§24. SHELLS OF THE WEALDEN. 401 



shell is found entangled in the fresh-water sediments ; but, 

 I repeat, the shells hitherto discovered afford no grounds 

 for invalidating my original statement,* that the "Wealden is 

 an ancient delta — a fluviatile formation. 



The most abundant shells of this division of testaceous 

 mollusca are the Paludince, of which the Sussex and Pur- 



Lign. 94. — Univalve shells of the Wealden. 



Fig. 1. Paludina Sussexiensis. 2. Melanopsis. 

 3. Neritina Fittoni ; a, natural size. 



beck marbles are almost wholly made up : the former 

 limestone differing from the latter, simply in the species 

 of which it is composed. 



Th^ Sussex marble is a congeries of paludina ; chiefly of 

 Paludina jluviorum and P. Sussexiensis (Lign. 94, fig. 1.). 

 In the coarse varieties of this stone, the shells are decom- 

 posed, and the interstices left by their removal filled up 

 with clay ; but in the compact layers and blocks, the shells 

 are transmuted into calcareous spar, and their cavities con- 

 tain indurated marl and limestone, of various shades of 

 grey, blue, yellow, &c. interspersed with pure white, 

 mottled with black ; the polished slabs (Lign. 95) dis- 

 play innumerable sections of the inclosed shells, and rival 

 in interest and beauty many of the foreign marbles. The 

 black and dark brown spots and veins in this and other 

 shelly marbles, have originated from the transmutation 

 of the soft bodies of the mollusca into a carbonaceous 



* In 1S22. 

 D D 



