Prof. Hohert C. Wallace — Gypsuin and Anhydrite. 275 



so that the organisms whicli entered these inimical waters might be 

 able to effect their escape. In the district in question sucli a barrier 

 would, in all probability, lie far towards the north. In the limestones 

 of Devonian age the fauna is distinctly European in type, and seems 

 not to have commingled with that of the New York and Michigan 

 Devonian seas till late Devonian times. These Devonian limestones 

 in the Mackenzie River basin overlap the Silurian beds on the edge 

 of the Archaean shield, and the character of the underlying Silurian 

 in that district has not been ascertained. At a single exposure, 

 however, on the Bear River, gypsum is found interstratified with 

 dolomites.^ 



To return to the main problem before us, the primary or the 

 secondarj' character of anliydrite. It is very evident that trans- 

 formations take place with extreme slowness. In the district to 

 which particular attention has been devoted in this paper, the 

 Cretaceous shales were elevated, and in all probability eroded, in 

 early Tertiary times. Since that time anhydrite has been the unstable 

 modification. Notwithstanding this, solid anhydrite may be found, 

 as already indicated, to a depth of over 90 feet, with no gypsum on 

 top. It is true that glacial erosion has taken place since that time, 

 but boulders of compact anhydrite which lie on the surface fully 

 exposed to atmospheric influence are surrounded by only a tliin film of 

 gypsum. Geological evidence points unmistakably to a very prolonged 

 lag in the transformation anhydrite -> gypsum, and it is not at all 

 likely that at temperatures not much above 30° C. the change 

 gypsum -> anhydrite goes on more rapidly. Indeed, the burying of 

 gypsum under considerable thicknesses of subsequently formed 

 sediment tends to maintain the gypsum as such, for there is an 

 increase of volume in the transformation gypsum -> anhydrite -f water, 

 and although the liquid phase will at once seek regions of lower 

 pressure, it is the volume relationships at the moment of trans- 

 formation that regulates transformation conditions. 



In the opinion of the writer, the anhydrite in the Upper Silurian 

 of Manitoba is primary, and the order gypsum-anhydrite-gypsum 

 represents the niineralogical sequence of deposition in Upper Silurian 

 times. The presence of gypsum underneath the anhydrite is rather 

 conclusive evidence against the secondary character of the anhydrite 

 zone. With regard to the upper gypsum, there has unquestionably 

 been a certain amount of transformation into gypsum of the top beds 

 of the anhydrite since Tertiary times. Analyses of the upper 

 anhydrite beds show a continuously increasing water value from the 

 middle of each bed to the margin, an evidence that a gradual change 

 is taking place. Besides, the folded structure of the gypsum beds is 

 most naturally explained as a ' Gekrose ' ^ phenomenon, due to an 

 internal increase of volume in the solid phase, consequent on 

 transformation. It is believed, however, that only the lowest beds of 

 the upper gypsum have so originated, and that the internal pressure 

 has been transmitted to the overlying gypsum, Avhich had been 

 precipitated as such at a late stage of the evaporation. 



I J. M. Bell, Geological Survey of Canada, 1899, xii, 25 C. 

 " Koken, Zentralb. f. Min., u.s.w., 1902, 3. 



