FOSSIL SPONGES, ETC. 27 



that of a series of cups one inside another, the largest 

 ones at the bottom, and the smallest at the top. The 

 topmost segment is usually crowned with an extin- 

 guisher-like cap, at the side of which we observe a 

 perforation, which is connected with the traces of 

 a pipe running down the centre of the chalky core 

 occupying the segments of this peculiar fossil. Each 

 segment is therefore hollow, or rather occupied with 

 a dense core of chalk. The country people use the 

 segments for flower-pots in their gardens, after they 

 have removed the chalky core. 



Fossil sponges are also obtained by hammering 

 the most likely and " rotten " of the flint pebbles 

 forming the gravel of the eastern and southern 

 counties of England. It is only when we are 

 familiar with the extent and thickness of these sheets 

 of gravel that we can form an idea of the extent to 

 which the chalk has been denuded ; for every one 

 of these flint pebbles is a broken-up, rounded, or 

 liberated portion of a flint nodule, which was origin- 

 ally formed by chemical segregation in the ooze of 

 the Cretaceous sea-bed. 



The comparatively recent application of the 

 microscope to rock structures has supplied us with 

 abundant materials for generalization. This instru- 

 ment enables us to detect plentiful traces of fossils 

 where, to the naked eye, the rocks appear quite 

 destitute of them. This is particularly the case in 

 limestone beds. Among the most numerous of the 



