254 PROCEEDINGS OF THE AMERICAN ACADEMY 



" These investigations show that the strata recognized in the State 

 of New York can be traced over this entire extent, and everywhere 

 recognized by their lithological and fossil characters. On St. Joseph's 

 Island, in the St. Mary's River, I have recognized the Chazy and 

 Birdseye limestones ; the Black River limestone, by its fossils, in a 

 thin band above the Birdseye ; and the Trenton limestone, preserving, 

 to a great extent, the same lithological characters, and containing the 

 same fossils, as in New York. I have traced the same strata, particu- 

 larly the Birdseye and Trenton limestones, across to the Mississippi 

 River, everywhere characterized by the same fossils. 



" The ' blue limestone ' of Cincinnati and other Western localities, 

 I have already proved to be a continuation of the Hudson River Group 

 of New York, and to be always above the Trenton limestone when 

 occurring at all. The Niagara limestone can be traced along the 

 entire distance, and across to the Mississippi River. At Milwaukie 

 and other places, it is characterized by numerous fossils identical with 

 those found in the same rock in New York. 



" After a more critical examination, I have satisfied myself that the 

 lead-bearing rock of Wisconsin and Iowa is not a part of the Niagara 

 limestone, as I had supposed, but a member of the Lower Silurian se- 

 ries, which, in the absence of the shales of the Hudson River Group, 

 succeeds the Trenton limestone proper, — as that rock is known in 

 New York and elsewhere, — and is a member of the series which I 

 had failed to recognize east of the Escanaba River. The fossils this 

 rock contains are of Lower Silurian types. The name Galena limestone 

 has been adopted for this rock. 



" I have also satisfied myself that the sandstones of the Upper 

 Mississippi are of the same age as the Potsdam sandstones, and that 

 the lower magnesian limestone of the Western geologists is identical 

 with the calciferous sandstone of New York, the next member of the 

 series above the Potsdam sandstone. The thin bed of sandstone 

 succeeding this rock cannot be identified as the Potsdam sandstone 

 by itself, but must be regarded as a repetition of the arenaceous de- 

 posits below, which likewise alternate with the calciferous sandstone 

 near its base. 



" All these investigations have proved the continuity and identity of 

 many of the most important formations, while others are wanting, and 

 thus allow two widely separated formations of the East to come in 

 contact, and apparently form one rock, at the West." 



