86 IV. N. LOGAN 



be true in the case of the other subdivisions of the Limestone 

 group, since the criteria are not so pronounced. 



In the Iowa-Nebraska area, according to descriptions given 

 of that region, the Benton limestone group seems to be entirely 

 wanting. This may be due to a misunderstanding in regard to 

 the proper position of the division line between the Benton and 

 the Niobrara. The following is a description of the Benton in 

 that region. 1 "Shales are more or less unctuous to the feel, 

 somewhat variable in color and texture, containing remains of 

 saurians and teleost fishes, the upper beds sometimes bearing 

 impressions of Inoceramns problematicus ;" while the Niobrara 

 which rests upon the above described Benton shales is described 

 as follows: "Calcareous beds consisting of chalk and thin 

 bedded limestones, containing shells of Inoceramus problematicus, 

 Ostrea congesta, and teeth of Odotus, Ptychodus, and other selachi- 

 ans ; thickness, thirty feet." 



It is not improbable that a part of these thirty feet of so- 

 called Niobrara should be assigned to the Benton, since, in 

 speaking further of the beds, the author says: "These (beds) 

 consist in part of soft chalky material and in part of fissile 

 limestone that divides under the hammer or on exposure to the 

 weather, into relatively thin laminae crowded with detached 

 valves of bwceramus problematicus." 



The above describes exactly the Inoceramus beds of the 

 Benton in the Kansan area. In that area, Inoceramus labiatus, syn. 

 problematicus, makes its appearance in the upper Bituminous 

 shale beds, and continues to increase in numbers until the Ino- 

 ceramus beds are reached, where it attains the acme of its 

 abundance. It then declines in numbers to the Blue Hill shale 

 horizon, where it disappears altogether, and if it reappears at all 

 in the Niobrara it is very rare. 



The geological range of the species is confined to the lower 

 Benton, and although its zone appears to be narrow its geologi- 

 cal distribution is exceedingly wide, as it is reported from nearly 

 all known Cretaceous areas. Mr. Gilbert 2 mentions it as the 



■Calvin, loc. cit. 2 Loc. cit. 



