290 THE GEOLOGY OP MINNESOTA. 



[Trenton period. 



Trenton proper is increased to a thickness of about 160 feet, and the 

 Galena formation only can he thrown with the epoch of the Hudson River, 

 as a possible equivalent of the Utica slate.* The proper arrangement 

 of the rocks of the Trenton period in the county may be expressed thus: 



1. Maquoketa shales of Iowa, seen about 15 ft. 



2. Galena limestone, vesicular and magnesian, and Upper Tren- 



ton, about 125 ft. 



3. Shales and shaly limestone, containing Columnariaalveolata, 20-25 ft 



4. Trenton limestone, 20-25 ft. 

 Of these, the first at least belongs to the Hudson River group of 



New York. 



There seems to have been little mention made of the "green shales," as 

 such, in northeastern Iowa, in any of the geological reports of that state, 

 although Prof. James Hall says that "a large admixture of shaly matter 

 often marks the Black River limestone, which in some of its bands contains 

 Ormoceras tenuifilum, and Gonioceras anceps" and that, "instead of alterna- 

 tions of calcareous and shaly laminae at the base of the group, there are 

 beds of shale of considerable thickness without defined limestone bands."! 



Dr. John Locke, also, noted a series of strata at Prairie du Chien, in 

 1839, of which he observed a thickness of thirty feet, made up of blue fos- 

 siliferous limestone, abounding with its characteristic fossils, and having 

 the usual external characters, alternating with blue clay-marl, the layers 

 of stone being very thin and "apparently corroded," which he believed to 

 .be identical with the rocks of the Cincinnati group, or the Ohio blue lime- 

 stone.:}: These beds lie, according to Dr. Locke, immediately above the 

 "buff limestone," which is twenty feet thick, non-fossiliferous, and lies upon 

 the St. Peter sandstone. This horizon of green shales is brought out dis- 

 tinctly after it enters the state of Minnesota, by reason of the'part it bears 

 in producing the peculiar mounds of the "mound limestone." 



The transition from the St. Peter sandstone to the Trenton is quite ab- 

 rupt. There is but little commingling of qualities from the Trenton down- 

 ward into the St. Peter, although a shaly layer of about two feet separates 

 them. The limestone always projects boldly beyond the sandstone, and the 



*8ee the reports on Goodliue and Rice counties. 

 tReporton the geological survey of Iowa, Vol. I., part I., p. 55 57. 



JOwen's report of a geological exploration of part of Iowa, Wisconsin and Illinois. 1839, as published by the 0. 

 senate in 1844, p 135. 



