50 Transactions of the 



while the superincumbent strata are perfectly flat. The beds imme- 

 diately above the limestone are triassic, composed of marls, some 

 of which are arenaceous, and others contain selenite and gypsum. 



The colour of the marls is due to the different states of the iron 

 which they contain ; both the blue and the red are essentially the 

 same in composition, but the blue contains protoxide of iron FeO, 

 while the red contains peroxide FejOa ; in both there are traces of 

 phosphoric acid and chlorine. It has been remarked that the 

 carbonic acid is greater in the blue marl, because the carbonic 

 acid, from decaying organic matter, has united with the pro- 

 toxide of iron to form the carbonate. 



That the selenite and gypsum should occur near together is not 

 strange, because they are simply the same mineral. They are both 

 sulphate of lime, containing parts sulphuric acid, 46'31 ; lime, 

 3290 ; water, 20"79. When gypsum is massive it is called alabas- 

 ter ; when transparent, selenite. Gypsum is unquestionably, in 

 many cases, a metamorphosed substance ; for sulphurous fumes 

 acting on beds of clay containing shells convert the lime into 

 selenite, and acting on limestone convert it into fibrous and com- 

 pact gypsum. Fibrous gypsum is found in profusion in Derbyshire, 

 and is known under the name of satinstone, and manufactured 

 into brooches and other ornaments. It is also found in veins and 

 masses in the red marls bordering the valley of the Trent. 



The stone commonly known as landscape marble or Cotham 

 stone is simply an irregular nodular formation of the lias. Cotham 

 Hill is a hummock, resembling Aust Cliff" in presenting the cap 

 of lias resting upon the base of red marl. 



The following are the measurements of these strata as given in 

 the " Memoirs of the Geological Survey of England : " — 



Ft. Inch. 



1. Gray argillaceous limestone with shale partings ; 



the bed, named Cotham Stone, among them ; 

 fossils — small Ostrea, Plagiostoma giganteum 

 and striatum, modiola, species of echinites, 

 bones, ..... 



2. Gray marly clays, .... 



3. Seam of limestone, .... 



4. Gray marls, ..... 



5. Argillaceous limestone, 

 G. Gray marls, ..... 



7. Gray marls, with three beds of nodular limestone, 



8. Gray marls, ..... 



9. Gray marls, with selenite, 

 10. Ai'gillaceous limestone, fish scales, elytra of in- 

 sects, modiola, terebratula, . . .08 



