GRIMSLEY. | Origin of Gypsum. aa 
Gypsum deposits are more wide-spread in nature than salt, but 
usually occur in thinner beds. 
This is the most generally accepted theory for the origin of 
gypsum, and probably applies to most of our larger gypsum 
deposits. 
DEPOSITION BY THERMAL SPRINGS. 
Gypsum is deposited by some thermal springs, as in Iceland, 
where it is formed by decomposition of volcanic tufa by acids 
dissolved in the water. The sulphurous acids become oxidized 
to sulphuric, and then convert the carbonates, especially of lime 
and magnesia, into sulphates. Through evaporation the sul- 
phate of lime is deposited, forming layers of fibrous and selenitic 
gypsum. 
DEPOSITION THROUGH VOLCANIC AGENCIES. 
Dawson,” following Lyell, explained the origin of the gypsum 
in Nova Scotia, as follows: First, there was an accumulation 
of numerous thin layers of limestone, either so rapidly or at 
so great depth that organic remains were not included in any 
but the upper layers. Second, there was an introduction of 
sulphuric acid, in solution or in vapor, which was a product of 
voleanic action. Then for a long time the acid waters acted 
upon the calcareous material without any interruption from 
mechanical detritus. The limestone and calcareous matter are 
changed to the sulphate, and gypsum of good quality accumu- 
lates in considerable thickness. 
HUNT’S CHEMICAL THEORY OF GYPSUM FORMATION. 
Hunt’s” chemical theory of the formation of gypsum is some- 
what complex, but he believed it applied to a large part of the 
gypsum deposits of marine and fresh-water origin. In his own 
words the theory is as follows: 
‘¢1, The action of solutions of bicarbonate of soda upon sea water separates, 
in the first place, the whole of the lime in the form of carbonate, and then gives 
rise to a solution of bicarbonate of magnesia, which, by evaporation, deposits hy- 
drous magnesian carbonate. 
‘¢2. The addition of solutions of bicarbonate of lime to sulphate of soda or 
sulphate of magnesia gives rise to bicarbonates of these bases, together with sul- 
35. ‘‘Origin of Gypsum Plaister Cove, Nova Scotia,’ Quar. Jour. Geol. Soc., vol. 5, p. 339, 1849. 
36. Quar. Jour. Geol. Soc., vol. 16, p. 154, 1859; Chem. and Geol. Essays, pp. 80-92, 1878. 
