12 ROYAL SOCIETY OF CANADA 



the alteration under consideration may take place is comparatively low; 

 it is lowered by pressure, and possibly in the lengthy processes of nature 

 no change may be needed at all, as is known to be the case for ferric, 

 aluminic, silicic and other hydrates. 



As to the reconversion of anhydrite into gypsum, by surface action, 

 the quarries at Hillsborough, in common with those of Windsor, Nova 

 Scotia, show a general tendency towards tiie occurrence of anhydrite 

 rather than gypsum with increace of depth, and Prof. Crosby has 

 informed me that at the locality last named every stage of the transition 

 may be traced. The only direct indications, however, of such a change, 

 as seen by the writer in the Hillsborough quarries, are to be found 

 in the concretions alluded to in the description of the varying aspects 

 of the plaster rocks, and of which the interior is invariably anhydrite 

 while the exterior is soft jDlaster or gypsum. Otherwise the two min- 

 erals are in this locality mingled in such a way as to' defy anticipation, 

 though even tliis may be favourable rather than the reverse to the view 

 that the one has been derived from the other. The matter is one of 

 great practical importance, as the recognition of any principle govern- 

 ing the occurrence of the hard and useless anhydrite as compared with 

 the merchantable gypsum would mean a very large saving in the cost 

 of production. 



The occurrence of albertite veins completely enclosed in pure 

 white and massive alabaster is an interesting feature of the deposits 

 at Hillsborough, and has been the subject of discussion by the vrriter 

 in Vol VII (second series) of the Transactions of this Society. As 

 the same mineral penetrates the overlying beds of the Millstone grit, 

 it could hardly have been contemporary with the enclosing gypsum, and 

 its presence in the latter, wholly disconnected with any outside source 

 of similar material, is not easy to understand. 



Another noticeable feature of the Hillsborough deposits is that of 

 the comparative paucity of selenite crystals. These are, indeed, numer- 

 ous in some parts of the quarries, but the}^ are quite local in their 

 distribution and of small dimensions, the greater part of the rock being 

 ■a cryptocrystalline or amorphous alabaster. Where most abundant they 

 also exhibit a decided tendency towards aggregation along lines of bed- 

 ding or lamination, which would seem to' indicate either that they 

 represent surfaces of original deposit, affording ample room for coarse 

 crystallization, or that they have been subsequently formed through the 

 agency of solutions traversing the rock along the lines referred to. 

 The selenite crystals are also usually much darker than the enclosing 

 alabaster, being sometimes almost black, a result probably due to dis- 

 seminated carbon. Groups of selenite crystals have been found cemented 



