; very uniform 

 on the whole, with the exception of the locally important feature of the 

 "Tombigbee Sand." The Cretaceous area of Arkansas, accordino- to 

 Owen's description, seems to fall within this group, as does also the 

 greater part of the Cretaceous area of middle and northern Texas. 



The Ripley Group consists of cr3'stalline, sandy limestones, alter- 

 nating with dark-colored glauconitic marls, containing finely preserved 

 fossils, and has a thickness of 300 to 350 feet. It is the equivalent of 

 the highest Cretaceous beds of New Jersey, and of the Fox Hills Group 

 of the West. The series of isolated Cretaceous outliers, which traverse 

 Louisiana, from the head of Lake Bisteueau, in a S. S. E. direction to 

 the great salt mass at Petite Ause, belong to this Group. 



Prof F. V. Hay den f said, that in Nebraska, the sandstones of the Da- 

 kota Group rest directh' upon rocks of the age of the Coal Measures. Al- 

 though they do not appear in full force until we reach a point near De 

 vSoto and beyond, yet remnants of.the sandstones make their appear- 

 ance within five or ten miles of Omaha, at any point north of the Platte 

 river. It is quite probable that they once extended all over Nebraska, 

 passing across into Iowa, and further eastward. The Coal-measure 

 limestones are thus exposed, in northeastern Nebraska, by the erosion 

 of the Cretaceous rocks. 



Near the entrance of the Big Sioux river, into the Missouri, the Dakota 

 Group disappears beneath the water-level, and is succeeded b_y a series 

 of black, plastic, laminated clays, with lighter colored arenaceous part- 

 ings and thin layers of sandstone. Near the mouth of the Vermilion 

 river the upper portion becomes more calcareous, and gradually passes 

 up into the next group, called the Fort Benton Group. It is often 

 immensely thickened, in the vicinit}- of the mountains, from the north 



* Proc. Am. Ass. Ad. Sei. 



t Hayden's U. S. Geo. Sur. of Wyoming. 



