272 Prof. Robert C. Wallace — Gypsum and Anhydrite. 



60°-90° C. in vacuo in contact \Yith ])hosphorus pentoxide, was also 

 studied. The results may be briefly tabulated as follows' : — 



Natural anhydrite transforms into gypsum — 



[a) At 30° C. in presence of a saturated solution of sodium chloride. 

 {h) At 66° C. in presence of water. 



Soluble anhydrite transforms into gypsum — 



(a) At 65° C. in presence of a saturated solution of sodium chloride. 



(b) At 89° C. in presence of water. 



Half -hydrate transforms into gypsum — 



(a) At 11° C. in presence of a saturated solution of magnesium chloride. 

 (6) At 76° C. in presence of a saturated solution of sodium chloride. 



(c) At 101-5° C. under atmospheric pressure. 

 {d) At 107° C. in presence of water. 



If, then, at a certain stage of evaporation an inland sea is saturated 

 as regards calcium sulphate and sodium chloride, and if the tempera- 

 ture is not below 30° C, anhydrite will form. The frequent 

 association of anhydrite and salt beds in nature would consequently 

 seem to indicate, at the time of deposition, rather high temperatures 

 in the bodies of water from which these minerals were precipitated. 

 This view has usually been taken in connexion with the Stassfurt 

 Beds, and was suggested by van't Hoff as a geological deduction from 

 his experimental work. If, however, gypsum beds, intercalated with 

 beds of salt, be in course of time buried to depths of 1,500 feet or 

 more, and if the average surface temperature be taken as 15° C, the 

 gypsum is, under such conditions, above transformation temperature, 

 and will presumably go gradually over into anhydrite. Arrhenius 

 and Lachmann^ believe that in the case of the Stassfurt sulphates the 

 faunal evidence is against the probability of temperatures so high as 

 30° C. in the inland seas of Northern Germany in late Permian times, 

 and that the anhydrite of Stassfurt is a transformation product from 

 originally deposited gypsum. The reverse reaction would necessarily 

 take place, and gypsum would again be formed, in the case of the 

 anhydrite horizon reaching a higher relative level owing to the 

 denudation of the overlying beds, or in the case of the abstraction of 

 the associated salt beds by solution. 



In the course of an investigation, for the Geological Survey of 

 Canada, of the gypsum and the salt horizons in the limestones of 

 Palaeozoic age in Manitoba, the writer has had occasion to examine 

 a surface exposure of gypsum, with associated anhydrite, at 

 Gypsumville, the present terminus of the Canadian Northern Railway 

 branch line which skirts the east side of Lake Manitoba. The 

 following section was obtained from the face of the quarry that is 

 now being operated : — 



(e) Surface capping (gypsite and soil) . 1-3 feet. 



{d) Eed argillaceous gypsum . . . '29 , , 



(c) White foliated gypsum . . . 75 ,, 



(b) Bluish-grey anhydrite . . . . 25 ,, 



[a] Hard reddish gypseous rock . . 5 , , 



^ TJntersuchungen ilber die Bildungsverhaltnisse der ozeanischen Salza- 

 hlagenmgen, 1912, p. 189. 



^ GeologiscJie Eundschau, Bd. iii, Hft. iii, p. 141. 



