1849.] DAWSON ON THE GYPSUM OF PLAISTER COVE. 339 



shows how little susceptible of induration by heat are the gypseous 

 rocks, and illustrates the reason of the absence of sections in the vici- 

 nity of many large masses of gypsum, in localities where the rocks in 

 general are hard and well-exposed. 



It must, I think, be evident from the facts above stated, that the 

 mass of gypsum under consideration is a regular aqueous deposit, 

 interstratified with the marls and limestone. This being admitted, 

 the only cause which appears competent to its production is that sug- 

 gested by Sir C. Lyell in his ' Travels in North America,' and subse- 

 quently illustrated by the writer, viz. the production of sulphuric acid 

 by volcanic action, and its introduction into the basins in which cal- 

 careous matter was being deposited. On this hypothesis, the history 

 of the deposit would be somewhat as follows : — 



First, the accumulation of a vast number of very thin layers of 

 limestone, either so rapidly or at so great a depth that organic remains 

 were not included in any except the latest layers. Secondly, the 

 introduction of sulphuric acid, either in aqueous solution or in the 

 form of vapour ; the acid being a product of the volcanic action whose 

 evidences remain in the neighbouring hills. At first the quantity of 

 acid was too small, or the breadth of sea through which it was dif- 

 fused too great, to prevent the deposition of much carbonate of lime 

 along with the gypsum produced ; and its introduction was accom- 

 panied by the accumulation on the sea-bottom of a greater quantity 

 of mechanical detritus than formerly : hence the first consequence of 

 the change was the deposition of gypseous marl. At this stage or- 

 ganic matter was present, either in the sea or the detritus deposited, 

 in sufficient quantity to decompose part of the sulphate of lime, and 

 produce sulphuret of iron ; and also to afford the colouring matter of 

 the nodules of black limestone found in the marl. Thirdly, the 

 prevalence for a considerable period of acid waters, combining with 

 nearly all the calcareous matter presented to them, and without 

 interruption from mechanical detritus. The anhydrite must have been 

 deposited with the common gypsum; but, under the circumstances, 

 it seems difficult to account for its production, unless it may have 

 been formed by acid vapours, and subsequently scattered over the 

 bed of the sea. Fourthly, a return to the deposition of marl, under 

 circumstances very similar to those which previously prevailed ; and 

 lastly, the restoration of the ordinary arenaceous and argillaceous 

 depositions of the carboniferous seas. 



Of the gypsum veins found in the marls, those which are white 

 and fibrous may have been nearly contemporaneous in their origin 

 with the marl itself ; those which are red and lamellar have been 

 subsequently introduced. The granular gypsum is in all cases a part 

 of the original deposit. The comparatively small quantity of red oxide 

 of iron in these marls and other associated beds is the most important 

 feature of difference between the deposit of Plaister Cove and those 

 of most other parts of this province. There is however a large quan- 

 tity of reddish and brown sandstone in the beds overlying the gypsum, 

 though on the whole these colours are less prevalent than in the car- 

 boniferous system of Nova Scotia proper. 



