78 SELECTED ESSAYS FROM LAY SERMONS 



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 GlobigerincB. 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. 



1 Elements of Geology, by Sir Charles Lyell, Bart., F.R.S., p. 23. 



