On the Silurian System of the Lake Superior Region. 189 
a part of the “cliff limestone” of the Ohio and Indiana reports, 
and is called by Dr. Owen, in his report on the Lead region, the 
“upper magnesian limestone.” From its position, as well as its 
lithological characters, it appears that this limestone, which is the 
principal lead-bearing rock in these states, is a continuation of 
that noticed on the Escanaba, lying above the fossiliferous beds 
of the Trenton limestone; but that it has increased in thickness, 
as traced westwardly, and becomes an important member of the 
series; and hence, we have designated it in the classification of 
the rocks, as the “Galena limestoue.” ¢ 
_ In the neighborhood of Galena, Dubuque, Mineral Point, and 
other places, there are numerous localities where a direct succes- 
sion in the beds may be traced. It is verv evident that this lime- 
stone diminishes in thickness eastwardly from these points, and 
becomes a very subordinate member of the series, losing, at the 
same time, its metalliferous character. From the general absence 
of fossils, and from its resemblance to the next succeeding lime- 
stone in lithological character, no distiuction has usnaily been 
made between them. In the localities cited, particularly in the 
neighborhood of Dubuque, the higher grounds are occupied by a 
limestone containing an aburdance of Catenipora, Heliolites, 
and other corals marking it as of the age of the Niagara. From 
the relative position of these ¢oral-bearing rocks to the lead-bear- 
ing strata, it has been inferred that they were: but parts of the 
same group, and they have heretofore been described as such, 
This was the state of our knowledge, when I examined this 
Series in 1841, and having satisfied myself that the coral-bearing 
limestone of Wisconsin and Iowa could be clearly identified with 
the Niagara group of New York, I expressed the opinion that the 
lower part of the cliff limestone was of the same age. Up to 
the present time (1850) 1 am not aware of any published evi- 
ence from an examination of the rock in place, to prove that the 
lead-bearing rock is of lower Silurian age.* The principal fossil 
resembles a Coscinopora; but is probably a Receptaculites. 
have found, however, in the same rock, the head of an Illenus, 
a Leptena, not unlike ZL. alternata, Spirifer lyn, and Airypa 
increbescens. ‘The still higher, thin-bedded, argillaceous lime- 
Stone contains a species of Lingula, undistinguishable from: L. 
subquadrata, Spirifer lynx, Pleurotomaria lenticularis, Murchi- 
* Mr. Conrad, in the proceedings of the Academy of Natural Sciences of Phila- 
: ne was of the age 
ined Mineral Point. These fossils are 
ftom the Trenton limestone proper, but no roductive veins are known to occur in 
roc: as I can learn i ; 4 
marked that the veins die out on reaching the “blue limestone.” This blue lime- 
of eg + apace: is no other than the Teen es large bs reciyel 
Je tossils peculiar to that pot, apt underlying vised 4 bead and 
