§ 4. ORIGIN OF ROCK-SALT. 541 



contained a much larger proportion of saline ingredients 

 than the present, — an inference for which there are no reason- 

 able grounds whatever. If we imngine successive subsidences 

 of a given area to have taken place, the alternations of beds 

 of marls with layers of salt of variable thickness, may be 

 explained ; but the difficulties above mentioned remain in 

 full force. As the chlorides of sodium and gypsum* are 

 often sublimed from volcanic vents, an igneous origin has 

 been ascribed to many of the beds of salt and sulphate of 

 lime. Gypsum is unquestionably, in most instances, a 

 metamorphic rock ; for sulphurous fumes acting on beds 

 of clay containing shells, convert the lime into selenite, 

 and limestone into fibrous and rock gypsum. Many ter- 

 tiary gypseous deposits have evidently originated from 

 this cause: but crystals of gypsum are also abundantly 

 found in beds containing pyrites, from the decomposition 

 of the sulphuret of iron, and the formation of sulphate of 

 lime from the action of the liberated sulphuric acid on 

 the calcareous materials, f In many parts of Sicily, vapours 

 charged with sulphuric acid are constantly emanating from 

 vents or fumaroles, as they are termed, and throw down 

 large deposits of sulphur and gypsum ; and the fumes of 

 sulphur and boracic acid escape in such quantities, that 

 the peasants put pots, and often bee-hives, over the fuma- 

 roles, and thus collect abundance of sulphur and boracic 



* Gypsum, or sulphate of lime, cousists of sulphuric acid, 46*31 ; 

 lime, 32*90 ; and water, 20*79. The massive gypsum is called .4 la baster; 

 the transparent gypsum, Selenite; powdered calcined gypsum forms 

 Plaster of Paris. The fibrous gypsum has a silken lustre, and is 

 used for ear-rings, brooches, and other ornaments. Fibrous gypsum 

 of great beauty occurs in Derbyshire : veins and masses of this sub- 

 stance abound in the red marls bordering the valley of the Trent. 



•f Recent railway embankments of shelly clays often exhibit this 

 phenomenon, and are liable to give way from their consolidation being 

 thereby prevented. 



