22 ON A PIECE OF CHALK i 



" The upper valve is almost invariably wanting, 

 though occasionally found in a perfect state of 

 presei-vation in the white chalk at some distance. 

 In this case, we see clearly that the sea-urchin 

 first Hved from youth to age, then died and lost 

 its spines, which were carried away. Then the 

 young Crania adhered to the bared shell, grew 

 and perished in its turn ; after which, the upper 

 valve was separated from the lower, before the 

 Echinus became enveloped in chalky mud."^ 



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 

 Glchigerinm. 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 Avhole coralline, since corallines do not live 

 embedded in mud.^ 



The progress of knowledge may, one day, enable 

 us to deduce from such facts as these the maxi- 

 mum rate at which the chalk can have ac- 

 cumulated, and thus to arrive at the minimum 



1 Elements of Geology, by Sir Charles Lyell, Bart. r.E.S., 

 p. 23. 



