4o8 STORY B. LADD 



on drying becomes even whiter on the surface. It is easily cut 

 and turned and under proper treatment takes a fine poHsh and 

 satin hister. 



Gypsum deposits have been formed by direct deposition, 

 and by the alteration of existing lime deposits. Most of the 

 gypsum deposits of the world are considered to have been 

 formed by the evaporation and concentration of sea water, 

 although the calcium sulphate is not deposited until about 80 

 per cent of the water has been evaporated. 



When such a body of water cut off from the ocean is evap- 

 orated, the calcium sulphate is deposited before the sodium 

 chloride, the latter being thrown down only after the removal 

 of 93 per cent of the water. With complete evaporation and 

 deposition there would be, therefore, first a deposit of g^^Dsum 

 and then a heavy deposit of salt, though the evaporation may 

 go far enough to deposit the gypsum, but not far enough to 

 deposit the salt ; or if the latter be deposited, it may be removed 

 subsequently by solution. Gypsum deposits, therefore, are 

 more widely spread than salt, but usually occur in thinner beds. 



When sulphuric acid, liberated by the decomposition of 

 pyrite, acts on calcium carbonate and converts it into calcium 

 sulphate, there is generally a gradual transition from the lime. 



Gypsum is deposited by some thermal springs, as in Ice- 

 land, where the sulphurous acids on becoming oxidized change 

 to sulphuric acid, which converts the calcium carbonate into 

 calcium sulphate, and this when evaporation takes place is 

 deposited in fibrous and crystalline forms. 



The theory of Dawson on the origin of gypsum at Plaister 

 Cove, Nova Scotia, is as follows: First, there was an accu- 

 mulation of numerous thin layers of limestone, either so rapidly 

 or at such great depths that organic remains were not included 

 in any but the upper layers. Second, there was an intro- 

 duction of sulphuric acid, in solution or in vapor, which was a 

 product of volcanic action. Then for a long time the acid 

 waters acted upon the calcareous material without interruption 

 from mechanical detritus, changing the calcium carbonate to 

 calcium sulphate, and gypsum of good quality accumulated 

 in considerable thickness. 



Gypsum is widel}^ distributed geologicall}', being found 



