INTRODUCTION. 
19 
are outliers of the Niagara limestone resting upon the shales of the Hudson- 
river group, which, in their denudation, have left the limestones above in 
columnar and castellated masses. 
On the west of the Mississippi river, the group continues to diminish; 
and towards the northern outcrop, I have obtained no sections beyond 
twenty-five or thirty feet in thickness. 
On descending the river, the shale augments, and the calcareous bands 
become characteristic. At the same time this change is accompanied by 
the presence of Orthis occidentalism Leptoena alternata, L. fditexta and Atrypa 
increbescens. The fossiliferous bands of the more northern localities contain 
abundance of Tcllinomya ( Nucula ) levata (which is often collected in great 
numbers from the soil along the outcrop), Lingula quadrata, a small species 
of Orthis , and sometimes many Orthoceratites. These fossiliferous bands are 
usually near the base of the formation. 
In Missouri, this group is estimated by the State Geologist at one 
hundred and twenty feet; and in some localities visited by myself in 
that State, I have seen from seventy-five to one hundred feet in thickness. 
Its most extreme attenuation appears to be in the northwesterly direction, 
where in some parts of Iowa it is less than fifty feet thick, and probably 
dies out entirely within the limits of that State. 
This group offers a very interesting exhibition of the phases presented 
by a sedimentary deposit, when traced over a wide extent of country. In 
the eastern townships of Canada, this group, including its sandstones and 
conglomerates, and the Sparry limestone of Eaton, is six or seven thousand 
feet in thickness. Following it to the southward, it gradually diminishes, 
but, according to the Geological Report of Pennsylvania, is still six 
thousand feet thick in that State. 
Upon the borders of Massachusetts, it would appear to constitute from 
two to three thousand feet of the elevation of some of the mountains, as 
Saddle mountain and others; while in New-York, its greatest thickness 
where undisturbed is probably not more than fifteen hundred feet, and 
where interrupted by the valley of Lake Ontario in Oswego and Jefferson 
