lO ROYAL SOCIETY OF CANADA 



posure to the effects of a warm, and arid climate, evaporation might 

 at intervals go so far as to lead to the deposition of the contained 

 salts. The waters of these lagoons, when highly concentrated or unduly 

 heated, would not be favourable to the presence of organic life, and 

 the absence of fossils in the limestones beneath the gypsums may be 

 due to this cause, as may also the vast numbers of fossil fishes found 

 in the shales of the old Albert mines only a ^ew miles distant from 

 Hillsboro, and whose sudden destruction may have been due to some 

 such cause. Under such conditions the salts of lime, as being the 

 least soluble, would, as in the artificial evaporation of brines, be the 

 first to separate, and, as in such artificial evaporation of water containing 

 calcic sulphate this invariably separates as hydrated sulphate or gypsimi, 

 we may suppose that the same was the case with the formation of 

 rock-plaster, the primary condition of the latter being therefore that of 

 true gypsum. This view is that taken by Prof. W. 0. Crosby, of the 

 Massachusetts Institute of Technology, who has further expressed his 

 belief that, the rock having been first deposited as gypsum, the burial 

 of this beneath a sufiicient mass of superincumbent strata would deter- 

 mine the conditions of low temperature thermo-metamorphism, dehy- 

 drating the gypsum as in the production of plaster of Paris, and giving 

 origin to anhydrite. Finally, as is well known, the latter mineral, 

 being, like other anhydrides, an unstable compound, tends to be recon- 

 verted, through atmospheric agencies, to the condition of gypsum, a 

 slow process which is still in operation. Thus, according to this view, 

 gypsum, as we now have it, is a hydrated anhydrite, the latter being 

 derived from previously existing and original deposits of gypsum. 



Tn this connection it may be observed that Van Hise, in his great 

 monograph on Metamorphism (page 357), says: "The main source 

 of anhydrite is by the alteration of gypsum," and again, that " the 

 chief alteration of anhydrite is to gypsum, with an increase of volume 

 of 60 per cent," citing as an example the anhydrite deposit of Bex, 

 Switzerland, where the transformation from anhydrite to gypsum has 

 taken place completely to a depth of from 18 to 30 metres, the materials 

 below this depth being anhydrite. 



On the other hand, there are those who maintain that both gypsum 

 and anhydrite may be deposited from the same solution, the production 

 of the one or the other depending upon the conditions prevailing at 

 the time, these conditions including temperature, depth of water, degree 

 of concentration, and especially the presence of other salts. Thus, 

 Adams observes, "Anhydrite may be formed from gypsum solutions at 

 various temperatures when the solutions contain other salts in sufficient 

 quantities. For example, it has been found that in the presence of a 



