18 
PALAEONTOLOGY OF NEW-YORK. 
three bands of the rock near its base, which are mostly made up of Telli- 
nomya, Lingula, Orthoceras and a small Orthis, with fragments of shells 
and some small concretions. The group has altogether diminished, so that 
it is scarcely one hundred feet thick ; and in many localities the cha¬ 
racteristic species of the group, Orthis, Leptcena, etc. are entirely absent, 
while in a single locality a few specimens only were found. 
The extreme attenuation of this group, and its position between the 
Niagara limestone above and the Trenton limestone below, has caused it 
to be overlooked. In Southern Wisconsin and Northern Illinois, as well 
as in some parts of Iowa, the shales of this group form the slopes of the 
mounds, such as Blue Mound, Sinsinawa Mound, Pilot Knob, etc. The 
base of the hills consists of the Trenton or Galena limestone, while the 
upper part is of the Niagara limestone, including perhaps some higher 
strata; while the slope, being superficially covered with fragments of 
limestone from above, has likewise been included in the same, and the 
whole elevation of these mounds set down as limestone*. I have learned, 
however, that in all these there is from 75 to 100 feet, and possibly in 
some instances more, to be estimated as shales of the Hudson-river pe¬ 
riod ; giving a geology to these parts of the country different from that 
heretofore known. 
The belt of outcrop of the shales, in that northwestern country, is 
alwaj^s marked by springs of water, by a greener vegetation in autumn, 
and by the presence of clay beds. These are often the locations of brick¬ 
kilns, as the decomposition of the shale produces a very adhesive clay. 
Where denuded of the limestones above, the outliers of these shales 
give the gentle eminences and graceful undulations to some parts of the 
prairie country, so well illustrated in some of the sketches accompanying 
the Reports of Dr. Owen, where the prevailing underlying rock is the 
Trenton or Galena limestone ; while the conspicuous outliers of the “Upper 
Magnesian limestone ” there represented, with abruptly sloping hills below, 
° Reports of Progress 1814 and 1848, and Final Report 1852, of Dr. D. D.Owen, upon the Chip¬ 
pewa Land District. 
