142 THE FORMATION OF FLINTS 



sometimes, on the contrary, the flint takes its own form, 

 uninfluenced, apparently, by that of the sponge. Occa- 

 sionally the flint fills partly or wholly the test of a 

 sea-urchin, sometimes exuding, as it were, from the 

 natural openings of this test, and sometimes more or less 

 completely enveloping it. Shells and fragments of shells, 

 derived from mussels, such as Inoceramus, or lamp- 

 shells, such as Terebratula, retaining their calcareous 

 composition, are not infrequently included in a nodule. 



The whiteness of the outer layer is to be explained in 

 the same way as the whiteness of snow. The minute 

 grains of which it consists are individually transparent 

 and colourless, but, owing to their extreme minuteness 

 and the immense number of little spaces between them, 

 they offer an infinite number of reflecting surfaces within 

 a slight thickness, and thus scatter the light that falls 

 upon them in all directions. The black flint is trans- 

 parent in very thin slices, and almost colourless, or only 

 slightly tinged with a faint brown colour, due to the 

 presence of a carbonaceous pigment, the last residuum of 

 the living protoplasm once present in the chalk. Light 

 penetrates the flint for a considerable distance, but is 

 absorbed by the brown pigment. The quantity of pig- 

 ment present differs in different cases, and this partly 

 explains why some black flints are blacker than others. 



The association of the remains of sponges with flints 

 was noticed as early as the middle of the eighteenth 

 century, and was especially emphasised some fifty, or 

 sixty years ago by Bowerbank, who was led by it to the 

 conclusion that flints are petrified sponges, and though 

 this cannot now be seriously maintained, yet it will 

 surprise no one who is familiar with the forms of these 

 objects, which are sometimes so singularly organic in 

 appearance as to seem purposely designed to deceive. 



It is scarcely necessary to remark that the sponges 



