262 THE CARBONIFEROUS SYSTEM. 



except Nova Scotia and Virginia. Different explanations may no 

 doubt apply to different countries and modes of occurrence. For 

 example, in the Upper Silurian of New York, gypsum occurs in such 

 circumstances that it has been supposed to have resulted from the 

 action of local sulphuric acid springs on limestone in situ (Dana) ; 

 while in the case of the gypsum occurring in rocks of similar age in 

 Upper Canada, Dr Hunt supposes that the mineral was deposited 

 from sea-water by its partial evaporation in lagoons, as it is now said 

 to be produced in some of the coral islands of the Pacific, — for instance, 

 Jarvis Island.* Again, there can be no doubt that detached crystals, 

 nodules, veins, and the disseminated gypsum of marls may have been 

 introduced by segregative processes, and by the percolation of 

 gypseous waters. I think it not improbable that there are in- 

 stances of all or most of these modes in the gypsiferous rocks of Nova 

 Scotia. But for the occurrence of the mineral in so thick and exten- 

 sive beds, interstratified with marl and limestone, there appears to 

 me to be but one satisfactory theory — that of the conversion of sub- 

 marine beds of calcareous matter into sulphate of lime, by free 

 sulphuric acid, poured into the sea by springs or streams issuing from 

 volcanic rocks. Modern volcanoes frequently give forth waters 

 containing sulphurous and sulphuric acids. In the volcanic region of 

 Java, for instance, there is a lake of sulphuric acid from which flows 

 a stream in which no animal can live. The water of this stream 

 being probably more dense than sea-water, will naturally flow for 

 some distance along the bottom of the sea, and if it meets with beds 

 of calcareous matter will convert them into gypsum. One of the 

 volcanoes of the Andes gives origin to a similar stream ; and the 

 volcanic mountain of Maypo, in the same range, is surrounded by 

 great masses of gypsum, probably produced by the action of sul- 

 phurous waters or vapours on the limestone of the region. We know 

 that in the Carboniferous sea of Nova Scotia there were great beds 

 of shells and corals. We also know that the volcanic action which 

 upheaved the metamorphic hills which formed the land of the period, 

 was not quite extinct when these shell-beds were growing. The 

 production of gypsum was a natural consequence of the action of 

 sulphuric acid, evolved from such volcanic regions, on the calcareous 

 beds and reefs. In accordance with this view, the gypsum is found 

 only in association with the marine limestones, though, as might have 

 been anticipated, these last sometimes occur without any gypsum. 

 In all other respects, except this conversion of part of the limestone 

 into gypsum, and some changes probably of similar origin in the 



* Hague, quoted by Dana. 



