Prof. Robert C. Wallace — Gypsum and Anhydrite. 273 



The immediately underlying beds are not exposed in this locality, 

 but from the evidence of a core section they appear to be a reddish 

 calcareous clay. The anhydrite forms heavy beds 2-4 feet thick, 

 vrhich show no evidence of disturbance. The overlying white 

 gypsum beds are 2-4 inches thick, and are sharply folded. At the 

 quarry there is a dip, which is only of local significance, towards the 

 north; consequently the anhydrite appears only at the south end of 

 the quarry. The line of contact between the gypsum and anhydrite 

 follows fairly closely the bedding-plane, wherever the top of the 

 anhydrite beds is exposed. A similar close association of the two 

 minerals may be observed in several of the gypsum exposures which 

 occur in the Gypsumville district, and which cover in the aggregate 

 an area of nearly 5^ square miles. In one locality, 6 miles north- 

 east of the quarry, anhydrite occurs at the surface, and is found by 

 boring to extend at least 90 feet vertically do\\nwards, entirely free 

 from gypsum ; while half a mile further east an exposure of gypsum 

 of considerable extent appears, with only occasional outcrops of 

 anhydrite. 



The horizon in which these surface exposures lie is near tlie top of 

 the Upper Silurian. The highest beds of the Silurian in Manitoba, 

 as determined by Kindle,' skirt the western side of the gypsum out- 

 crop, and consist of thin-bedded magnesiau limestones, rather poor in 

 fcjssils, but characterized by the presence of Leperditia hisiiigeri. These 

 beds overlie the gypsum. Underlying the gypsum horizon are 

 dolomites and dolomitic limestones, in which but few fossils have been 

 found. They belong to the Stonewall Series, which is probably the 

 western equivalent of the Niagara and Guelph horizons in the east. 

 The Palaeozoic beds, which form in this locality a western fringe to 

 the Canadian Shield, dip almost imperceptibly towards the south- 

 west. The topography is relatively flat, the elevations of the 

 surfaces of Silurian and Devonian formations ranging from 750 to 

 900 feet above sea-level. An escarpment of soft Cretaceous shales 

 rises rather sharply from the Upper Devonian Limestones on the 

 west side of Lake Winnipegosis and Lake Manitoba to a height of 

 2,500 feet above sea-level. The distance from the Gy])sumville quarry 

 (elevation 850 feet) to the top of the escarpment, measured in an east 

 and west line, is approximately 80 miles. Almost certainly these 

 Cretaceous shales formerly extended eastwards over the gypsum 

 exposures, which were consequently buried to a depth of 1,700 feet 

 at least. While salt has not been found in association with the 

 gypsum horizon, the presence of giauberite (Ca S 0^ . Nao S 0^^) from 

 a core obtained by drilling at the quarry shows that the concentration 

 of sodium ions was high in the solution from which the calcium 

 sulphate was precipitated; and on the west side of Lake Winnipegosis 

 numerous salt springs occur in the Upper Devonian, so that the 

 gypsum horizon, when deeply buried, was proliably saturated with 

 a brine solution. Conditions may then have been favourable for tlie 

 transformation of gypsum into anhydrite when the Upper Silurian 

 was buried beneath the Cretaceous shales. Tlie problem in this case 



' Geological Survey of Canada, Summary Eeport, 1912. 

 DECADE VI. — VOL. I. — NO. VI. 18 



