Belated Formations. 



13 



The Galena limestone in its north-eastern extension commences as a 

 comparatively thin formation and augments in volume to the west and 

 south-west. In the north and north-western exposures, it is essentially 

 a connecting link or bed of passage between the Trenton and Hudson 

 formations. Distinct, lithologically and pal?eontologically, when 

 viewed by itself, as is the Utica slate, still, on the same grounds, 

 it is united to the formations above and below. To the underlying 

 Trenton it is connected by beds of passage and the presence of fifty- 

 six species of fossils found in the Trenton, twenty-nine of which do 

 not pass upward into the Hudson River formation. In the north- 

 eastern portion it partakes of the character, in its upper beds, of the 

 succeeding beds, and thirty species continue on into the Hudson 

 River formation, while nineteen species of fossils are limited to the 

 Galena. 



The following table gives the number of species in each of the 

 two formations, viz. : Utica and Galena. 



Total number of species/ 



Number of species limited to the formation, 



Number of species limited to the formation and Trenton group, . 



Number of species limited to the formation and the Hudson River 

 formation, 



Number of species common to the Trenton, Hudson River, and U. 

 and G. formations, 



Number of species passing from the Trenton formation to Utica 

 and Galena, 



Number of species passing from the Utica or Galena to the Hud- 

 son River formation .... 



Utica. Galena. 



The table shows that a greater change took place in the fauna of 

 the Utica slate than in that of the Galena limestone ; the former 

 having fifty-four species limited to its boundaries, and thirty-six 

 derived from the Trenton ; while the latter has nineteen species 

 peculiar to it, and fifty-six passing up from the Trenton formation 

 beneath. This diversity is undoubtedly owing to the greater variation 

 in the character of the sediments of the Utica slate as compared with 

 the Galena, when the change from the Trenton limestone-forming 

 deposit occurred. One of the strongest arguments in favor of the 

 Galena being placed with the Utica slate in a distinct epoch is the 

 fact that, notwithstanding its being a limestone formation, it has a 

 fauna of twenty-two species, which do not occur in the Trenton and 

 but three pass to the Hudson River formations. If the Utica slate 



^This does not include the undefined species from the Galena, mentioned in 

 the Oeology of Wisconsin^ ii, 1877. 



