158 THE FORMATION OF FLINTS 



flint to have the same size as the sponge, the calculation 

 is easily made : it would require the silica of eight 

 sponges to form the substance of one such flint, and if 

 the life of each sponge is limited to five years, this gives 

 us forty years for the period sought ; this is the time that 

 would be necessary for the accumulation of sufficient silica 

 to form a layer of flints five centimetres in thickness. 



In the middle of the last century geologists had escaped 

 so recently from the restrictions of a six days' creation 

 that they were not uncommonly occupied in proving to 

 themselves and others that the age of the earth must be 

 very great. Thus so late as 1868 Huxley says in his 

 lecture, " On a Piece of Chalk," * "... Geologists find 

 in the chalk a fossilised sea-urchin, to which is attached 



FIG. 46. A Fossil Sea-Urchin (Ananchytes) from 

 the chalk with lower valve of a brachiopod 

 (Crania) attached. Reduced one-half, b. 

 Upper valve of the Crania detached. Taken 

 from. Lyell's " Student's Elements," with 

 kind permission of Messrs. A. & J. Murray. 



the lower valve of a Crania (Fig. 46). This is a kind of shell- 

 fish, with a shell composed of two pieces, of which, as in 

 the oyster, one is fixed and the other free." " The upper 

 valve is almost invariably wanting, though occasionally 

 found in a perfect state of preservation in the white chalk 

 at some distance. In this case we see clearly that the 

 sea-urchin first lived from youth to age, then died and 

 lost its spines, which were carried away. Then the 

 young Crania adhered to the bared shell, grew and 

 perished in its turn, after which the upper valve was 

 separated from the lower before the sea-urchin became 

 enveloped in the chalky mud." f 



* "Lay Sermons," p. 190. 



f " Elements of Geology," by Sir C. Lyell, p. 23. 



