$98 Geological Society. 
The plastic clay of Paris basin also agrees very accurately 
with the corresponding deposit in the Isle of Wight basin. 
They are both of various colours and qualities, and both 
appear destitute of organic remains. The English deposit 
however contains two or three distinct beds of carbonized 
vegetable matter, which does not occur in that of Parts. 
The blue clay, the most important of any of the English 
beds above the chalk, does not make its appearance as such 
jn the Paris basin; nor, on the otber hand, does the calcaire 
grossier, a bed of equal importance in the latter basin, make 
its appearance among our English strata. The calcaire 
grossier is composed of beds of limestone inclosing beds 
of zoned hornstone, alternating with clays and marls of 
various descriptions. Of the fossil shells contained in this 
deposit nearly 600 species have been described by M. La- 
marck, many of which, and particularly those which are 
considered as the most characteristic, have been found by 
Mr. Webster to occur in the blue clay at Bracklesham, in 
Selsea, in Stebbington Clitf near Portsmouth, and other 
parts of the Isle of Wight and London basins. Hence it 
appears probable that the blue clay and the lower beds of 
the calcaire grossier are parts of the same deposit, and 
that from local circumstances the clay has prevailed in the 
one, and the calcareous matter in the other. 
The lower fresh-water formation lies on the upper black 
clay, and is most distinctly to be seen in the section of 
Headen Hill, which forms the northern boundary of Allum 
Bay. It here consists of a series of beds, chiefly of sandy, 
calcareous and argillaceous marle, mixed more or less with a 
brownish coaly matter. Of these beds some appear to consist 
almost wholly of fragments of shells, many of which how- 
ever are sufficiently entire to show that they are fresh-water 
shells, belonging to the genera Lymnea, Planorbis, and 
Helix. Some of the beds are sufficiently indurated to form 
a coarse building stone. 
At Cowes, and at Binstead near Ride, are also beds con- 
taining fresh-water shells lying over sand and blue clay, 
and differing from those of Allum Bay in little else than 
that instead of being marl they are limestone. 
An analogous formation occurs in the Paris basin, like 
that of the Isle of Wight already described: it contains 
beds of marl inclosing fresh-water shells of the genera 
Lymnea and Planorbis, but differs from the English in 
containing also three gypseous deposits. Of these the 
lowest is the thinnest, and abounds in crystallized selenite : 
the next is thicker, and includes a bed of indurated clay 
containing 
