26 _ Messrs. Foster and Whitney on the 
Silurian epoch, at least two lines of volcanic foci, from which 
flowed numerous streams of lava. These, mingling with the 
detrital deposits then in progress of accutnulation, formed a mass, 
whose united thickness far surpasses the height of the loftiest 
summits in this region. We refer to the trap ranges of Kewee- 
naw Point and Isle Royale, described in a previous report. 
* * * * * 
The limits of this oceanic basin are but imperfectly defined. 
It stretched east and west, from the shores of the Atlantic to the 
flanks of the Rocky mountains ; to the south, it extended beyond 
the Rio Grand, and north, to the Arctic ocean. 
Within this basin, the granite axes between Lake Superior and 
Lake Michigan, and between Lake Superior and Hudson’s Bay, 
rose above the waters of the Silurian sea. 
A large portion of North America is embraced within this 
oceanic bed, constituting its fairest and most productive tracts. 
The uplifting foree, by which this sea-bottom was converted into 
land, must have beeu gradually applied ; since the strata, for the 
most part, repose in a nearly horizontal position, and exhibit few 
marks of derangemeut. e meet with no mountain chains, and 
no transverse valleys, except such as have been excavated by 
running water. The whole region is spread out in gently undu- 
lating plains; or, if ridges exist, they are due to accumulations 
of drift, or to the greater coherence between the particles 10 
certain groups, which enabled them to resist the general de- 
nudation, which has everywhere left such incontestible evidence 
of its action. 
rom the MS. of Mr. Hall, we append some general remarks 
on the identity of the members of the Silurian groups, as developed 
in different parts of this basin. 
‘Ihe observations of the past season have served to show that 
| 
extend uninterruptedly from the more easterly points, where 
they have been investigated, through the northern peninsula of 
Michigan, as far west as the Mississippi, and even beyond. 
These observations form a connecting link between those here- 
tofore made in New York and Canada, and those made in the 
southern peninsula of Michigan, Wisconsin, lowa, Minnesota, 
and other portions of the West, and enable the geologist to form 
a correct idea of the range, extent, and fossil contents of these 
groups, as developed in the northern portion of the United 
States. We believe that these results will render some points 0 
resemblance, heretofore obscure, clear and distinct ; and remove 
any doubts that may have been entertained, as to the identity of 
certain members of the Silurian system, in their eastern and 
western prolongation. If these results are attained, we shall be 
satisfied that we have not labored in vain. 
Carbs esate SRN Ht 
