194 F. H. Knoivlton — Evolution of Geologic Climates. 



rupted by layers of other solids, wMch it seems is ordi- 

 narily not the case. The deposition of a bed of pure 

 gypsum from the evaporation of a single filling of this 

 postulated inclosed basin is understandable, but when we 

 must account for beds 30 to 60 feet thick, calling for the 

 evaporation of a body of water from 8 to 15 miles deep, 

 the proposition becomes top-hea^^. A further compli- 

 cation also arises when it is recalled that many of the 

 deposits containing gypsum are ob^dously continental 

 deposits. 



Of course deposition by evaporation from impounded 

 water is not the only way in which gypsum may be formed. 

 Thus, some deposits are now explained as deposition from 

 solution in ground water and others as deposits produced 

 by alteration, by action of sulphuric acid on calcium car- 

 bonate. Bedded limestone may be changed to bedded 

 gypsum by contact with sulphuric acid derived from 

 ground water from pyritic shales. The gypsum deposits 

 in the Silurian of New York are thus explained. 



In an abstract of a paper on ^^Some conclusions in 

 regard to the origin of g^^DSum," just published, F. A. 

 Wilder^ says : 



''While admitting that this [salt-pan] theory best explains 

 some gypsum deposits it seems probable that many important 

 bodies of gypsum owe their origin to other causes and conditions. 

 . . . Present day gypsum deposits are, for the most part, afflo- 

 rescent deposits, periodic lake deposits, spring deposits, and 

 deposits due to the alteration of carbonate to sulphate. There 

 is reason to belicA^e that many important gj^psum deposits of 

 earlier periods owe their origin to similar causes." 



Professor Coleman says that to dispute the formation 

 of gypsum beds '*by evaporation in times of dry heat'' 

 is unwarranted. As to the matter of dry heat, I have 

 been under the impression that evaporation depended on 

 the relative humidity and pressure of the atmosphere, 

 that is to say, if the air contains less moisture than it 

 is capable of holding there will be evaporation quite 

 irrespective of the temperature. 



From the above discussion of the processes by which 

 gypsum may be deposited and the difficulties that seem 

 to beset its accumulation by the evaporation of impounded 

 sea water, it appears to me that the blanket statement 



* Wilder, F. A., Geol. Soc. Am., Bull., vol. 32, p. 67, 1921. 



