PALAEONTOLOGY OF NEW-YORK. 
which include the limestones of the Trenton period, few affinities are 
observed; until we find in the shales of the Hudson-river group some 
remarkable species which show analogies with those below. Altogether, 
however, depending upon the fauna we possess, we are ready to conclude 
that a total organic change supervened to the final deposition of these 
• lower masses. 
In Iowa and Wisconsin, the junction of the Potsdam and Calciferous 
sandstones is marked by alternating beds or bands of the two rocks; 
showing a repetition at short intervals of the previously existing con¬ 
ditions, and that the causes giving rise to the arenaceous deposit of the 
Potsdam sandstone did not cease with the commencement of the calcareous 
formation succeeding it. These alternating bands of the two rocks, though 
marked in the outcrop as very distinct, are nevertheless found on closer 
inspection to present gradations from one to the other which are nearly 
imperceptible, except where the deposition succeeding the sandstone is of 
cherty matter. This being a chemical rather than a mechanical deposit, 
presents in consequence an abrupt change in the characters of the material. 
Again, farther to the south, in the State of Missouri, we find exhibited 
results of the operation of similar causes in a far more extreme degree*. 
Here the Calciferous sandstone (or, as it is termed in the Report, the 
Magnesian limestone), instead of occurring mainly as one interrupted 
formation with some comparatively unimportant alternations at its base, 
is subdivided into four principal masses. These are each separated from 
the others by beds of saccharoidal sandstone, having a thickness of from 
50 to 100 feet; while the intervening calcareous masses have a thickness 
respectively of 190, 230, 350, and 300 feet. We have here the most satis¬ 
factory evidence that conditions producing the Potsdam sandstone recurred 
at long intervals throughout the period from the commencement to the 
final deposition of the Calciferous sandstone. 
At the close of this period, although important modifications occur in 
See the Report of Prof. Swallow upon the Geological Survey of Missouri, 1855. 
