CANANDAIGUA AND NAPLES QUADRANGLES T 



dark when fresh, but becoming light ashen gray on exposure. 

 These beds are both underlain and overlain by thin light gray 

 magnesian limestones or platten dolomites. Entire thickness 50 

 feet. 



On account of the meagerness of the exposures throughout the 

 northern area of the Canandaigua quadrangle the exact position 

 of the contact line between the red or Vernon shales and the 

 gypseous Camillus shales, which is the equivalent of the rock beds 

 of western New York, is not apparent. The lowest rock exposures 

 are along Mud creek below Brownsville and in the bed of Ganar- 

 g'ua creek just to the north of the north line of the sheet. Here are 

 two outcrops, one just above and the other about 25 rods below 

 the bridge, which show a few feet of very fine hard dark bluish 

 drab limestones characterized by needle cavities or styliolites, 

 which mark the magnesian limestones of the gypseous deposits of 

 the Salina group elsewhere. These layers are easily broken into 

 small and regularly shaped blocks. Between the dolomites are 

 thin layers of bluish clay shales. In the Goose Egg, an oval hill 

 1 mile south of Brownsville on the west side of Ganargua creek 

 there occurs the most northerly outcrop of the upper gypsum or 

 plaster bed. The exposure is a small and isolated one and is 

 obscured by drift and disintegrated shale. Gypsum was formerly 

 quarried here. One mile farther south the gypsum outcrops at 

 the foot of the declivity on the west side of the Ganargua creek 

 channel and " land plaster " has been quarried here for many 

 years and ground in Conover's mill near by. In consequence of the 

 expense attending the stripping of the heavy covering of drift, 

 30 to 40 feet thick, the small amount of plaster produced in 

 recent years has been mined, access to the bed being had through 

 a horizontal tunnel at the base of the hill. The breast of the mine 

 is 14 feet high. The gypsum is purest at the bottom. This bed 

 is a continuation of the one from which "Onondaga land plaster '* 

 is obtained in Onondaga county ; " Cayuga ])laster '' in the 

 vicinity of Union Springs, Cayuga co., and " Vienna plaster '* 

 along the Canandaigua outlet in the western part of the town of 

 Phelps. It is 30 to 40 feet thick in this region and is composed 



