Belated Formations. 



7 



laceous matter. The upper portions are usually thinner bedded and 

 more argillaceous, and often pass hy a gradual accession of argil- 

 laceous m,aterial into the shaly rocks of the group above. ****** 

 In some localities there are repetitions of the lower calcareous strata 

 above the base of the Galena limestone or alternations of thin fossil- 

 iferous beds of the Trenton limestone with the dolomitic limestone of 

 the Galena period." 



In the second volume of the Geology of Wisconsin, Prof. T. 

 C. Chamberlain gives a detailed description of the Galena forma- 

 tion and a list of the species of fossils occurring in it. From this we 

 learn that towards the north-western portion of the state the forma- 

 tion begins to undergo a change. 



"The modification consists mainly in the introduction of more 

 clayey material in the form of shaly leaves and partings. This 

 changes the color from the usual buff to a greenish or bluish-gray. 

 There is also an increase of fossils. ********* Xhe change 

 in the Galena limestone is gradual and progressive for forty or fifty 

 miles, beyond which its nature as modified, becomes constant for 

 nearly one hundred miles, to the limits of the state." 



The formation has still more the character of beds of passage 

 between the Trenton and Hudson River formations in its north- 

 western extension. The following extracts are taken from Prof. 

 N. H. Winchell's report,^ as giving information of the Galena lime- 

 stone, and also an account of reference being made to the nomencla- 

 ture of the period to which it beloags: 



In New York the Trenton limestone is succeeded by a mass of 

 shales with the local designations, Utica slate, Frankfort slate, shales 

 and sandstones of Pulaski, and Lorraine shales. These were all em- 

 braced in the term Hudson River group, which had before been 

 applied to a mass of shales that are now known to be much lower. 

 On account of this error the term Cincinnati group has been generally 

 substituted." 



" On the other hand in Iowa and southern Wisconsin and Minne- 

 sota, the Trenton limestone is found to pass into the Galena by slow 

 stages and to be followed, at least in Iowa, by a greatly reduced 

 representative of the Cincinnati group, named by Dr. White the 

 Maquoketa shales. Leaving Iowa and passing into Minnesota the 

 Trenton increases in thickness, and the Galena diminishes, the latter 



^ Sixth Ann. Rep. Geol. and Nat. Hist. Survey of Minnesota, pp. 82, 83. 1878. 



* The erroneous reference of the shales of the Hudson River group in the 

 valley of the Hudson to a lower geological horizon is one that, since the correc- 

 tion of the error by Prof. James Hall and more recently by Prof. J. S. New- 

 berry, will undoubtedly be dropped as an argument in favor of retaining the 

 term Cincinnati group. The only grounds upon which it can be retained with 

 any degree of reason are those advanced by Prof. Newberry in the Geology 

 of Ohio. 



