66 SELECTIONS FROM HUXLEY 



between the death of the sea-urchin, and its burial by the 

 Globigerince. For the outward face of the valve of a Crania, 

 which is attached to a sea-urchin (Micr aster} , is itself over- 

 run by an incrusting coralline, which spreads thence over 

 5 more or less of the surface of the sea-urchin. It follows that, 

 after the upper 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 coralline, since corallines do 

 not live embedded in mud. 



10 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 



15 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 



20 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 



25 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 



30 years. 



The foundation of all this calculation is, of course, a knowl- 

 edge 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 



