138 , NEW YORK STATE MUSEUM 



trust. After three years the company was dissolved, and the 

 property went into other hands. 



The oldest quarry is that of Mr Severance, which has been 

 worked since 1812. The bed of gypsum in this quarry is about 

 60 feet thick and consists of eight layers varying in thickness 

 from 18 inches to 30 feet. It is covered with about 40 feet of 

 shaly rock, together with hydraulic limestone. 



The plaster rock varies in color from light drab in the cap to a 

 dark brown (iron layer), all forms becoming lighter on exposure 

 to air. The so called " slate " consists of fibrous, scaly, and 

 other forms of gypsum; the remaining beds consist almost en- 

 tirely of compact gypsum. 



The specific gravity varies from 2.68 in the 9 foot layer to 2.33 

 in the slaty layer. 



The composition is nearly uniform, consisting of gypsum from 

 80^ to 90^; calcium carbonate, a trace; magnesium carbonate, 

 5^ or less; clay, 10^ or less. The amount of carbonates is great- 

 est in the iron layer, showing effervescence in the mass. It is 

 nearly as abundant in the cap layer; it is least in the crystalline 

 layer (the so called slate) ; but in none of the layers is the 

 quantity sufficient to prove detrimental. To quarry this gyp- 

 sum, the slate rock above must be stripped. This stripping is 

 accomplished after a portion of the quarry has been worked 

 out, by blasting out a layer of shale about 4 feet in thickness 

 immediately above the plaster rock. When sufficient has been 

 blasted, the overlying slate falls down into the hole from 

 which the gypsum has been removed, and the new part exposed 

 can be quarried without removing very much of the strippings. 

 Mr Severance claims that in this way he has to remove only 

 about one third of the stripped rock. 



When quarrying the gypsum, three beds are recognized by the 

 quarrymen. The upper bed is the lightest of the three and is 

 about 30 feet thick. The lower beds are of darker color, show- 

 ing more marked strata of light and dark gypsum, and are said 

 to be richer in sulfate of lime. 



