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more generally shows itself, not in the shape of an efflorescence 

 on the surface like the others, but dispersed in minute granules 

 throughout the body of the marl. Many of these calcareous 

 granules are grains of dolomite, analogous in composition to the 

 magnesian variety of the limestone, which overlies the greensand ; 

 whence probably the true source of the sulphate of magnesia 

 above referred to. When the traces of shells are very numerous 

 in the bed, and their conversion into the sulphate of lime has 

 happened on the large scale, the gypsum forms a conspicuous 

 part of a soft white clayey matter derived from the shells and 

 interspersed among the green grains. The mixed mass of car- 

 bonate and sulphate of lime is then usually in a yellowish white 

 chalky condition. Sometimes we may detect the gypsum in the 

 marl in the shape of small regular crystals of transparent selenite, 

 at times so minute as only to be detected by the magnifier. 



Various fossil shells and other marine organic remains, amount- 

 ing to considerably more than one hundred species, are scattered 

 through the greensand. They do rot occur very evenly distri- 

 buted, but lie together in groups or colonies, forming layers, often a 

 few feet in thickness, which extend over moderately large spaces. 

 These collections of fossils would seem to be most abundant in 

 those parts of the stratum which consist largely of the greensand. 



The water percolating through the overlying sands, and also 

 through the pervious greensand itself, has effected, and is daily 

 effecting, important changes in the condition of the shells and 

 other fossils ; sometimes replacing their carbonate of lime with 

 oxide of iron, sometimes removing it altogether, and leaving a 

 mere mould forming either an inner or an outer cast, and some- 

 times obliterating nearly every trace of their former presence. 

 We can perhaps nowhere meet with a better exemplification of 

 the various alterations induced by the infiltration of water through 

 porous strata, than are to be witnessed in these greensands and 

 their associated deposits, where numerous substitutions of the 

 elements are continually in progress, and where every species of 

 dissolving and cementing action is hourly going forward upon 

 an extensive scale. 



The oxide of iron, the chief sources of which, as I have hinted, 

 are the sulphuret of iron in the clays of the greensand and the 

 ferruginous particles of the overlying yellow sands, is frequently 



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