GEOLOGY OF THE PENN YAN-HAMMONDSPORT QUADRANGLE S^^ 1 



and the lighter shales carry a few specimens of Styliolina 

 fissurella Hall ; Liorhynchus quadricostatum 

 Hall ; Pterochaenia fragilis Hall ; Probeloceras 

 lutheri Clarke ; Lingula spatulata Vanuxem ; B a c - 

 trites aciculum Hall ; Orbiculoidea lodensis 

 Hall ; Pleurotomaria r u g u 1 a t a Hall. 



Genundewa limestone 



On the next quadrangle at the west and thence continuously 

 to Lake Erie the Genesee black shale is overlain by a band of thin 

 limestone composed almost entirely of the shells of a minute 

 pteropod, Styliolina fissurella and designated the 

 Genundewa limestone from its favorable exposure at Genundewa 

 Point on Canandaigua lake. The horizon of this limestone is 

 entirely covered by drift in the western part of the Penn Yan 

 quadrangle and in the eastern part the limestones are not found 

 although there are a* few exposures of the horizon. 



There is, however, a small outcrop of this rock in a small ravine 

 on the east side of the Potter swamp, i^ miles north of the north 

 line of this quadrangle and another i mile south of Fir Tree point 

 on the west side of Seneca lake, 6 miles north of Wat kins. On 

 the west side of Seneca lake the horizon is plainly marked by a 

 band of light gray calcareous shale 6 to 8 feet thick in which large 

 flat concretions are common and near Fir Tree point form a con- 

 tinuous layer. The representative fossils of the Genundewa lime- 

 stone are abundant in the concretions and they also contain a 

 number of additional species. The latter are in strong contrast to 

 those of the preceding rocks and mark an invasion of an entirely 

 new congeries of organisms into this region. 



This gray band may also be traced along the east side of Seneca 

 lake and on both sides of Cayuga lake. It is finely displayed in 

 the west bank of Salmon creek near the forks 2 miles south of 

 Genoa. This formation is described and its interesting fauna cited 

 in full in Museum memoir 6. 



West River black shale 

 The West River shale, formerly known as the Upper Genesee 

 shale, is composed of fine bluish black or dark gray shale with a thin 

 layer of black slaty shale at intervals of a few feet. Spheric and 

 oblong concretions, many of which are septaria, are common 

 throughout the beds singly or in rows and thin sandy flags occur 

 specially in the upper part. 



