• PALa:ONTOLOQY 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, 1865. 



