GEOLOGY OF THE PEXN YAN— HAMMONDSPORT QUADRANGLES 47 



Lithologically they all have the same general character and ap- 

 pearance except as to the proportion of sandy sediments to the 

 argillaceous and this is nowhere constant even in the same horizon. 

 Very little assistance can be obtained in the determination of 

 horizons from the fossils of these beds, specially in the Hatch 

 shales and flags, for this is in the region, though on its western 

 border, where extensions of the Naples fauna from the west and the 

 Ithaca fauna from the east succeed each other, or are intermingled 

 in a few thin layers. Correlation is therefore more a labor of careful 

 tracing of formations from the type localities in the Naples valley 

 than of detailed study of vertical sections. 



Undulations of the strata cause frequent changes in the dip and 

 make accurate measurements of the thickness of formations well- 

 nigh impossible. The figures given for these upper divisions are 

 therefore estimates or arbitrarily assigned. 



Hatch shale and flags j 



This formation is composed principally of shales, mostly soft and 

 light blue or olive in color, but with frequent intercalations of layers 

 of black shale from a fraction of an inch to 4 feet in thickness. Flags 

 and thin sandstones are liberally though very irregularly distributed 

 throughout the beds but appear more frequently in the middle and 

 upper parts. These sandstones vary from an inch to 2 feet thick 

 and are usually compact and hard on the lower surface but become 

 shaly on the upper side. Some of them are laminated and on ex- 

 posure separate into thin plates. Layers of this character occur 

 in the Cashaqua shale and in all the formations on these quad- 

 rangles above the Rhinestreet shale. 



The lower beds of the Hatch shale and flags are very much like 

 the Cashaqua beds in appearance but in the upper part the changes 

 from dark to light and hard to soft are more pronounced and in 

 many old exposures the frequent flags projecting beyond the softer 

 shales produce a coarsely stratified appearance. 



The proportion of sandstones is much greater at some outcrops 

 than at others in the same horizon. The formation is 300 feet thick 

 on the western border of these quadrangles and increases slowly 

 toward the southeast. It is the surface rock over a large area in 

 the Penn Yan quadrangle but appears to be somewhat thinner and 

 softer in the more northern exposures. It is well exposed in the 

 upper part of the ravines at Friend ; the Belknap gully at Guyanoga ; 

 the Wagener gully at Pulteney. The upper beds may be seen in 

 the large ravine in the western part of Hammondsport and at that 



