NIAGARA LIMESTONE. 



377 



THE GUELPH BEDS. 



The Guelph limestone constitutes the tippermost snbdivision of the 

 Niagara Group in Wisconsin. In its lithological character, it does 

 not differ essentially from the Eacine limestone, being in general a 

 rough, thick bedded, irregular dolomite, usually quite free from im- 

 purities, and of buff, gray, or blue color. The distinction between the 

 two subdivisions is a paleontological rather than a physical one. In 

 the latter respect there is less difference between these. than either of 

 the other members of the group. There was evidently no marked 

 change in the physical history of the region, but the same conditions 

 continued from the beginning of the deposit of the Eacine limestone 

 to the close of the formation of the Guelph beds. In the interval, 

 however, the life underwent a change by the introduction of the 

 species that characterize the Guelph horizon. This introduction was 

 gradual, so that many localities show a mingling of the two faunas. 

 The right hand white line on the maps is intended to bound this 

 formation, and is drawn so as to include localities presenting these 

 mingled species. In doing so, however, it was necessary to include 

 some of the reefs and sandrock deposits whose physical history is un- 

 questionably identical with the Eacine reefs. The propriety of doing 

 tliis may be questioned. 



Local Details. The mound and quarry in Granville and tlie Guelph beds neai- 

 Cedarbnrg and Grafton have, been mentioned in the description of the Racine Hmo- 



