b. Middle Eocene, or the Bagshot Beds, composed of sands and clays. The beautiful coloured sands of Alum Bay, the sands of the Surrey and Hampstead Heaths, are familiar examples of the beds of this age. Very few fossils indeed have been found in them. The clay-beds on the contrary as seen at Barton and Hordwell on the Hampshire coast and again in the Isle of Wight, abound with shells belonging to genera such as Conus, Voluta and Venus, that inhabit warm seas. With them are the Nummulites, looking externally very like buttons, but on the inside divided into innumerable chambers in which the complex animal that formed the nummulite dwelt.
c. Lower Eocene, the well-known London clay, may almost be said to compose this division, for the underlying sands, gravels, and clays are in mass comparatively insignificant. The London clay contains plenty of fossils, only as they are disposed in layers (zones) at a considerable distance apart, they are not often hit upon. Layers of Septaria or cement-stones are of frequent occurrence. Sheppy is the great locality for London clay fossils, as the sea annually washes down large masses of the cliffs and breaks them up on the beach. A great many fossil fruits and seeds, remains of crabs, shells of Nautili, Volutes, and other mollusca, besides turtles, a species of snake, a bird with teeth, and a tapir-like animal, have at different times and in various places been found in this deposit, which sometimes attains a thickness of over 400 ft. The "Bognor Rock" is a local variety of the basement bed of this formation.
Aturia Zic-zac (from the London clay).
The Mesozoic or Secondary rocks embrace a series of limestone, clays, sands, and sandstones that on the whole are well consolidated. The main mass of them lies to the west of a line drawn across the map of England from the mouth of the Tyne, in Northumberland, southwards to Nottingham, and thence to the mouth of the Teign in Devonshire. In the south-eastern counties they underlie the tertiary rocks of the London and Hampshire basins, as they are called, at no great depth from the surface. Outlying patches of secondary rocks occur in Scotland, where they are found near Brora on the east coast, and in the islands of Skye and Mull on the west. In Ireland they are scantily represented round about the neighbourhood of Antrim. The secondary rocks are divided into—
Ammonites various (from the chalk).
1. Cretaceous.
a. The Chalk is too well known to need description, though technically it may be described as a soft white limestone chiefly built up of the microscopic shells of Foraminifera, and characterized in its upper part by nodules and bands of flint. These flints frequently inclose casts of fossils (sponges, sea-urchins, etc.), and sometimes shells themselves. Fossils, too, are fairly abundant, scattered throughout the mass. Amongst the commoner may be noticed the sea-urchins, such as the "sugar loaf" (Ananchytes) and the heart-shaped Micraster, the Brachiopods or Lamp-shells (Terebratula, Rhynchonella), a "Thorny Oyster" (Spondylus spinosus), besides Ammonites, Belemnites (part of the internal shell of a kind of cuttle-fish), and the teeth of several species of sharks. Altogether the chalk is about 1,000 feet thick.
b. Upper Greensand is a series of greenish-grey sands and sandstones. The green colour, on close inspection, is seen to be due to the presence of innumerable small green grains of a mineral called glauconite. These are frequently casts of the chambers of the very same foraminifera that the chalk is so largely composed of.