FOREIGN CORRESPONDENCE. 



395 



live in the present sea. The most common of these shells is that of the 

 cockle ( Cardium edide), seen also, as a fossil, in the argile grise, on 

 which the corn and potatos grow so well in the neighbourhood of 

 Ostend ; but the rock contains an infinity of others, all common on the 

 coast or in the neighbouring sea. 



Upon submitting this rock to a careful examination, I found it to be 

 a tufaceous limestone, of a grey colour, containing a considerable 

 admixture of clay and sand, some few lamellae of mica, vegetable 

 debris of peat, and recent shells. Some fragments of it are light and 

 fragile ; others, harder and denser, consist of layers superposed in such a 

 manner as to show that they have been formed successively, but which 

 do not obliterate the compact appearance of the larger masses. This 

 structure is frequent in tufaceous limestones. 



On examining the argile grise which exists all along the coast of 

 Flanders, I found that it is in reality an argillaceous marl, producing 

 effervescence in acids, and containing a notable proportion of carbonate 

 of lime (at least on the coast of Ostend); so that the tufaceous rock in 

 question may owe its existence to a gradual absorption of carbonate of 

 lime by this argillaceous stratum ; but it is, no doubt, to the cretaceous 

 rocks which are bathed by the waters of the Eaglish Channel (such as 

 the chalk cliffs of Dover, &c.), that the carbonate of lime in our recent 

 rock is owing/^ 



Fresh- water tufa, or travertine, as it is sometimes called, is very 

 common in limestone countries, as all our readers are doubtless aware — 

 there is hardly a stream in such districts but deposits carbonate of lime 

 by the loss of the carbonic acid in the water which held it in solution. 

 Marine tufa, however, has been looked upon, perhaps erroneously, as 

 of much rarer occurrence. It has been observed in Sicily, at the Island 

 of Ascension, in Australia, &c. Every one has heard of the famous 

 rock discovered in the Antilles by M. Moreau de Jones,f and which the 

 negros have ctiristened maconne-hon-Bieu. It became celebrated from 

 being found in Guadaloupe to contain human bones. Another such 



* Since I found this marine tufa on the coast of Ostend, I have seen, in the 

 geological collection of the Jardine des Plants, at Paris, a rock called " Gres 

 coquillier de Beauchamp (Seine et Oise)," containing both marine and fresh- water 

 shells, and which appears to me to be identical with the tufa described in the text 

 above.— T. L. P. 



t Hist. Physique des Antilles ; also Humboldt Relation Historique. — T. L. P. 



2 G 2 



