I ON A PIECE OF CHALK 23 



duration of the chalk period. Suppose that the 

 valve of the Crania upon which a coralline has 

 fixed itself in the way just described, is so attached 

 to the sea-urchin that no part of it is more than an 

 inch above the face upon which the sea-urchin 

 rests. Then, as the coralline could not have fixed 

 itself, if the Crania had been covered up with 

 chalk mud, and could not have lived had itself 

 been so covered, it follows, that an inch of chalk 

 mud could not have accumulated within the time 

 between the death and decay of the soft parts of 

 the sea-urchin and the growth of the coralline to 

 the full size which it has attained. If the decay 

 of the soft parts of the sea-urchin ; the attachment, 

 growth to maturity, and decay of the Crania ; and 

 the subsequent attachment and growth of the 

 coralline, took a year (which is a low estimate 

 enough), the accumulation of the inch of chalk 

 must have taken more than a year : and the 

 deposit of a thousand feet of chalk must, conse- 

 quently, have taken more than twelve thousand 

 years. 



The foundation of all this calculation is, of 

 course, a knowledge of the length of time the 

 Crania and the coralline needed to attain their 

 full size ; and, on this head, precise knowledge is 

 at present wanting. But there are circumstances 

 which tend to show, that nothing like an inch of 

 chalk has accumulated during the life of a Crania ; 

 and, on any probable estimate of the length of 



