GYPSUM DEPOSITS OF NEW YORK 1 5 



attains its greatest hardness with small amounts of water, 33 per 

 cent being sufficient, of which 22 per cent remains in the hardened 

 plaster. Plaster made with a large excess of water, as much as 

 200 per cent being often used, must necessarily be porous and less 

 coherent as the crystals are not so tightly interlaced. It is also 

 more absorbent of moisture and more liable to disintegration under 

 change of temperature. 



Other uses. Plaster of paris is also used in various printing 

 processes.^ Gypstereotyping is the process for the production 

 from movable types of a solid printing plate of type metal. The 

 printing foirm to be cast is secured in a metal frame or ^' chase," 

 the type metal oiled and the space above it filled with plaster paste 

 struck off even with the upper edge of the frame. After allow- 

 ing it to set 15 minutes there remains a plaster mold into which 

 the molten type metal can be poured. 



In galvanoplastic work plaster molds saturated with stearin or 

 wax and coated with graphite are used, and in rubber stamp mak- 

 ing the rubber substance is pressed into a plaster mold and 

 vulcanized. 



GENERAL GEOLOGY 

 Occurrence of gypsum in New York State 



The workable gypsum deposits are restricted to the Salina stage 

 of the Upper Siluric or Ontaric system. The Salina includes also 

 the rock salt beds of the State and is the equivalent practically of 

 the Onondaga salt group as described in the early reports by Hall 

 and Vanuxem. According to present nomenclature it is the basal 

 subdivision of the Cayugan group, the uppermost of the three 

 groups which together constitute the Upper Siluric succession in 

 this region. 



The Salina strata occupy two main areas within the State. The 

 larger area contains the original sections which have become the 

 types for comparison, and is the more important from an economic 

 standpoint. It is represented by a belt that extends with uninter- 

 rupted continuity from Albany comity on the east through central 

 and western New York to the Niagara river and thence into the 

 Province of Ontario, Its approximate limits are shown on the 

 sketch map [pi. i]. 



The belt terminates within or near the town of Knox, Albany 

 CO. by the thinning out of the strata, which in this part consist of 

 only a few feet of shale. From Albany county westward the Sa- 



Scientific American Sup.. '1907. ^63:26033, 



