59 
THE GEOLOGIST. 
1. The Thanet Beds. — This formation, almost a purely KentisL. one, 
being but little shown in any other county, thins out a little west of 
Loruion. 
Under London it is from 30 to 40 feet thick, and consists of fine, soft, 
light-coloured sand, without fossils. It continues the same through West 
Kent, but with a thickness increasing to 60 feet or more. In East Kent 
a gradual change sets in, the beds get more clayey, and fossils occur in 
parts ; until the Isle of Thanet, and to the south, there are little else than 
marls, mostly hard, and clays — with a little sand at top however — often 
full of fossils, and from 80 to 100 feet thick. At the bottom, everyiohere 
there is a bed of clayey greensand, with green-coated llints lying at once 
on the Chalk. 
The number of kinds of fossils in these Thanet beds is small. They 
are all marine, and seem to be such as would have lived in the sea of a 
more or less temperate climate ; the chief genera are Cyprina, Cytherea, 
Plioladomya, Cucullsea, Thracia, Nucnla, Corbula, Sanguinolaria, all bi- 
valve shells, and AmpuUaria. The cliffs and the shore, west of the Recul- 
vers, yield those of the uppermost and more sandy part of the formation ; 
whilst at Pegwell Bay the lower marly part is also shown. At both places 
there are fossil-bearing sandstones in the upper part. The Bekesbourne 
cutting of the London, Chatham, and Dover Eailway 5nelded many fossils 
to Mr. Dowker, one of the members of the Society, who has described this 
section in a former volume of the ' Geologist.' 
Where a section shows a good thickness of the Thanet beds above the 
Chalk, the junction of the two formations is even ; where, however, there 
is but little of the former, the junction is generally more or less wavy, and 
there are often large funnel-shaped hollows, known as " pipes." These 
have been slowly formed by the action of water containing carbonic acid 
in solution, which has dissolved awa}- the Chalk (carbonate of lime) in an 
irregular and unequal manner, and the beds above have fallen down into 
the hollows thus left. It is where the Chalk is but thinly covered, and 
therefore where water would the more easily filter through to it, that one 
would expect this action to occur to the greatest extent, and such is the 
case. 
2. The Woolwich Beds are remarkable for their ever-changing structure, 
especially in Kent, in which county, however, there is none of the brightly- 
coloured mottled plastic clay, that occurs so generally in this formation in 
the western part of the London basin. 
In the " far west " of Kent, near London, the Woolwich beds are about 
50 o:.' 60 feet thick, and consist of alternations of sands, beds of pebbles, and 
clays with shells ; and at the bottom, a bed of greenish sand with pebbles, 
sometimes resting somewhat unevenl}'- on the underlying Thanet Sand. 
The shell-beds must have been deposited in a river or estuary, for some 
of the shells are of a kind that could not have lived in the sea, — such as 
the bivalve genera Unio, Cyrena, and the univalve Paludina, Neritina, Me- 
lania, and Melanopsis. There are also oysters, and other genera that show 
that the beds are not altogether freshwater, but partly of brackish-water 
origin. As is usually the case with freshwater beds, fossils are most abun- 
dant in numbers, but not in kind, — few species, but hosts of individuals. 
Air. Prestwich infe* s that the river which deposited these shells, and the 
clay that contains thorn, flowed down from an island, which he supposed 
to have occupied at that tim^e part of our present Weal den district. The 
climate indicated by the shells is that of a temperate region, and the remains 
of plants that are found here and t'nere, point the same way. These shell- 
beds are well shown in the large ballast pit, at Charlton, near Woolwich, 
