499 THE MEDALS OF CREATION. Cwap. XI. 
deposit of shells of this genus is the Wealden formation ; 
throughout which there are extensive beds of marble, coarse 
limestone, and clays, almost wholly composed of Paludine, 
and minute fresh-water Crustaceans, of the genus Cypris, 
which will be described in a subsequent chapter. The com- 
pact paludina-limestone of Sussex, called Petworth or Sussex 
marble, is principally made up of one species, the P. flu- 
vorum, Lign. 133, fig. 1, and is an ageregation of Paludine, 
held together by crystallized carbonate of lime; the cavities 
of the shells, and their interstices, being often filled with 
white calcareous spar. A polished slab, displaying sections 
of the enclosed shells, is figured in Wond. p. 402. Upon 
examining slices of this marble with the microscope, the 
cavities of the shells are found to contain myriads of the 
cases of Cyprides.* The Wealden limestone of the Isle of 
Purbeck, Lign. 134, 
known as Purbeck 
marble, is, in like 
manner, composed 
of Paludine, but of 
a much smaller spe- 
cies. Both these 
marbles were in 
Lien. 134. great repute with 
the architects of the 
middle ages, and 
there are but few of our cathedrals and ancient churches 
which do not still contain examples, either in their columns, 
monuments, or pavements, of one or both varieties. The 
polished marble columns of Chichester Cathedral, and those 
of the Temple Church, in London, are of Purbeck marble; 
in other words, they are composed of the petrified shells of 
snails, that lived and died in a river, flowing through a 
PoLISHED SLAB OF PURBECK MARBLE. 
* For a particular account of this marble, see Geol. 8. E. pp. 
182—187. 
