As we have traced the world's history written in the rocks we have seen an old continent gradually submerged, a deepening sea flowing over this part of the earth's surface. Now we shall find evidence of the deepening of the sea to something like an ocean depth. We are coming to the great period of the Chalk, the time when the material was made which forms the undulating downs of the south east of England, and of which the line of white cliffs consists, which with sundry breaks half encircles our shores, from Flamborough Head in Yorkshire, by Dover and the Isle of Wight, to Bere in Devon. Across the Channel white cliffs of chalk face those of England, and the chalk stretches inland into the Continent. Its extent was formerly greater still. Fragments of chalk and flint are preserved in Mull under basalt, an old lava flow, and flints from the chalk are found in more recent deposits (Boulder Clay) on the East of Scotland, pointing to a former great extension northward, which has been nearly all removed by denudation. In the Isle of Wight the chalk cliffs of Freshwater and the Culvers are the grandest features of the Island; while all the Island is dominated by the long lines of chalk downs running through it from east to west. Now what is the chalk? And how was it made? The microscope must tell us. It is found that this great mass of chalk is made up principally of tiny microscopic shells called Foraminifera, whole and in crushed fragments. There are plenty of foraminifera in the seas to-day; and we need not go far to find similar shells. On the shore near Shanklin you will often see streaks of what look like tiny bits of broken shell washed into depressions in the sand. These, however, often consist almost entirely of complete microscopic shells, some of them of great beauty. The creature that lives in one of these shells is only like a drop of formless jelly, and yet around itself it forms a complex shell of surprising beauty. The shells are pierced with a number of holes, hence their name (fr. Lat. foramen, a hole, and ferre, to bear). Through these holes the animal puts out a number of feelers like threads of jelly, and in these entangles particles of food, and draws them into itself. Now, do we anywhere to-day find these tiny shells in such masses as to build up rocks? We do. The sounding apparatus, with which we measure the depths of the sea, is so constructed as to bring up a specimen of the sea bottom. This has been used in the Atlantic, and it is found that the really deep sea bottom, too far out for rivers and currents to bring sand and mud from the land, is covered with a white mud or ooze. And the microscope shows this to be made up of an unnumerable multitude of the tiny shells of foraminifera. As the little creatures die in the sea, their shells accumulate on the bottom, and in time will be pressed into a hard mass like chalk, the whole being cemented together by carbonate of lime, in the way we explained in describing the making of limestones. So we find chalk still forming at the present day. But what ages it must take to form strata of solid rock of such tiny shells! And what a vast period of time it must have required to build up our chalk cliffs and downs, composed in large part of tiny microscopic shells! With the foraminifera the microscope shows in the chalk a multitude of crushed fragments, largely the prisms which compose bivalve shells, flakes of shells of Terebratula and Rhynchonella, and minute fragments of corals and Bryozoa. Scattered in the chalk we shall also find larger shells and other remains of the life of the ancient sea. The base of the cliffs and fallen blocks on the shore are the best places to find fossils. Much of the base of the cliffs is inaccessible except by boat. The lower strata may be examined in Sandown and Compton Bays, and the upper in Whitecliff Bay. A watch should always be kept on the tide. The quarries along the downs are not as a rule good for collecting, as the chalk does not become so much sculptured by weathering.
The deep sea of the White Chalk did not come suddenly. In the oncoming of the period we find much marl—limy clay. As the sea deepened, little reached the bottom but the shells of foraminifera and other marine organisms. How deep the sea became is uncertain: there is reason to believe that it did not reach a depth such as that of the Atlantic.
It is difficult to draw the line between the Upper Greensand and the Chalk strata. Above the Chert beds is a band a few feet thick known as the Chloritic Marl, which shows a passage from sand to calcareous matter. It is named from the abundance of grains of green colouring matter, now recognised as glauconite; so that it would be better called Glauconitic Marl. It is also remarkable for the phosphatic nodules, and for the numerous casts of Ammonites, Turrilites, and other fossils mostly phosphatized, which it contains. This band is one of the richest strata in the Island for fossils. It differs, however, in different localities both in thickness and composition. It is best seen above the Undercliff, and in fallen masses along the shore from Ventnor to Niton. It is finely exposed on the top of Gore Cliff, where the flat ledges are covered with fossil Ammonites, Turrilites, Pleurotomaria, and other shells. The Ammonite (Schloenbachia varians) is especially common. The sponge (Stauronema carteri) is characteristic of the Glauconitic Marl. As the edge of the cliff is a vertical wall, none should try this locality but those who can be trusted to take proper care on the top of a precipice. When a high wind is blowing the position may be especially dangerous.
PL. III
 (Pecten) |  Neithea Quinquecostata | |||
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| (Sea Urchins) | ||||
| Micraster Cor-Anguinum | Echinocorys Scutatus (Internal cast in flint) | |||
LOWER AND UPPER GREENSAND AND CHALK
The Chloritic Marl is followed by the Chalk Marl, of much greater thickness. This consists of alternations of chalk with bands of Marl, and contains glauconite and also phosphatic nodules in the lower part. Upwards it merges into the Grey Chalk, a more massive rock, coloured grey from admixture of clayey matter. These form the Lower Chalk, the first of the three divisions into which the Chalk is usually divided. Above this come the Middle and Upper, which together form the White Chalk. They are much purer white than the lower division, which is creamy or grey in colour. The Chalk Marl and Grey Chalk are well seen at the Culver Cliff, and run out in ledges on the shore. The lower part of this division is the most fossiliferous, and contains various species of Ammonities, Turrilites, Nautilus, and other Cephalopoda. (Of Ammonites Schloenbachia varians is characteristic. Also may be named S. Coupei, Mantelliceras mantelli, Metacanthoplites rotomagensis, Calycoceras naviculare, the small Ammonoid Scaphites æqualis; and of Pectens, Æquipecten beaveri and Syncyclonema orbicularis may be mentioned). White meandering lines of the sponge Plocoscyphia labrosa are conspicuous in the lower beds. The Chalk Marl is well shown at Gore Cliff, sloping upwards from the flat ledges of the Chloritic Marl. It may be studied well, and fossils found, in the cliff on the Ventnor side of Bonchurch Cove,—which has all slipped down from a higher level.
The uppermost strata of the Lower Chalk are known as the Belemnite Marls. They are dark marly bands, in which a Belemnite, Actinocamax plenus, is found. The hard bands known as Melbourn Rock and Chalk Rock, which on the mainland mark the top of the Lower and Middle Chalk respectively, are neither of them well marked in the Isle of Wight. In the Middle Chalk Inoceramus labiatus, a large bivalve shell, occurs in great profusion; and in the Upper flinty Chalk are sheets of another species, I. Cuvieri. It is hardly ever found perfect, the shells being of a fibrous structure, with the fibres at right angles to the surface, and so very fragile.
There is a striking difference between the Middle and Upper Chalk, which all will observe. It consists in the numerous bands of dark flints which run through the Upper Chalk parallel to the strata. The Lower Chalk is entirely, and the Middle Chalk nearly, devoid of flint. Though the line at which the commencement of the Upper Chalk is taken is rather below the first flint band of the Upper Chalk, and a few flints occur in the highest beds of the Middle Chalk; yet, speaking generally, the great distinction between the Middle and Upper Chalk, the two divisions of the White Chalk, may be considered to be that of flintless chalk and chalk with flints.