i>o4 



Repojbt of the State Geologist. 



The third division of the Onondaga group, designated by Vanuxern 

 the "Gypseous deposit," is locally known under the name of the "Gypseous 

 shales.' 1 It is the surface rock over a belt having an average width of two and 

 one-half to three miles, parallel with the foot of the " Helderberg escarpment," 

 with long extensions toward the south where the Limestone, Butternut, 

 Onondaga, Marcellus and Skaneateles creeks have broken through and worn 

 away the' heavy limestones of the Corniferous and Low er Helderberg forma- 

 tions and uncovered the gypsum beds. 



The contact line with the second deposit, that is, with the horizon of the 

 salt beds along the north side of this belt, lies mostly in the low alluvial 

 plains previously mentioned, and not far from the course of the Erie 

 canal. 



The contact line with the hydraulic limestones of the Lower Helderberg 

 group in the south," is usually found at varying heights in the face of the 

 Helderberg escarpment. The territory embraced between these lines is much 

 of it broken and hilly, and the rocks are abundantly exposed throughout 

 nearly its whole extent, though the exposures are not usually continuous for 

 a very long distance. 



The "Gypseous deposit 11 is composed in part of beds of tine grained 

 magnesian or dolomitic limestones, generally in thin layers, sometimes so 

 finely laminated as to become a slaty, calcareous shale. It also contains thick 

 masses of gypsum and soft gypsiferous shale in two courses, separated by a 

 bed of limestone forty or fifty feet thick. In the thicker and more compact 

 layers of limestone, freshly broken blocks show the rock to be very dark, 

 almost black, in the interior; but after exposure the color changes to an ashen 

 grey or medium dark drab, sometimes showing a slight pink shade. It is very 

 like hydraulic limestone in appearance, but the proportion of clayey admixture 

 is so large as to injure or destroy its cohesive qualities. In the middle and 

 Lower beds it is frequently more or less porous or cellular. The cavities are 

 sometimes an inch or more in diameter, very irregular and ragged in shape, 

 and lined with a fine brown dust. When of this character, they are very 

 unevenly distributed through the rock and most frequently are found on the 

 surface of a layer, or opening into a joint. 



In other layers, the cavities are found to be much more numerous, occu- 

 pying in the aggregate nearly half the space of the rock, and having the form 

 of circular cells, w ith the diameter ranging from one-fourth of an inch down 

 to a needle point. Usually the cells in a particular layer of limestone have a 

 considerable degree of uniformity in shape and size, but occasionally the con- 



