CHAP. X.J CONTINUITY OF THIS CHALK. 481 



of land and in the path of slight currents, whilst the 

 extreme purity of the white chalk of Sussex would 

 seem to indicate that it had been laid down in deep 

 still water far from land. A considerable proportion 

 of the silica of the chalk-mud, however, consists of 

 the spicules of sponges, of the spicules and shields 

 of radiolarians, and of the frustules of diatoms ; 

 and this organic silica is uniformly distributed 

 through the whole mass. Taken in connection with 

 the absence of diffused silica in the ^vhite chalk, 

 we have the singular fact of the presence of regular 

 layers of flinty masses of nearly pure silica, pre- 

 senting frequently the external form of more or 

 less regularly-shaped sponges, and frequently filling 

 up the cavities of sea-urchins or bivalve shells. If 

 we take the simple instance of pure grey flint filling 

 up entirely the cavity of an urchin, such as Oale- 

 rites albo-galerus, or AnancJiytes ovatus, and showing 

 at the oral opening of the shell a little projecting 

 knob, like a bullet-mould filled with lead, we have 

 no escape from the conclusion that after the death 

 of the urchin the silica has percolated into the shell 

 in solution or in a gelatinous condition, and the silica 

 must have previously existed in some other form, 

 either in the chalk or elsewhere. In the chalk which 

 contains not a trace of silica we often find the moulds 

 and outlines of organisms which we know to have been 

 silicious, from which the whole of the silica has been 

 removed ; and I have more than once seen cases in 

 which a portion of the delicate tracery of a silicious 

 sponge has been preserved entire in a flint, while the 

 remainder of the vase which projected beyond the 

 outline of the flint appeared in the chalk as a trellis- 



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