FOREIGN CORRESPONDENCE. 
395 
live in the present sea. The most common of these shells is that of the 
cockle ( Cardium edule), 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 lamellaj 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 English Channel (such as 
the chalk olilfs of Dover, Sec), 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 solittion. 
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 Jone3,f and which the 
negros have christened maconne~hon-Dieu. 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 tlie text 
above.— T. L. P. 
t Hist. Physique des Antilles ; also Humboldt Relation Historique. — T. L. P, 
2 a 2 
