ERIE DIVISION. 171 



the kind, in part, Ithaca was considered a good point for the rocks below those of its imme- 

 diate locality, now called the Portage group, and for the rocks above the Chemung group ; 

 the latter underlying the Old redsandstone or Catskill group, upon which the Coal group or 

 formation rests, in all the middle and eastern portions of its range in Pennsylvania. 



The rocks at Ithaca presented a different mineral appearance from those below, and from 

 those above them ; being darker colored, and the shaly part coarse, harsh, dull, and less dis- 

 posed to be in layers than either of them ; containing numerous fossils, which were wanting 

 in the flags, etc. below ; whilst those of the lower part of the mass at Ithaca appeared to be 

 different from those of the Chemung group, the latter also containing fossils which were not 

 found at Ithaca ; for example, the very common Avicula pectenoides, which will be figured in 

 the Report of the Fourth district, and the Avicula triradiata, figured at the head of that group 

 in this Report. 



Few places offer more powerful attractions to the naturahst, than Ithaca. The village is 

 seated at the head of a lake, which is encircled apparently on all its other sides by a wall of 

 rock, rising from the water's edge to a height of several hundred feet. The rock is favora- 

 bly exposed in Fall creek, Cascadilla, and other water courses, which flow over its sides ; and 

 also at the inclined plane south of the village, where the products of the group, both fossil 

 and mineral, can be examined to the greatest advantage. 



In commencing this report, it was the intention of the writer to unite the Sherburne and 

 the Ithaca masses, not having discovered in the district those leading characters by which they 

 could be readily distinguished. They may, however, exist there ; but finding, on the contrary, 

 that Mr. Hall was desirous to unite the Ithaca and the Chemung groups, from the little or 

 no difference which he could perceive between them in his district, and that the lower masses 

 merited a distinct name, the original arrangement was retained, and the name of Sherburne 

 was changed to Portage or Nunda group ; not having the facts required to make any other 

 disposition of them at present, as the specimens of the two districts were not arranged for 

 satisfactory examination at the time of writing this Report. It is very certain that the old 

 arrangement is inconvenient for both districts, but it is the only one which can be made pre- 

 vious to the investigation of their fossils. The question, no doubt will be settled in the Report 

 on the Palaeontology of the State. In the geological map of the State, the three groups, or 

 upper part of the Erie division, are united together by the small scale of the map, and are 

 colored of a light burnt umber. As they are thus arranged, they appear to correspond with 

 the Devonian system of Mr. Phillips, the Ordinance Geological Surveyor of England, who 

 separates the Old redsandstone or Catskill group from this system. 



There are difficulties in the beginning of most if not all subjects ; those in the three groups 

 under consideration, arise from the little difference in their mineral characters, which for dis- 

 tant points cannot be relied upon. The change too in the fossil character is not so marked 

 as to be at this time available, from the very imperfect manner in which as yet they have 

 been investigated ; more especially at certain points to be noticed, where one or more groups 

 have thinned out at the east end of the district, and disappeared, bringing those of very diffe- 



