VARIOUS FORMS OF FLINTS 141 



flint obliterates the cavity and pervades the canals and 

 interstices of the skeleton. When the flint encloses a 



cav 



FIG. 38. Examples of Flints, broken open to show the structure. 



1. In this the black flint, 6, has been converted into white flint, w, 



towards the exterior by weathering. Towards the interior it 

 passes into incompletely silicified chalk or grey flint, g. This 

 is succeeded by a cavity, further towards the middle, coated 

 with crystals of quartz, q. 



2. Here the black flint is coated with a very thin film of white 



flint. It includes the skeleton of a sponge, sp, from which it 

 it is separated by a layer of chalk, ch. 



3. This flint also includes a sponge, one of the Ventriculites, which 



branches below into spreading roots, a necessary provision for 

 support in the soft sea-ooze. 



4. Is similar, but the sponge is of a different kind, and separated 



from the surrounding flint by a cavity which, no doubt, was 

 once filled with chalk. 



sponge skeleton it sometimes faithfully follows the form 

 of the latter, so that a glance at the exterior of the 

 nodule is sufficient to show that there is a sponge inside ; 



