GEOLOGY OF THE THIRD DISTRICT. 
rent levels near to each other. A remarkable instance of this was given when treating of the 
Hudson river group; a fact which should be borne in mind in all investigations of fossils, 
where superposition is in question. What has caused Geology to advance with rapid strides, 
has been a knowledge of fossils; and when those of the upper part of the Erie division shall 
have been fully examined, and the kind determined which are limited to a group, and those 
which are not, then the difficulties will be at an end. 
The course which appears best, and will be followed, is to give a cursory account, under 
their respective heads, of each of these three groups, as they appear along the common 
boundary of the third and fourth districts, and of other localities when well determined ; and 
then to treat of them as they are connected together upon the map, as the upper part of the 
Erie division, as contradistinguished from the Marcellus shales and Hamilton group, which 
form the lower part of the same division. 
25. PORTAGE OR NUNDA GROUP. 
Cashaqua Shale , Gardeau and Portage Groups , and Sherburne Flagstone and Shale of the 
Reports. 
43. 
Graphic fucoid, (F. graphics.) 
This group, which embraces the above mentioned masses of the Reports, may be considered 
as the commencement of the upper part of the Erie division, considering the Tully limestone 
