BENTON FORMATION IN SOUTH DAKOTA 571 



The top of the Benton shale ends abruptly and evenly against the Niobrara 

 chalk, which is quite hard and massive below. 



Thickness. Depth. 

 Carlile : 



Bine shaly clay, with selenite crystals 32 32 



Blue shaly clay, with a zone of smaller lenticular calcareous concretions 



above and of larger ones below 8 40 



Blue shaly clay, with some concretions showing Prionoeyclus, Ostrea con- 

 gesta,- Inoceramus deformis (?), Serpula, Pinna, etcetera, grading into next.. 125 ± 165 ± 



Greenhorn : 



Blue chalk, weathering white, /. labiatus, shading into next 4 160 



Hard slaty blue limestone, sometimes hard calcareous shale or slabby 



limestone, with many J. labiatus, shading into next 12 181 



Blue chalk, with some /. labiatus, shading into next 12 193 



Graneros : 



Darker shaly clay 30-50 243 



Black carbonaceous shale, inconstant 2 245 



Sandy shale and clay, interstratified in thin layers 13 258 



Dakota (?) : 



Soft massive yellow sandstone, with many root marks above 7 265 



The irregular thickening toward the north is shown on the Big Sioux as well as 

 along the Missouri. 

 From our own observations and those of other students of this field we may 



generalize as follows : 



Feet. 



3. Carlile shales. Gray shales and shaly clays, with several zones of ferruginous and cal- • 

 careous, lenticular concretions, and occasional thin limestone layers, with Prionoey- 

 clus woolgari, a few small oysters, probably O. congesta, but rarely showing deep valve, 

 Inoceramus deformis, sometimes 15 inches across; Serpula and Pinna skeletons of 

 large saurians near the top. A quite extensive rusty irregularly bedded sandstone, 

 10 to 50 feet thick, appears in the upper portion from the vicinity of Mitchell west- 

 ward 140-175 



2. Greenhorn limestone. Thin bedded or slabby bluish chalky limestone, shading above 

 and below, through 5 to 10 feet of bluish chalk, into shale, the whole weathering white 

 or cream color, and abounding in Inoceramus labitus and the scales and teeth of 

 fishes, but small oysters rare and not showing much age. The whole formation quite 

 persistent, though apparently thinning to the north and west 30-45 



1. Graneros shales. Dark pyritiferous and carbonaceous shaly clays, often plastic, more 



sandy below, sometimes developing into rusty sandstone strata 45-100 



The limit between this and the Dakota is quite uncertain, for not only are 

 there inconstant sandstone layers, sometimes several feet in thickness in the lower 

 part of the Benton, but the upper part of the Dakota is much occupied by strata 

 of clay shale of variable thickness. 



B. The thickness of the Benton is much greater in this region than has hitherto 

 been reported. This increase along the Missouri has been made by addition of 

 the Greenhorn, which had been called Niobrara, and the overlying Carlile, and 

 on the James by the inclusion of the sandstone formerly called the Dakota. 



Hayden gave 50-100 feet as its thickness near Sioux City,* and 150-200 its 

 maximum | for the region. 



Meek with greater care says that "along the Missouri in Nebraska, below the 

 Great Bend, it probably does not attain a thickness of more than 100 feet." J 



* Final Report of Nebraska, p. 42, 1872. 



f Second Ann. Report U. S. Geol. Survey of Terr., p. 90, 1870. 



JHayden's Final Report, vol. ix, p. xxx, 1876. 



