26 
GEOLOGY OF THE FOURTH DISTRICT. 
CHAPTER IV. 
'Enumeration of the rocks and groups below those of the Fourth Geological District , 
in the order of succession. 
The rocks occupying the Fourth District commence at about the termination of the lower 
great division of the System ; a point above which, few or none of the previously existing 
organic forms are continued. The rocks and fossils of the lower division are described in the 
reports of the First and Third districts ; and they are merely enumerated here in the order of 
succession, for the purpose of showing their connexion with those of the Fourth district, which 
terminate above with the conglomerate of the carboniferous period. The report will thereby 
be rendered more complete and satisfactory in itself, affording means of reference to the rela¬ 
tive situation of the lower rocks, and of comparing the genera and species of their fossils 
with those above. 
The want of some previously acknowledged base line, from which to make the starting 
point, has been felt during the whole of the survey; and to supply in some degree this defi¬ 
ciency, I have examined the lower rocks upon the north side of Lake Ontario. These show 
a continuation of the lower groups as developed in the eastern part of the State, with the 
exception of that portion occupied by the basin of the lake. This, between the Fourth dis¬ 
trict and the Canada shore, has been excavated partially from four of these groups, viz. the 
Utica slate, Hudson-river group, and Grey sandstone, as well also as a large portion from the 
Medina sandstone, which forms the southern border of the lake from Oswego to its western 
extremity, and its northern margin for many miles near the western termination. 
From these facts, it will appear that a section extending across the Fourth district, and 
embracing the rocks upon the north side of the lake as far as the Primary range, will give a 
connected view of all the rock formations presented farther east. 
The following woodcut will illustrate these remarks. It shows the relative position of all 
the rocks of the Fourth district, as well as their relations to the lower ones, in a section ex¬ 
tending from the Primary of Canada, across the lake, and thence across the State of New- 
York from the mouth of Genesee river to the Pennsylvania line. 
