292 GENERAL GEOLOGY. 



principal exposures of these strata are at Sergeant's Bluff's, 

 seven miles below Sioux City, at Sioux City, and at intervals 

 along the bluffs of the Big Sioux river to the northwest corner 

 of the county. Near the last named point, a fine exposure of 

 them is to be seen in connection with the Inoceramus, or 

 chalky beds, of which a sketch maybe seen in the description 

 of the Geology of Woodbury county, in Part III, of this 

 report. 



Economic Value. A large part of the sandstone of this 

 formation is impure and shaly, but some of the layers are 

 firm and compact. At Sioux City, and other points, these 

 hard layers are quarried for building purposes, and furnish 

 the only material of that kind to be found in that part of the 

 State. It is, at best, suitable only for purposes of common 

 masonry. 



At the town of Woodbury, the clayey layers are pure 

 enough to use as potter's clay for the manufacture of common 

 stone pottery, an establishment for which is in operation 

 there. 



At several points in Woodbury county, a distinct carbo- 

 naceous band, sometimes approaching true coal in character, 

 is to be seen among these strata. This has been regarded by 

 many as a hopeful indication of the existence of profitable 

 beds of coal in that part of the State, but after giving the 

 subject as careful attention as possible, the conclusion is 

 arrived at, that there is no good ground for hope that coal will 

 be found in paying quantity among the strata of Cretaceous 

 age in Iowa, while the known productive coal-measure 

 strata probably thin out before reaching so far north as 

 Sioux City. 



The position of this carbonaceous band in relation to 

 the other strata, may be seen in the illustrations which 

 accompany Prof. St. John's report on Woodbury county. 

 The greater part of the material of this formation is of no 

 economic value. 



Fossils. Detached scales of a Lejndoganoid species have 

 been detected in the strata of this formation, but no other 



