File:The Quarterly journal of the Geological Society of London (13206277784).jpg

362
PROCEEDINGS OF THE GEOLOGICAL SOCIETY. .May 26,
This therefore forms a well-marked and characteristic division, which
will aid materially in taking the next and more debateable step.
London Clay.
Immediately overlying these mottled clays, or separated from them
only by a thin subordinate bed of sand, there is found in the Hamp-
shire district a thick and important mass of fossiliferous brown and
grey clay, well-exhibited in the sections at Alum and White-Cliif Bays,
and proved to exist in the same position in the well at Southampton
(see Plate XIV. Gen. Sect. fig. 2 to 4). This deposit is well-marked
and persistent in its lithological characters. At Alum Bay it is 200
feet thick, and at White-Cliff Bay and Southampton about 300 feet,
apparently therefore thickening gradually in its range northward and
eastward. Its local composition at the former two places I have
described in my last paper : it is there termed " the Bognor beds.""
I will therefore now confine myself to its more permanent and cha-
racteristic features.
In the first place it is almost uniformly preceded by a thin seam of
sand, frequently green, and containing small rounded black flint peb-
bles, and occasionally also, when the underlying mottled clays have
been sufficiently hard, rolled and water-worn pebbles of the mottled
red clay itself, pointing to, as I have before observed, a sudden alter-
ation of hydrographical conditions, and a change in the ancient sea-
bottom. This fact is particularly noticeable in White-Cliff Bay.
Fig. 2. — Junction of the London Clay (1 to 5) and mottled clays at
White-Cliff Bay .
1 . Dark grey and brown clay at top ; in descending
becomes mucli mixed with greensand ; few fos-
sils in this part of it.
2. Tabular Septaria (4 inches thick) with greensand ;
full of shells ; amongst them the most abundant
are, Pyrula tricostata, Pectunculits breviros-
tris, Cardium (Plumsteadiense ?), Natica glau-
cinoides, Osfrea, and a few Ditrupa plane,.
3. Similar to (5), but finer grained 6 inches.
4. Dark green sand, full of the Ditrupa plana ; a
few flint pebbles 6 inches.
5. Brown sandy clay much mixed with greensand,
and passing downwards into a conglomerate
with rounded pebbles oi flint, chalk, and 7'ed
clay, A few Ditrupa plana are dispersed
throughout 2 feet.
6. Dark red mottled clays (upper part of).
I dwell upon this fact, because it indicates a well-marked com-
mencement of a different order of things from that which previously
existed. (See also Plate XIV. Comp. Sec. 3 to 11, point ^.")
The lowest beds of the London clay are generally more or less sandy,
but the sand gradually disappears, passing into tough brown clay with
layers of Septaria (and concretionary conglomerates at Southampton),

• This is best exhibited on the shore between high and low water level.