BENTON FORMATION IN SOUTH DAKOTA 573 



connecting the Inoceramus limestone with the Niobrara. This would virtually 

 follow from Hay den's interpretation, though he did not announce it. Bain dis- 

 tinctly formulated it in his Iowa report* 



3. Another mistake arose from the difference in composition of the Benton in 

 different localities. A rusty sandstone is found in the James River valley, some- 

 times 30 feet or more in thickness. It has much of the aspect of the Dakota. 

 Where first met with it was separated from the Niobrara chalk by about 50 feet of 

 clay, about the thickness of the Benton near Sioux City, according to the then 

 accepted interpretation. It was considered Dakota and was so published in Water 

 Supply and Irrigation Paper number 34, and was so printed on the geologic maps 

 of the Olivet and Mitchell folios; but further investigation showed that farther 

 north the clay nearly disappeared and the sandstone was brought up closely to 

 the Niobrara chalk and often in such a way as to suggest unconformity. Further- 

 more, distinctly marine shells like the Benton were found 250 feet or more below 

 the Niobrara chalk and 100 feet above the main artesian flow from the Dakota. 

 In short, the fossil horizon corresponds fairly well to the Greenhorn limestone. 

 Moreover, the sandstone seems to be limited on the south and probably on the 

 north, but extends far west. This appears chiefly from the distribution of soft- 

 water wells, which are known to be supplied from this horizon. That it extends 

 westward at least as far as the Black hills is attested by the copious water and its 

 increase of pressure toward the west, as in the Dakota strata lower down. 



In fact, the existence of thick masses of shale between it and the main water- 

 bearing sandstone which all recognize as Dakota, together with the inconsistency 

 of having the Benton virtually absent so near to the thick beds of it around the 

 Black hills, forms alone a strong reason for revising the earlier opinion. 



Harmonious with the modified view is the occurrence in the same region of a 

 meager water horizon between the two, which not improbably corresponds to the 

 Greenhorn horizon. That formation is known to be water bearing where it has 

 been weathered, as is attested by copious springs along the Big Sioux. Probably 

 its porosity is less where not weathered, though it may still convey a perceptible 

 amount of water. 



Its geographical Extent 



The Benton extends therefore up the Missouri as far as Yankton, and without 

 doubt lies immediately below the drift up the James with considerable width as far 

 as the north line of Davison county, up Enemy creek to a point south of Mitchell, 

 and up the Firesteel to a point north of Mount Vernon. It extends up the Ver- 

 milion between cliffs of chalkstone, all buried in till, probably to Centerville; up 

 Brule creek, exposed in patches from Richland into the southwest corner of town- 

 ship 94 north, range 50 west, and under the drift possibly to the north line of 

 Union county. Along the Big Sioux it appears here and there nearly to Canton. 



The top of the Greenhorn limestone disappears below r the Missouri at Ionia Ferry 

 landing, altitude 1,115 feet above tide, and below the Big sioux, on section 4, town- 

 ship 94 north, range 48 west, 1,150 feet above tide, and below Brule creek, section 

 7, township 92 north, range 49 west, 1,110 feet above tide. 



The Carlile shales, weathered to a waxy gritless clay, are frequently exposed 

 along smaller streams. Between the two latter streams the Niobrara is struck in 



*Iowa Geol. Survey, vol. iii, p. 103, 1893; and vol. viii, p. 329. 

 LXXIV— Bull. Geol. Soo. Am., Vol. 15, 1903 



