232 PICTURESQUE SKETCHES 



surrounding water, replace these horny fibres, so that 

 the frame- work becomes more or less entirely made 

 up of them, and assumes a different character. A body 

 in this state when exposed to the influence of water 

 in which siliceous or flinty matter is abundantly pre- 

 sent, would, as we know by observation in similar 

 cases, attract to itself the particles of siliceous matter 

 in the water, and become fossilised and surrounded 

 with silica. Such has been, perhaps, the history of 

 those singular layers of flint which characterise the 

 upper beds of chalk ; and it is quite certain that care- 

 ful examination under the microscope often shows 

 very distinctly in such flints a fine net-work of fibres 

 like those which we know to exist in sponges. The 

 annexed figure (82) will give an idea of the appear- 

 ance of such fibres in flint when greatly magnified. 



Fig. 82 



FOSSIL SPONGE 



The forms of the sponges themselve are not unfre- 

 quently preserved in that of the flint into which 



