110 MICROSCOPICAL OBJECTS FOUND 



animals do not contain above 1 per cent of phos- 

 phate of magnesia, to about 62 of carbonate of 

 lime. I am not aware that shells and microscopic 

 organisms contain any appreciable quantity. 

 Hence it is clear, that in the case of Dolomites, 

 there must have been in operation other causes 

 than those dwelt upon in the preceding pages, 

 which have separated the magnesia from the 

 water, and precipitated it in an insoluble form. 

 The chemical agents which would accomplish 

 this, would produce the same effect on solutions 

 of lime ; and hence, as sea-water contains both, 

 and there is every reason to believe that the 

 magnesia was so thrown down, it almost renders 

 it certain that lime has, in some cases, also been 

 a chemical precipitate. 



Another question closely allied to the preceding 

 is, — From what source have the flints and cherts 

 of Chalk and limestone been derived ? It is 

 well known that almost every calcareous deposit 

 contains more or less of siliceous matter in some 

 form or other, but most frequently aggregated either 

 as nodules, or concretions, or horizontal layers. 

 Ehrenberg remarks, — " In the south of Europe, 

 the beds of marl which alternate with the Chalk, 

 consist of siliceous shells of Infusoria, and flints 



