396 Geological Society. 
Next above the chalk marl lies a thick bed of dark reé 
clay, often mottled with yellow and white, and forming an 
excellent material for bricks and coarse pottery. Then 
comes a bed of yellowish white sand; and another thick 
bed of blue clay, containing green earth and’ nodules. ‘of ai 
dark coloured limestone inclosing shells. Above this he 
several beds of differently coloured sand, more or less ferru- 
ginous and clayey, in which no organic remains have hitherto 
been found. "These are again covered by a numerous suc- 
cession of strata, consisting of irregular alternations of 
white, yellow and gray pipe clay, with white and variously 
coloured sands : neither the clays nor sands contain organic 
remains ; bat in the upper part of this series are three strata, 
from six inches to a foot in thickness, of a difficultly com- 
bustible and sulpbarcous coal, the vegetable origin of which 
is apparent from the branches and fruits’ that may still be 
observed in it. Above this lies another extensive series of 
white and differently coloured sands. ‘The different beds 
which have now been enumerated as lying above the chalk 
marl are very interesting, not only in a geological but m an 
ceconomical point of view. The fine white sand of Allam 
Bay used in the manutactories of flint glass, as well as the 
white clays of the Isle of Wight and Dorsetshire, so ex-' 
tensively used in our finer kinds of pottery, belong to this 
series. It is plainly to be traced in varions parts of the 
Isle of Wight basin, and may be seen in the London basin, 
on the east bank of the Medway near Rochester, in the 
neighbourhood of Reading, and elsewhere. 
Above the strata just faentioned lies the great deposit of 
the blue or London clay. Its thickness im many places 
amounts to 200 or 300 feet, and in some parts even exceeds 
500 feet. It consists for the most part of a very tough 
blackish clay, inclosing flattened spheroids of indurated 
marl, which when broken exbibit compartments formed by’ 
the intersection of veins of yellowish calcareous spar, and 
are hence called Septaria. In some parts the clay 1s mixed 
with green earth and sand, and more or less impregnated 
with carbonate of Ime, with pyrites, and with gypsam, 
this latter being probably the result of the vitriolization of 
the pyrites. and the subsequent decomposition of the sul- 
phate of iron by the carbonate of lime. 
On account of these saits the water contained in this: 
bed is bur little fit fer domestic purposes ; wells are there-. 
fore, if possible, sunk through it into the sand below, the: 
water of which is very abundant and of good quality. 
One distinguishing characteristic of this deposit is the: 
number, ' 
