60 
THE aEOLOGTST. 
genera Cytlierea, Cyprina, Cardium, Pectunculus, Natica, and Lamna 
teeth) ; but there are also a few of the estuarine shells of the Woolwich 
beds, and in abundance Cyrena, Cerithium, and Melania. 
This bed (or rather the set of sands, etc., tliat are usually classed with 
it) does not occur for some distance to the east, owing to the extent of the 
denudation of the Tertiaries ; coming on again, however, near Sittingbourne 
with the same general characters and a little thicker. It has been observed 
in the cutting north of that town, on the Sheerness branch-line. Near 
Boughton and Canterbury this sand is again shown, and it contains flint- 
pebbles and ironstone. At the mouth of the tunnel, on the Whitstable 
Railway, near the second place it may be well seen, and the thick pebble- 
bed at ibhe base, cemented into a hard mass by the ironstone, is remarkable. 
At kShottenden Hill there is a great thickness of the flint-pebbles. 
The fine cliff-section between the Eeculvers and Heme Bay shows this 
sand-bed very clearly. It has here an average thickness of 20 feet ; is 
crowded with fossils ; contains large blocks of hard tabular sandstone, often 
almost made up of fossils or casts of fossils, and smaller blocks of fossil- 
bearing iron-sandstone ; and has a well-marked pebble-bed, from a few 
inches to more than a foot thick at the bottom. The fineness and loose- 
ness of the sand of this division of the Tertiary series is here well shown : 
on windy days many tons' weight of this sand are blown away from the 
face of the cliff, the wind indeed being very powerful in wearing away 
these cliffs. The fossils are of a London Claylype; but many of them 
occur also in the beds below. Sharks' teeth and vertebrae are plentiful, 
and amongst the shells the bivalve genera C3^prina, Cytherea, and Car- 
dium, and the univalves Natica and Aporhais. 
In all these East Kent sections, it is to be noted that the London Clay 
is sharply divided from, and nowhere passes into the sands below, and that 
it nearly always has a few pebbles, green grains, and sharks' teeth at the 
bottom, which is rather sandy, and not unlike the undoubted basement-bed 
in the western part of the London basin, and this may perhaps be the real 
basement-bed. The underlying sands are more like the beds below them 
in mineral structure ; but more allied to the London Clay by fossils. 
4. The London Clay is not only of much greater thickness, but also of 
more uniform mineral character than the lower formations. It consists al- 
most throughout its whole thickness of stiff dark-bluish-grey clay, weather- 
ing brown, with now and then a bed of nodular masses of clayey limestone 
(known as " septaria," from having septa or divisions of carbonate of lime), 
from which Homan cement is made. The lower part is generally roughly 
laminated, and abounds in transparent crystals of selenite (sulphate of 
lime). The upper part, which however rarely occurs, having been for the 
most part denuded, is also somewhat laminated and sandy, and generally 
passes up into the overlying Bagshot Sand (present only in the Isle of 
Sheppey). 
The full thickness of the London Clay in the most western part of Kent 
is calculated to have been about 450 feet; so great a thickness occurs no- 
where in West Kent, however, the highest beds of the formation having 
been denuded. Shooter's Hill, indeed, is the only place where the greater 
part of the London Clay has been left. In the Isle of Sheppey the thick- 
ness is calculated at 480 feet, the greatest known. As might be expected 
from its greater thickness, the London Clay occurs at the surface over a 
wider belt of country than the thinner underlying formations. 
The fossils, in some places abundant, but often " conspicuous by their 
absence," show that the clay is a sea-deposit. Moreover, as the many 
species occur for the most part from top to bottom of the formation, and as 
