OUTLINE OF ROCK FORMATIONS. 59 



layers of gray shale and thin layers of hard sandstone. Above the Portage 

 in Erie County, Pa., is a shale formation about 225 feet in thickness to 

 which White has given the name Girard shale, from Girard, Pa. It imme- 

 diately underlies the tyjjical Chemung formation and is regarded as a 

 transition from the Portage to the Chemung. 



The Chemung group of western New York is described by Hall as con- 

 sisting of a "series of thin-bedded sandstones or flagstones with intervening 

 shales, and frequently beds of impure limestone resulting from the aggre- 

 gation of organic remains." Occasionally a coarse conglomerate appears 

 as in the "rock cities" near Salamanca and Panama, ,N. Y. The name 

 of the group is taken from the Chemung River, along which this group 

 of rocks is finely displayed. The southern tier of counties in western 

 New York is largely occupied by this formation, and it outcrops for a short 

 distance soiithward in northern Pennsj^lvania. It forms the highest eleva- 

 tions in the eastern part of that region, attaining a height of about 2,600 

 feet above tide and 600 to 1,000 feet above the larger valleys which 

 traverse it. At the western limits of the State the altitude of its surface 

 has decreased to about 1,800 feet. The thickness of this group in the 

 vicinity of the Chemung River was estimated by Hall to be not less than 

 1,500 feet, but it apparently decreases westward. There is some difference 

 of opinion concerning the limits of the Chemung in northwestern Penn- 

 sylvania, it being uncertain whether it should include the Venango oil 

 sands. 



The area of outcrop of the undoubted Chemung in northwestern 

 Pennsylvania is restricted to a narrow belt lying a few miles south of Lake 

 Erie in Erie and northwestern Crawford counties and in the larger valleys 

 of Warren and McKean counties. The Venango sands also have outcrops 

 on ridges in the midst of the Chemung- area and in a narrow strip on its 

 south border. At the Pennsylvania-Ohio line the Chemung outcrop extends 

 less than 20 miles south from the shore of Lake Erie. This formation 

 constitutes the main part of the escarpment south of Lake Erie in north- 

 western Pennsylvania and northeastern Ohio. 



The shales of northeastern Ohio as mapped by Read ^ include the 

 Venango sands of the Pennsylvania survey, as well as the undoubted 

 Chemung and the transition beds between the Chemung and Portage 



1 Geology of Ohio, Vol. I, 1873, p. 483. 



