16 GEOLOGY OF THE THIHD DISTRICT. 



lowest member is coextensive with it. It is but a thin mass near the Hudson, exposing merely 

 its edge or nortiicrn outcrop, but thickens and spreads out in its progress west, rapidly expos- 

 ing more and more surface in its onward course from the west of Oneida ; its breadth averag- 

 ing in tlie counties of Madison, Onondaga and Cayuga, about ten miles. The Helderberg 

 division is the termination upwards of all the limestone masses of any considerable thickness 

 or purity, or which r^ge through the district. With this division a system of north and south 

 valleys and lakes commence, which are parallel to each other, and arc found every few miles 

 upon its surface, from the east to the west of the district. 



Erie Division. This commences on the top of tlie Helderberg division, and ranges con- 

 tinuously with it throughout its long course, being the mass under which that division disap- 

 pears. It has no existence whatever, north of the lower division, in the district or State, and 

 its several members appear in regular succession, in going towards the Pennsylvania line, 

 though with no regularity whatever in their line of outcrop or appearance. It covers the 

 whole surface south of the great range, or Helderberg division, with tlie exceptions of portions 

 of the counties of Otsego, Chenango, Broome and Tioga, which contains the Catskill group. 



Catskill Group. This is the terminal member of the New-York system, founded upon an 

 important geographical character. It is restricted altogether in the district to the four coun- 

 ties above named. It covers the southern part of Otsego ; the middle and eastern portion of 

 Chenango ; the east and south part of Broome, and the greater part of the area in Tioga 

 which extends from the Pennsylvania line to near the Susquehanna river. It is the uninter- 

 rupted extension west of the Catskill mountain range, which diminishes in height and thick- 

 ness entering the district. 



