118 
GEOLOGY OF THE FOURTH DISTRICT. 
to the salines from which nearly all the salt in the State is manufactured. It extends east 
beyond the limits of the district, and west beyond the Niagara river into Canada ; maintaining 
the same general characters, and containing beds of gypsum. Some of the shaly and porous 
limestones of the upper part of the group appear still beyond, forming the island of Mackinaw, 
if we may judge from similarity in character, though the connexion with rocks above or below 
is not there visible. Its resources, except in certain places, are at this time undeveloped; 
and it is only when its limits and extent shall become known, that its immense value will be 
appreciated. 
From the character of the group, it has been deeply excavated by the ancient denuding 
agency, and it occupies a depression throughout the greater part of the Fourth District; while 
from the accumulation of drift upon its surface, and the absence of deep streams, there are 
few points where good sections can be obtained. The absolute contact of this group with the 
one below has nowhere been observed in the district; yet from the small space intervening, 
and no rock being known in this place, there remains no doubt of the order of succession. 
In the Third District, its connexion with the Niagara group is seen in several places, both 
together being exposed in the banks of ravines. 
Localities. — By reference to the Geological Map, it will be seen that this group occupies 
the southern part of Wayne and the northern part of Ontario and Seneca counties ; the 
southern part of Monroe county extending southward for a short distance into Livingston on 
the Genesee river; the northern part of Genesee and Erie counties, and a small portion of 
the southern part of Niagara. Its greatest width in the district is at its eastern extremity, 
where the denudation of the higher rocks give this formation a greater southern extension by 
several miles than it has farther west. 
The best section of the group which I have been able to obtain, is along the country south 
from Rochester. The lower portions are developed only at a few points, but their characters 
are known from several borings which have been made to great depths for the purpose of ob¬ 
taining water. The higher portions of the group with the gypsum beds and the porous lime¬ 
stone are exposed on Allen’s creek at Garbutt’s mill. From these observations the following 
section has been constructed. 
