THE COLORADO FORMATION 85 



tral-interior province. And although it varies much in thickness 

 its faunal and lithological characteristics are remarkably uni 

 form. 



The Limestone group. — The Limestone group is recognized in 

 all the areas except the Iowa-Nebraska and Eastern Dakota 

 areas. Subdivisions have not been designated for the group in 

 any of the areas except the Kansan. Although the subdivisions 

 are persistent the divisional lines are arbitrary for the beds grade 

 into each other. The Lincoln Marble is more easily differen- 

 tiated on both palaeontological and lithological grounds. It has 

 a remarkably interesting and unique fauna. In the shallow 

 marine waters where its beds were deposited, foraminifera, corals, 

 and oysters were associated with sharks, fish, turtles, and sau- 

 rians. The rock is composed almost wholly of foraminiferal 

 remains, in which are imbedded sharks' teeth, fish teeth, shells, 

 and other animal remains. So abundant are the teeth in certain 

 portions of the rock that it gives a reddish hue to the surface 

 on which they have been rendered visible by weathering. 



The beds are composed of thin layers of a compact close- 

 textured limestone, having an average thickness of three or four 

 inches, and susceptible of moderate polish. On account of the 

 last-named characteristic it is called, locally, marble. The lime- 

 stone layers are separated by thin beds of shale of about the 

 same thickness. The assumption that these beds are of shallow 

 water origin is based on the presence of quantities of carbon- 

 aceous matter in the limestone and the highly carbonaceous 

 character of the intercalated shale beds. Numerous fragments 

 of fossilized trees and charcoal have been discovered in the beds 

 in the Kansan area. 



The Lincoln marble is notably persistent in the Kansan area. 

 The same is true of the Black Hills area. In the latter area the 

 layers of limestone are somewhat arenaceous, but the faunal 

 characters are similar, as fish teeth and saurian bones have been 

 noticed. It is presumable that when a careful study is made of 

 the Benton stratigraphy in other parts of the province that the 

 Lincoln marble will be differentiated. Such is not so likely to 



