in 



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 arc 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 

 Uieir 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. 



Graphic fucoid, (F. gnphica.) 



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 



