12 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [NoV. 19, 



that surrounds tliem, that at first sight they might be supposed to 

 consist of a different material altogether, — in place of being the 

 earthy gypsum to which its water has been restored, and the mole- 

 cular arrangement of which has been altered. 



This unusual instance impresses strongly upon us the mystery 

 attendant on dry crystallization. An old plastered wall is covered 

 with long spicula of nitrate of lime, or actual nitre : — pieces of 

 pyrites mixed with clay become decomposed, and long needles of 

 sulphate of iron spring out of them. In the centre of the beautiful 

 nests of laumonite are little groups of almost microscopic spicula 

 springing from a common centre, radiating outwards, and to all 

 appearance in the act of extending themselves. We have here not 

 only a rearing in the molecules of the solid mass, but we have cry- 

 stals built up, where there is no solution or menstruum around, of 

 material not soluble in atmospheric air. 



Along with these specimens of gypsum-crystals, I forward some 

 specimens in which an alteration of another description in molecular 

 arrangement is apparent. 



The whole of India is surrounded by a border of marine deposit 

 recently upheaved from the ocean ; the organic remains contained in 

 it being all recent. The arrangement of the beds is generally this : 

 1 st (lowest) . Old rock, fresh or decomposed on the outer surface ; but 

 the decomposed material, where such exists, undisturbed, as if its sub- 

 mergence had occurred suddenly but tranquilly, and it had never 

 been worked upon or abraded by the action of breakers at all. 2nd. 

 Blue clay — obviously detrital trap, — the same as forms the sludge or 

 silt of our present estuaries and bays ; this abounds with shells, and 

 contains the roots of mangroves and other littoral bushes, to all 

 appearance imbedded where they grew. 3rd. Above this clay is a 

 mass of rough gravel, obviously rolled by the surge, and which must 

 have been deposited under circumstances altogether different from 

 those in which the light soft mud, now constituting the blue clay, 

 was laid down, — turbulent and angry breakers having succeeded the 

 pre-existing state of stagnation and repose. 



The clay-beds are not unfrequently omitted, — the shell-gravel 

 reposing at once upon the rock ; and this gravel is for the most part 

 cemented together into a solid mass, which forms a very serviceable 

 building-stone, and is known by the name of "littoral concrete." 



The larger shells, though perfect externally, and having the epi- 

 dermis quite fresh, have lost the whole of the material of which they 

 were originally composed, its structure having been entirely altered. 

 The walls of the shell are changed into selenite, or into a fine, highly 

 crystalhzed carbonate of lime, — and this transformation, although so 

 entire, has in no way affected their general form, either externally or 

 internally. 



The blue clay also abounds with concretions of impure lime, 

 which forms an excellent cement, and is known by the name of 

 "kunkur," — a term derived from a Sanscrit word meaning "nodule." 

 The calcareous matter avails itself of any nucleus — such as a sci'ap 

 of wood or piece of shell — which presents itself, and forms castings 



