118 NEW SOUTH WALES 
The Royal Commission (Ireland) say, fruitful oyster mud may vary 
within very wide limits, from almost pure sand to almost plastic clay. 
In the very sandy grounds, there must however be always a sufficient 
quantity of highly hydrated clay to render the sand adhesive and to 
preserve it from becoming a mere loose running mass. 
In the clayey grounds there must always be calcareous mud to make the 
clay porous and prevent it becoming too hard,—clay marls, with some 
intermixed sand, being perhaps the best of all materials for oyster grounds. 
The earth known as the London clay appears to be the soil peculiarly 
adapted for oysters. It may be well here to explain that the term 
‘London clay’ is employed in.a general and a special sense. In the 
former it is used as a collective name for a number of beds of the old 
tertiary formation, consisting of gravels and sands below and of clays 
above. In the special or limited sense, it is applied to the bluish or 
blackish clay, sometimes mixed with a greenish coloured earth and white 
sand, which forms the upper parts of the beds just mentioned. London 
clay is plastic clay, not differing much in chemical composition from 
ordinary potter’s clay. All fruitful oyster muds contain organic matter, 
always due in part to the presence of ,infusoria, and sometimes in part 
to small alge or conferve, remains of shell-fish and other marine 
creatures. 
Bertram says, one of the most lucrative branches of oyster farming in 
France is the fattening of oysters in claires, at Marennes, which have 
been brought from the Ile de Ré breeding parcs. In the claires the 
oysters become green, and of considerably more value than the white 
oyster. The peculiar colour and taste of the green oyster are imparted 
to it by the vegetable substances which grow in the claires. The industry 
carried on at Marennes consists chiefly of the fattening in claires ; and 
the oysters operated upon were at one period of their lives as white as 
those which are grown at any other place; indeed, it is only after they 
have been steeped a year or two in the muddy ponds (claires) of the 
river Seudre that they attain their much-prized green hue. The ponds 
(claires) for the manufacture of these green oysters—the oyster par 
excellence, according to all epicurean authority—require to be watertight, 
for they are not submerged by the sea, except during very high tides. 
Each claire is about 100 ft. square; the walls for retaining the waters 
require, therefore, to be very strong. They are composed of low banks 
of earth, 5 or 6 feet thick at the base, and about 3 feet in height. 
These walls are also useful in forming a promenade, on which the 
watchers or workers can walk to and fro and view the different ponds. 
The floodgates for the admission of the tide require also to be thoroughly 
watertight and to fit with great precision, as the stock of oysters must 
always be covered with water, but a too frequent flow of the tide over 
the ponds is not desirable, hence the walls, which serve the double 
purpose of both keeping in and keeping out the water. A trench or 
ditch is cut in the inside of each pond, for the better collection of the 
green slime left at each flow of the tide, and many tidal inundations 
are necessary before the claire is fit for its stock. The oysters placed in 
them are a year or sixteen months old, and it is two years at least before 
they are properly greened. 
