NATURE OF PRESENT-DAY DEPOSITS 393 



seems that man}^ if not most, of the gypsum heds have been formed by 

 the hydration of sedimentary anhydrite." While this statement may be 

 broad, many gypsum deposits have evidently been derived from anhydrite. 

 This is probably true of Virginia gypsum and of the important Nova 

 Scotia deposits about Windsor. In the light of the latest drilling in New 

 York, Newland"^ is inclined to the opinion that the more important de- 

 posits of gypsum in that State were originally anhydrite. 



Briefly reviewing the statements that have been made, it seems that as 

 a result of Stieglitz's work we must eliminate from the list of gypsum de- 

 posits formed directly by the evaporation of sea-water or by evaporation 

 in stream-fed inland basins all deposits that contain less than .5 per cent 

 of calcium carbonate. Unless we are willing to assume an atmosphere 

 with less carbon dioxide than at present, we must eliminate all deposits 

 containing less than .9 per cent of carbonate. 



Circulating water which has taken up gypsum disseminated in small 

 quantities through beds traversed by the water is a primary factor in the 

 formation of recent gypsum deposits. Gypsum so concentrated is often 

 concentrated farther by the action of the wind and by evaporation in 

 inland lakes. 



Many gypsum deposits were originally anhydrite. The conditions 

 which determine whether calcium sulphate will be deposited as gypsum 

 or anhydrite are not fully understood, in spite of the work of van't Hoff 

 and others. The chemist is seldom familiar with the field conditions 

 which are associated with deposits of these minerals. If Udden is right 

 in believing that extensive bodies of anhydrite in Texas have been formed 

 as the result of reaction between magnesium sulphate in circulating solu- 

 tions and calcium carbonate in the original sediment, the process may 

 admit of broader application and may be applied to gypsum deposits 

 which are distinctly bedded, yet which are so nearly free from calcium 

 carbonate that they cannot be regarded as salt-pan deposits. Anhydrite 

 so formed might later be altered to gypsum. 



2s Mineral Industry, 1920, vol. 28, p. 333. 



