170 GEOLOGY OF THE THIRD DISTRICT. 



Preliminary Observations upon the Upper Part of the Erie Division op the 



New-York System. 



The Erie division embraces the rocks above those of the Helderberg division, extending to 

 the Catskill group. It presents through several counties on both sides of the boundary of the 

 third and fourth districts, two well defined parts, separated by the Tully limestone and the 

 Genesee slate ; these latter are boundary masses of the two parts, being comparatively very 

 thin, and of no great extent of range. The lower part of the division consists of the Marcellus 

 shales and the Hamilton group ; and the upper portion contains the different sandstones and 

 shales below the rocks at Ithaca, subsequently to be mentioned, and the Ithaca and Chemung 

 groups. 



The distribution of the rocks of the upper part of the Erie division under the heads of 

 Sherburne flags, Ithaca group, and Chemung group, was founded upon observations made with 

 Mr. Hall, commencing along Cayuga lake, going south from Ludlov>rville by Ilhaca, and from 

 thence to the Pennsylvania line. The rocks, therefore, along that section, especially the upper 

 ones, are the standard of reference, or type for those of their name. 



The Genesee slate, which exhibits itself so prominently along the lake shore by its uniform 

 black color and fine grain, and possesses all the characters of an ancient mud deposit of con- 

 siderable tenuity, is succeeded by a series of layers of sandstone and shale, usually coarse ; 

 the sandstone layers being of unequal thickness, and some with smooth even surfaces, sepa- 

 rated by more or less shale ; the whole having the usual southwest dip, and disappearing near 

 the head of the lake, under the succeeding or Ithaca group. These layers of sandstone and 

 shale, being similar in mineral character to those near Sherburne in Chenango county, ex- 

 amined the first year of the survey ; the same peculiar fucoid being found in both ; and both 

 holding the same relative position to the Hamilton group, though the intermediate rocks were 

 not seen near Sherburne, were considered to be, as they are, the same mass, and the same 

 name was consequently applied to both. Fevv or no fossils were observed in it at either locahty, 

 except the fucoids before mentioned ; the position which it held, the absence of fossils, and 

 the thickness of the mass not being less than one hundred and fifty feet, were the motives 

 which led to its separation, or rather to consider it apart from the overlying rocks at Ithaca. 



In such investigations, the mind is placed between two antagonist universal powers : a 

 generalization or synthesis, the extreme of which destroys all individuality ; and an indivi- 

 dualization or analysis, whose extreme equally annihilates all gcnerahty : and consequently 

 science cannot exist where either power is in excess, but arises from their mutual equipoise, 

 being the middle term, the harmonizing principle, and performing the same useful office in 

 what are termed the higher mental operations, that common sense discharges in ordinary life. 



In commencing investigations of any kind, where the objects are numerous, lines must 

 occasionally be drawn ; they are to the mind, like the steps or rounds of a ladder : we have 

 then a place to stand and look aroimd, and to ascertain fully our position. From a motive of 



