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 occidentalis, Leptana alternata, L. filitexta and Atrypa 

 increbescens. The fossiliferous bands of the more northern localities contain 

 abundance of Tellinomya {Nucula) levafa (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 Slate, I have seen from seventy-five to one hundred feet iu 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 Jefierson 



