592 HENRY SHALER WILLIAMS 



sure; this range is increased on passing eastward, and at the Ithaca 

 meridian is some 300 feet. The thickness of the Ithaca member, 

 estimating it from the upper Reticularia Igevis zone of the Sherburne 

 to the upper Ithaca Reticularia laevis zone is approximately 460 feet 

 at Ithaca. 



Lithologic definition of the Nunda formation. — Thus is established 

 a definite stratigraphic subdivision of the Devonian system of for- 

 mational rank, of which the technical name is "Nunda formation," 

 composed of the members Cashaqua, Gardeau, and Portage in its 

 typical outcrop in the Genesee valley, and of the Sherburne, Ithaca,, 

 and Enfield members in the Ithaca section. 



Lithologically it is defined as an irregular combination of fine 

 fissile shales, either light or dark in color, with, generally, thin- 

 bedded and occasionally thicker beds of sandstone. The shales, 

 in general, are more conspicuous in the lower half; the thicker 

 sandstones are more frequent in the upper part. The light-greenish- 

 colored shales seen in the western part of the state are inconspicuous 

 in the eastern sections. The tough, thin-bedded, often wave-marked 

 sandstone, called "flagstone" from its common use in paving 

 sidewalks, is more frequent in the eastern than in the western 

 outcrops. 



THE BLACK SHALE BANDS 



The fissile black shales, similar lithologically to the Genesee 

 black shales below, appear throughout the vertical thickness of the 

 formation at various horizons, sometimes of 20 to 50 feet thickness, 

 presenting locally great uniformity. These fissile black shale 

 masses rapidly change their appearance laterally by the intrusion 

 of sandstone bands interrupting the shales, and altering the aspect 

 of exposures to that of the ordinary type of shale and thin-bedded 

 sandstone so characteristic of the whole mass. A typical example 

 of this type of shale is seen at the base of the Ithaca group, near 

 the foot of the hillsides at Ithaca. Locally it seems conspicuous, 

 and I defined and indicated it as the "Ithaca Lingula shales" in 

 the Bulletin No. j of the United States Geological Survey in 1884, 



Clarke has defined other local expressions of it under the names 

 Middlesex black band, Rhinestreet black band, and Dunkirk black 



