§ 25. PURBECK MARBLE. 403 



The Purbeck marble differs from that above described, 

 in being composed of smaller shells of the same genus 



(Paludina elongata) ; it 

 also contains abundance 

 of cyprides ; and in some 

 layers, small bivalves 

 (Uniones), replaced by 

 white spar, give a va- 

 riety to the markings 



Lign. 96.— Polished slab oe Purbeck exposed in the sections. 



. , M ^ RBL • The polished cluster- 



(Composea of a small species of Paludina.) A 



columns in the Temple 

 Church in London, and in Chichester Cathedral, and 

 many of the monuments in Westminster Abbey, are 

 of this marble ; in other words, they are constructed of 

 conglomerated masses of petrified shells of snails, which 

 lived and died in the rivers that flowed through a country 

 inhabited by the Iguanodon and other extinct colossal 

 reptiles, ages before a single layer of the chalk was 

 deposited ! 



A few species of an elongated spiral freshwater univalve 

 (Melanopsis), are found in great perfection in some of the 

 clays at Pounceford (Lign. 97, a), associated with the 

 characteristic bivalves of the wealden (Lign. 97, b and c). 

 A few specimens of a small elegant Nerite (Neritina 

 Fittont, Lign. 94, Jig. 3), have been found in the calciferous 

 grit of Tilgate Forest and Hastings. 



25. Bivalve shells. — We have already had occasion to 

 observe that many kinds of the bluish grey limestones of 

 the Wealden, are largely composed of small bivalves of the 

 genera Cyclas and Cyrena ; some layers form a compact 

 marble that will bear a good polish. In the clay the shells 

 are often beautifully preserved ; as in these specimens from 

 Pounceford (Lign. 97), in which the epidermis and liga- 

 ment remain. 



dd2 



