IK.] ON A PIECE OF CUALK 191 



The progress of knowledge may, one clay, 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 the length 

 of that life, the chalk period must have had a much 

 longer duration than that thus roughly assigned to it. 



Thus, not only is it certain that the chalk is the mud of 

 an ancient sea-bottom ; but it is no less certain, that the 



