PHYSICAL AND FAUNAL EVOLUTION 77 



Lake and other regions; and that these dunes were subsequently 

 altered, by replacement, to iron ore. 



The series of limestones overlying this basal bed, or resting 

 directly upon the Ordovicic, is for the most part richly fossiliferous. 

 Some of the beds, as the Racine and the Coral Beds, are characterized 

 by reefs of Stromatoporoids and other corals, widely distributed and 

 connected by more or less barren lime sands (calcarenytes) which 

 resulted from the erosion of the reefs. Some beds are of shallow- 

 water origin and bear the marks of periods of exposure, resulting 

 in the formation of mud cracks, etc. The fauna is more or less uni- 

 form throughout, and the series represents continuous deposition, 

 recording only minor oscillatory movements. Southward we find 

 these beds extending through northern Illinois, Indiana, and Ohio, 

 with a more or less uniform fauna, while further south, in the Cin- 

 cinnati and western Tennessee regions, part of the limestones is 

 replaced by shales and new faunal elements appear. 



The typical Niagaran fauna. — This is to be found in the strata 

 of the Wisconsin section and in their continuations in northern 

 Illinois, Indiana, Michigan, and Ohio. It is an exceedingly rich 

 fauna, and, as Weller has ably demonstrated, has many elements 

 in common with the Mid-European Siluric. The Stromatoporoids 

 abounded on the reefs of the Coral Beds and the Racine. They have 

 not been much differentiated in Wisconsin, but from other sections, 

 especially Canada and Ohio, a considerable number of genera and 

 species have been recognized. Corals abound, especially Halysites 

 and Favosites, while Bryozoa are most common in the shales of 

 New York and the southern area, Fistulypora making extensive 

 reefs in western New York. The brachiopods, except the large 

 Pentamerus, are likewise more characteristic of the shales. Crinoids, 

 Cystoids, and Trilobites appear to be most common in the lime- 

 stones of the interior. 



The Guelph fauna.— Thi?, fauna demands a special notice, 

 because it is so distinct in its eastern manifestations. The peculiar 

 aspect of the fauna is produced by the great Trimerelloid brachiopods 

 {Trimerella grandis, T. ohioensis, Monomorella prisca, etc.); the 

 peculiar corals Pycnostylus; the large pelecypod Megalomus canaden- 

 sis; the gastropods Pycnomphalus solarioides; and the genera 



