Proceedings of the American Association. 249 
the lower Silurian period, by another in the lower stage of the 
upper Silurian, and by two or more species in the higher strata of 
this period. It is however unknown in the Devonian period. 
~The exhibition of the geological range of species as shown in 
the preceding table, proves that the lower stage of this division 
of the system is susceptible of being separated from the upper, 
being almost disconnected by its fossil contents, Of the one 
hundred species in the strata below the Trenton period, only seven 
are known to ascend into that rock and the succeeding strata. 
Still there is no strong line of demarkation, and there often ap- 
pears to be a mingling of the materials of the two portions at the 
point of junction, as well as of the organic forms. . 
‘The larger number of species of the Trenton limestone period, 
which pass upwards into the succeeding rocks, notwithstanding 
their dissimilarity of composition, show a very intimate relation, 
proving that but a single period is indicated by the fauna. This 
is further proved by examinations farther westward, where the 
distinctions of Trenton limestone, Utica slate, and Hudson River 
group, are not recognizable, the entire composition being calca- 
S. 
At the termination of this period to which more particular allu- 
sion is now made, we find a strong lithological line of demarka- 
tion. 'The number of species which passes beyond this limit is 
exceedingly few, and their appearance in a subsequent period is, 
or the most part, so rare that they give no characteristic feature 
to the strata, nor are they ever likely to produce any confusion 
in their identification. We have not, as is said to be the fact in 
uurope, a mingling of the fossil contents of the upper and lower 
Silurian periods, at the junction of the strata of these two epochs. 
On the other hand, we are able to demonstrate over a considera- 
ble portion of country, an entire lithological change at this point, 
and an examination of the organic remains proves these evidences 
not less unequivocal than the others. 
IL. Prof. Gale on the Natchez Bluff Formation.—This paper 
Consists chiefly of an inquiry respecting the geological character 
of fossil human bones, announced by Dr. M. W. Dickerson as 
discovered by him in the above formation, and so reported to the 
Association at their sittings in 1846. The report above referred 
to stated that the fragment of bone was diseovered in the same 
mation, and almost in juxtaposition, with the cranium of the 
Megalonyx laqueatus of Harlan. So extraordinary and impor- 
tant a fact, if such it should prove to be, led the writer to inves- 
tigate the circumstances connected woe “serpent _ the 
report present ‘to be the result of the investigation. 
ne falireved a para the geology of the Natchez bluff 
> that the portion containing the fossils in ques- 
