504 BULLETIN: MUSEUM OI COMPARATIVE ZOdLOGT. 



some particular epochal subdivision of 1 1 * * - Cincinnati group. This 

 non-committal usage of the term has continued to th<- pre enl time, 

 bo that now as formerly, the Maquoketa comprises those late Ordovi- 

 cian sediments thai occur an Iowa, Wisconsin, Minnesota, and Illinois, 

 underlain bj the massive Galena and overlain by the equally ma 

 dolomite of the Niagara. 



The exposures of this formation extend over a long narrow aria in 

 the states mentioned, usually bordering the Mississippi River. It 

 follows the same northwest, southeast strike as the Galena and 

 Niagara. The most southerly exposure is in Jackson County, Iowa. 

 South from there, it disappears and the Galena and Niagara meet. 

 To the north the Maquoketa thickens and attains a maximum of 

 200 ft. (Calvin). In northeastern Iowa it again decreases and slowly 

 thins out in southern Minnesota, where in some places it is discernible 

 with a thickness of hut a few inches. 



The great difficulty encountered in attempting to correlate the 

 Maquoketa shale is due to the fact that the formation has not been 

 correlated either lithologically or palaeontologically within itself. 

 Equivalent strata of different but not very widely separated localities 

 often show entirely dissimilar lithologic and fauna! characters. For 

 example, the heavy dolomitic beds at Clermont have no vestige of 

 similarity to the strata of the typical region at Graf, Iowa. The 

 Ctenodonta and Orthoceras beds of Dubuque County are not present 

 in Fayette or Winneshiek Counties. In the light of this fact it can he 

 appreciated why investigators have had difficulty in correlating these 

 beds with any part of supposed equivalents in Ohio or New York. 



Origin: — The heterogeneity of the Maquoketa shale has led writers 

 to infer a shallow water or shore origin. The great localization of 

 faunas and the local lithologic variation is the basis of this decision. 

 In many places the muddy facies of shore deposition is _ evident. 

 Shrinkage cracks on some of the layers, the broken condition of most of 

 the fossils and a certain irregularity in bedding in Winneshiek County, 

 Iowa, led members of the state Survey to call this an old beach de- 

 posit (1905). The Maquoketa appears to have been laid down in 

 comparatively shallow water, just how deep it is impossible to say. 

 The land-locked Galena sea with its arid shores and salty waters gave 

 way to a body of water freely communicating with the open ocean. 

 Mechanical sediments, indicative of general rainfall, were laid down 

 in water of normal density. The lithologic and faunal differences 

 were no greater than those to be seen in the sediments and life of 

 shallow seas todav. 





