182 GEOLOGY OF THE ISLE OF WIGHT. 



simply incrusted by silex : such is the state of 

 numerous sponges which are as it were invested 

 by the flint, and have their pores and tubes filled 

 by the same material ; the original tissue appear- 

 ing as a brown earth. In other examples the 

 sponge has been envelo|)ed by a mass of liquid 

 silex and has subsequently perished ; in this man- 

 ner have been formed those hollow nodules, which 

 on being broken present a cavity containing only 

 a little white powder, or some fragments of sili- 

 cified sponge : while in other instances the cavity 

 is lined with quartz or chalcedony. It frequently 

 happens that part of the zoophyte only is per- 

 meated by silex, while the other portion is 

 a friable calcareous substance imbedded in the 

 chalk. Certain zoophytes (Ventriculites, see 

 *' Medals of Creation," vol. i. p. 274,) are often 

 found in this state ; hence the protean forms of 

 the flints that have been moulded on portions of 

 these organisms. 



But there are innumerable flint nodules which 

 exhibit no traces of spongcous structure ; and 

 veins, dikes, and sheets of tabular flint that 

 may be regarded as pure, and free from organic 

 remains, excepting such as must inevitably have 

 become entangled and imbedded in a stream of 

 mineral matter flowing over a sea-bottom. 



