180 GENEKAL GEOLOGY. 



the species of which probably also occur in the Trenton lime- 

 stone. 



CINCINNATI GEOUP. 



THE MAQUOKETA SHALES. 

 Syn— Hudson River Shales of Hall. 



Area and General Characters. The surface occupied by 

 this formation is comprised within a singularly long and 

 narrow area, seldom reaching more than a mile or two in 

 width, but more than a hundred miles long within the State. 

 It lies like a narrow sinuous band upon the surface between 

 the regions occupied respectively by the Galena and Magara 

 limestones; having, like them, a northwestward and south- 

 eastward trend. Its most southerly exposure is in the bluffs 

 of the Mississippi river near Bellevue, in Jackson county, 

 and the most northerly one yet recognized is in the western 

 part of Winneshiek county. 



The whole formation is largely composed of bluish and 

 brownish shales which weather into a tenaceous clay upon 

 the surface, and the soil derived from it is usually stiff and 

 clayey. The shales are sometimes slightly arenaceous, and 

 sometimes calcareous bands compose a considerable part of 

 its bulk. The latter is the case at the typical localities on 

 the Little Maquoketa river about twelve miles westward from 

 Dubuque. These calcareous bands have really a large pro- 

 portion of magnesia in their composition, as shown in the 

 report of Prof. Emery. The shales are sometimes bitumi- 

 nous, but there is no reason to suppose that they contain 

 enough for the profitable distillation of petroleum. 



Economic Value. Its economic value, being wholly com- 

 posed of soft or fragmentary materials, is very slight, 

 principally consisting of the brick clay into which it disin- 

 tegrates upon the surface. 



Geological Age. The fossils contained in this formation, 

 together with its position in relation to the underlying and 



