PORTAGE AND NUNDA QUADRANGLES ; 55 



the formation is displayed in a most striking manner, as a heavy 

 band of light blue gray in the middle of the wall contrasting strongly 

 with the black bands above and below it. If 



The mass of the unique " Hogback " is composed of this shale 

 capped by the black Rhinestreet shale. 



The ravine at Gibsonville affords an excellent opportunity for 

 examination of the upper beds and good exposures of the entire 

 formation may be found in the ravine i mile west of the mouth 

 of the gorge; in Buck run ravine, i mile southeast of Mount 

 Morris; along Cashaqua creek for 2 miles south of Sonyea, and on 

 the east side of the Canaseraga valley in the large ravines nearly 

 opposite Sonyea. The smaller ravines on both sides of the valley 

 present many good exposures of the upper beds and the contact with 

 the Rhinestreet shale appears at the roadside on the hill 1^4 rniles 

 southeast of Groveland station, near the east line of the Nunda 

 quadrangle. , 



Rhinestreet black shale 



/The Cashaqua beds from Schuyler county on the east to Lake 

 Erie are succeeded by a band of black shale with a few thin lighter 

 and mostly arenaceous layers intercalated at some localities 

 altogether differing materially in both structure and fauna from the 

 Gardeau group in which it was formerly included as .described in 

 the reports of the Geological Survey of the fourth district. Its 

 strong contrast with the light blue Cashaqua beds below it, and the 

 flags and sandy ferruginous shales above it, makes it a distinct and 

 noticeable feature in the stratigraphy of western New York. It 

 was referred to by Clarke in United States Geological Survey 

 bulletin 16, as the " Second Black Band " in the Portage group. In 

 New York State Museum bulletin 63, it is described as a Portage 

 unit, and on account of its constant exposure in the vineyard 

 region north of Naples known as " Rhinestreet " the name here used 

 was applied to it. 



It is 21 feet thick at Naples, but increases toward the west at an 

 average rate of about 2 feet per mile and on Lake Erie has a thick- 

 ness of 185 feet. In the Genesee river section the assigned thickness 

 is 53 feet. 



Fossils, except lignites, fish remains, conodont teeth and occasion- 

 ally a Spathiocaris and a few small lingulas are almost entirely 

 absent from these beds. The lighter interlaminated shales at the 

 top and bottom occasionally contain specimens frorn the Cashaqua 

 fauna. 



