THE PURBECK STRATA. 353 



The Purbeck strata. — The uppermost Pur- 

 beck bed is a green, calcareous, and for the 

 most part friable, deposit, containing shells of a 

 small species of Unio ; beneath this is a stratum 

 of the well-known marble, composed of an aggre- 

 gation of the small river snail -shells already de- 

 scribed (Paludina elongata, P/.VI. fig. 2); a polished 

 slice of this marble is figured in lign. 29. Some of 



I.1GN. 2'J. POLISHED SLAB OF PURBECK MARBLE. 



( Composed of petrified river snail-shells. ) 



the layers contain interspersions of the large palu- 

 dinae (P. jltiviorum, PL VI. fig. 3) of the Sussex 

 marble, and also a few small river-mussels. I 

 have seen polished slabs of this kind, in which 

 sections of bivalves (Uniones) formed the princi- 

 pal markings ; these shells were associated with 

 cyprides and very small fragments of bones, and 

 vegetable detritus. The white and cream-coloured 



