7 58 microscopic geology: — chalk; flints. 



the shells of Cytherina, a marine form of Entomostracous Crusta- 

 cean (§ 498). Different specimens of Chalk vary greatly in the 

 proportion which the distinctly Organic remains bear to the 

 amorphous particles, and which the different 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 propor- 

 tion which the siliceous remains of Diatomacea? bear in the former, 

 compared with that which is mingled in the latter with the calca- 

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

 extent 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. 144) ; and it is surmised by Prof. 

 Ehrenberg that the layers of Flint which the British Chalk con- 

 tains, have been derived by some metamorphic process from 

 similar layers of siliceous Diatomaceaa 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 forma- 

 tion ; so that little support is furnished by their phenomena to his 

 hypothesis. But whatever may have been the origin of the Sili- 

 ceous material, it may be 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 super- 

 ficial markings often highly characteristic of those organisms, but, 

 when sections of them are made sufficiently thin to be trans- 

 parent, a Spongeous texture may be most distinctly recognized in 

 their interior.* It is curious that many such sections contain 

 well-preserved specimens of Xanthidia, which are Sporangia of 

 Desmidiacecb (§ 206) covered with long spinous projections, often 

 cleft, and sometimes furnished with hooks at their extremities ; 

 and we occasionally also find upon their surface, or even imbedded 

 in their substance, Foraminiferal shells (especially Rotalice), in 

 which not only the substance of the shell has undergone silifica- 

 tion, but also that of the soft animal body, the shrunken form of 

 which may be recognized in the dark Carbonaceous hue imparted 

 to the central portion of the Silex which fills each chamber. 



590. In examining Chalk or other similar mixed aggregations, 

 whose component particles are easily separable from each other, it 

 is desirable to separate, with as little trouble as possible, the 

 larger and more definitely-Organized bodies from the minute amor- 

 phous particles ; and the mode of doing this will depend upon 

 whether we are operating upon the large or upon the small scale. 



* See Dr. Bowerbank's Memoirs in the "Transact, of the Geolog. 

 Society," 1840, and in the " Ann. of Nat. Hist.," 1st Ser., Vols, vii., x. 



