ix.] ON A PIECE OF CHALK. 165 



Cases of this kind are admirably described by Sir Charles 

 Lyell. He speaks of the frequency with which geologists find 

 in the chalk a fossilized sea-urchin, to which is attached the 

 lower valve of a Crania. This is a kind of shell-fish, with a 

 shell composed of two pieces, of which, as in the oyster, one is 

 fixed and the other free. 



&quot; The upper valve is almost invariably wanting, though occa 

 sionally found in a perfect state of preservation in the white 

 chalk at some distance. In this case, we see clearly that the 

 sea-urchin first lived from youth to age, then died and lost its 

 spines, which were earned 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. 1 



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 Globigerince. 

 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 

 1 &quot; Elements of Geology,&quot; by Sir Charles Lyell, Bart. F.K.S., p. 23. 



