300 GEOLOGY OF THE ISLE OF WIGHT. 



abundant univalve shells are four or five kinds of 

 Paludina ; a common river snail, of which there 

 are species in the tertiary freshwater strata of 

 Headon Hill and Calbourne (see PL I. fig. 6). 

 Numerous beds of marble, coarse limestone, clays 

 and shales, abound in, or are wholly made up of 

 paludinae, and minute crustaceans. The compact 

 Sussex limestone, which from its taking a good 

 polish, is called marble, principally consists of the 

 shells of one large species, the Paludina flwiorum 

 {PL VI. fig. 3), held together by an infiltration of 

 crystallized carbonate of lime ; the cavities of the 

 shells, and the interstices between them, being 

 often filled with white calc-spar. Upon examin- 

 ing thin slices of this marble under the microscope, 

 the shells are found to contain myriads of the 

 cases of Cj/prides (Jign. 25). The Purbeck marble 

 differs from the above in being composed of a 

 much smaller species — the Paludina elongata 

 {PL VI. fig. 2). 



These marbles were in 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 of both varieties, either 

 in their columns, sepulchral monuments, or pave- 

 ments. The polished cl nster-eolunuis of the Tei 1 1 ] > 1 1 • 

 Church in London, and ninny of the monuments 



