INTRODUCTION. 
i 5 
coralline limestone, covering the greater part of this area, we may form 
some conception of the immense extent of that ancient ocean, whose 
quiet waters admitted of such a vast area of undisturbed coral reefs. 
Succeeding the Upper Helderberg group, we have to contemplate a 
renewal of conditions almost precisely similar to those which followed 
the epoch of the Trenton limestone. In that earlier period, the fine 
sediments of the Utica slate, and the green shales of Frankfort in 
New-York, were followed by alternations of fine and coarse material, 
constituting the Hudson-river group. At this latter epoch, succeeding 
the upper members of the Upper Helderberg group, we have a black, 
fine-grained shale, with some bands of limestone ; again followed by a 
soft bluish calcareous shale, above which are alternations of argillo- 
calcareous and shaly arenaceous bands, the mass varying in composition 
at different localities, and terminating above in more arenaceous 
deposits. These constitute the Hamilton group with Marcellus shale at 
base, the Portage group, and the Chemung group. 
In like manner we find the source and origin of these strata to corre¬ 
spond precisely to that of the Hudson-river group; and we see repeated, 
after this long interval, all the phenomena which accompanied the 
Lower Silurian formation. 
The greatest accumulations of material in the period of the Hamilton, 
Portage, and Chemung groups, lie in the direction of the Appalachian 
chain. In Gaspe, we have the authority of Sir William Logan for saying 
that there are seven thousand feet of strata, which must be referred to 
a period intermediate between the limestones and the Coal measures. 
In Pennsylvania, Prof. Rogers has computed, in the corresponding 
Formations viii and ix, eleven thousand feet of thickness*; while in 
Western New-York the Hamilton group alone is less than one thousand, 
and, including the Portage and Chemung groups, the whole together 
would scarcely exceed three thousand feet. We have, therefore, the 
clearest evidence that the strata thin out in a westerly direction. 
* In No. ix is included a part of our Catskill-mountain series, so that a portion of this thickness 
is to be taken from the estimate of the Hamilton and Chemung groups. 
