ON A PIECE OF CHALK 61 



valve of the Crania fell off, the surface of the attached 

 valve must have remained exposed long enough to 

 allow of the growth of the whole corraline, since coral- 

 lines do not live imbedded in mud. 



The progress of knowledge may, one day, enable us 

 to deduce from such facts as these the maximum rate 

 at which the chalk can have accumulated, and thus to 

 arrive at the minimum 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, consequently, 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 



