632 



MICROSCOPIC GEOLOGY. 



proportion which the distinctly organic remains bear to the 

 amorphous particles, and which the diiFerent kinds of the former 

 bear to each other; and this is quite what might be anticipated, 

 when we bear in mind the predominance of one or another tribe 

 of animals or plants in the several parts of a large area. True 

 Chalk seems to differ from the Levant Mud, in the small pro- 

 portion which the siliceous remains of Diatomacese bear, in the 

 former, to that which is mingled in the latter with the calcareous 



Fio. snfi. 



Microscopic Orfjanisms in Clialk fri m IMpudoii : pnrlly seen as opaque, and partly as 

 transparent objects. 



shells of Foraminifera, &c. ; and it seems doubtful to what ex- 

 tent they were present in the seas of that epoch. Such remains 

 are found in abundance, however, forming marly strata which 

 alternate with those of a chalky nature, in the South of Europe 

 and the North of Africa (Fig. 101) ; and it is surmised by Prof. 

 Ehrenberg, that the layers oi flint which the British Chalk con- 

 tains, have been derived by some metamorphic process from 

 similar layers of siliceous Diatomaccfe which have disappeared. 

 It is now certain, however, that the deposits referred to by Prof. 

 Ehrenberg are of an age later than that of the great Chalk for- 

 mation ; so that little support is furnished by their phenomena 

 to his hypothesis. But whatever may have been the orio-in of 

 the siliceous material, it may lie stated as a fact beyond all 

 question, that nodular flints and other analogous concretions 

 (such as agates) may generally be considered as fossilized 

 Sponges or Alcyonian Zoophytes; since not only are their 

 external forms and their superficial markings often highly cha- 



