GEOLOGICAL TIME 159 



"A specimen in the Museum of Practical Geology in 

 London still further prolongs the period which must have 

 elapsed between the death of the sea-urchin and its burial 

 by the Globigerinse. For the outward face of the valve 

 of a Crania, which is attached to a sea-urchin (Micraster), 

 is itself overrun by an incrusting coralline, which spreads 

 thence over 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 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/' 



No doubt this famous illustration, which, in its mode- 

 ration, is beyond all criticism, was simply intended to 



