F. H. Knowlton — Evolution of Geologic Climates. 193 



of which 30 to 60 or more feet in thickness are not uncom- 

 mon. R. W. Stone^ of the U. S. Geological Survey has 

 recently reviewed the subject and, after pointing out that 

 calcium sulphate constitutes only about 3.6 per cent, of 

 the 3.5 per cent, of mineral salts held in solution in sea 

 water, says : 



"With these facts in mind, it is difficult to account for the great 

 thickness of some gypsum beds. The quantity of water of nor- 

 mal salinity which would have to be evaporated to make a gypsum 

 deposit 30 or 60 feet or more thick is so great that no known ocean 

 basin would hold it. From 1,000 feet of normal sea water about 

 0.7 foot of gypsum would be precipitated before the point of 

 saturation for sodium chloride would be reached ; to precipitate 

 30 feet of gypsum would require about 43,000 feet of water." 



The explanation offered then is the evaporation of sea 

 water in an inclosed basin which is shut off by a low 

 barrier from continuous access to the sea but which is 

 continually replenished by periodic incursions of the 

 sea. Can Professor Coleman visualize the physical 

 setting necessary to account for gypsum deposits in all 

 parts of the world at all geologic horizons, and so nicely 

 adjusted as to admit just the right amount of water at 

 just the right times and just the right degree of concen- 

 tration to keep the deposition a continuous and appa- 

 rently uninterrupted process! Should not the layers 

 of gypsum, if accumulated under these conditions, be of 

 fairly uniform thickness and widely extended? It is well 

 kno^vn, however, that gypsum deposits thicken and thin 

 laterally within very short distances in a very discon- 

 certing manner. Why are gypsum deposits unfossilifer- 

 ous 1 Because the water had already reached such a 

 degree of concentration as to be unfavorable to life, we 

 are told, yet is it not rather remarkable that during the 

 hundred of times the impounded water must have been 

 replenished from the adjacent sea that no marine life or 

 impurities of one kind or another found entrance 1 Stone 

 states that 80 per cent of this impounded sea water must 

 be evaporated before the deposition of gypsum begins, 

 and seemingly every fresh incursion of sea water would 

 so dilute it as automatically to stop the deposition of 

 gypsum until the earlier deposited solids had been thrown 

 down. The beds of gypsum should therefore be inter- 



^ Stone, R. W., Gypsum deposits of the United States, U. S. Geo. Survey, 

 Bull. 697, pp. 22-26, 1920. 



