190 On the Silurian System of the Lake Superior Region. 
sonia bellicincta, and another species with angular volutions, a 
large Orthoceras, and fragments of Idlenus. Up to this point, I 
found no corals of the Niagara period, and though the fossils are 
not numerous, they are all of lower Silurian forms, and furnish 
the best evidence we have of the age of this limestone. 
This lead-bearing rock, as before observed, rests upon fossilife- 
rous strata of the Trenton age, which can be recognized as the 
identical group traced over several hundred miles. The galena 
sometimes penetrates the Trenton series, in films or sheets, but 
does not form veins, as in the grey, heavy-bedded limestone 
above. 
From all the evidence, therefore, the lead-bearing, or Galena 
limestone, Must be regarded as a distinct member of the lower 
Silurian system, which is not recognized at the east. F'rom its 
gradual diminution in thickness, its source appears to have been 
towards the west. The conditions of the ocean, though favora- 
ble to the deposition of this immense mass of calcareous materi- 
als, were unfavorable to the development of organic life ; for, al- 
though there remain a few species which continue through the 
period of its deposition, the greater number known in the group 
below did not survive beyond the commencement of this. The 
occurrence of more highly fossiliferous strata above, still of lower 
Silurian age, would show that the Galena limestone does not form 
a series of transition beds between the upper and lower Silurian. © 
To the south and southwest, this rock is of limited extent, 
probably nearly coincident with the lead region; the universal 
testimony showing that no productive lead mines exist int 
western states, out of the range of this rock. 
This fact sets at rest all speculations as to the probable metal- 
liferous character of certain rocks which have been supposed to 
be identical with the Galena limestone. The lead-bearing rock 
is a peculiar one, holding a certain place in the series, and of lim- 
ited geographical extent. It is metalliferous throughout the 
greater part of its known limits, whereeit has considerable thick- 
ness. ‘l'he lead veins are almost wholly confined to it, and evl- 
dently have their source in it. The small quantities sometimes 
found below its base are in seams that die out, as they penetrate 
the inferior rock, and it often takes the form of interlaminations 
