DESCRIPTION OF THE GEOLOGICAL MAP OF THE ISLE OF WIGHT.

To render this map instructive to the general reader, it may be necessary to explain the principle on which geological charts are constructed. The intelligent observer is of course aware that the surface of every country is more or less covered by vegetable soil, and that beneath this superficial layer of earth there are beds of loam, clay, sand, gravel, &c., of various thickness and extent, which often abound in boulders and large waterworn masses of stone, derived either from the strata of the surrounding district, or from the rocks of distant countries, and have been transported at some remote period to their present situation; they frequently contain bones of extinct species of elephant, horse, ox, deer, &c. These accumulations of alluvial detritus were formerly distinguished by the general name of Diluvium, or diluvial deposits, and their formation was ascribed to the effects of an universal deluge; but they are now called "Drift," a term significant of their origin from aqueous or glacial transport. (See ante, p. 40.)

These heterogeneous deposits are distributed over the fundamental strata of the country, which they conceal from observation, except where the latter are exposed to view by natural sections as sea-cliffs, ravines, and precipitous escarpments or by artificial openings, as quarries, railway works, well-sinkings, &c. The geographical distribution of the regular deposits beneath the superficial drift and vegetable earth is indicated by the colours on the map; the

section from West Cowes across the Island to the Undercliff, shows the order of succession of the three groups of strata composing the Isle of Wight (ante, p. 41); or, in other words, the manner in which these have been deposited upon each other. This order of superposition determines the chronology or relative antiquity of each formation, for it is obvious that the lowermost beds must be more ancient than those which lie upon them. The strata, though originally deposited in horizontal layers, are now more or less inclined, in consequence of the subterranean disturbances which have taken place since their original deposition and consolidation; hence the varied aspect of the physical geography of the district.

The geological map is intended to demonstrate the order and succession of the principal groups of deposits, and the lithological structure of the rocks distributed over a given area, and the nature of the organic remains they may contain. The three natural groups or formations composing the Isle of Wight are expressed by distinct colours. Thus the WEALDEN beds are indicated by the sienna tint, the CRETACEOUS by blue, and the TERTIARY or EOCENE by pink: the subdivisions are defined by lines of shading. The following summary of the characters of the several formations described in the body of the work, will enable the tourist to comprehend at a glance the geological phenomena of the Island.

I. THE WEALDEN (ante, p. 197). This formation consists of an extensive series of clays, sands, sandstones, and limestones, the aggregated thickness of which is upwards of 1,000 feet. The strata are characterised by the abundance of univalve and bivalve shells, and minute crustaceans, of species that exclusively inhabit rivers, lakes, and pools of freshwater. With these are associated bones, teeth, and scales of marine and fluviatile fishes; and the bones and teeth of gigantic terrestrial and aquatic reptiles; and the

stems, foliage, and fruit of pines, firs, cypresses, cycadeous plants, ferns, and other land vegetables. But few vestiges of marine plants and animals have been observed.

The line of cliffs extending from the west of Atherfield Point to near Compton Bay, and that from Redcliff in Sandown Bay to the Fort (coloured sienna on the map), are natural sections of the Wealden deposits; and along the shores in these localities bones of enormous extinct reptiles are continually exposed by the encroachments of the sea on the wasting cliffs.

II. The CRETACEOUS or CHALK FORMATION (ante, p. 126), comprises the strata which overlie and conceal the Wealden, and constitute the southern half of the Island, as shown by the blue colour. The lowermost series (the greensand) consists of sands and clays, with bands of cherty limestone; above this group is a thick bed of stiff blue marl or clay, termed galt, or Folkstone marl; on this are strata composed of marl, with an intermixture of particles of green silicate of iron, the consolidated layers of which constitute the limestone provincially called firestone, a term now generally employed by geologists to designate these arenaceous marly deposits. The uppermost series of the cretaceous system consists of strata of white limestone, composed of the substance generally known as chalk, with layers and nodules of flint. The four subdivisions of this extensive formation, namely, the lower greensand, galt, firestone, and chalk, amount in aggregate thickness to nearly 1,500 feet. They are severally expressed on the map by different lines of shading.

The fossils of the cretaceous system are entirely marine, with the exception of a few vestiges of terrestrial reptiles and plants, analogous to those of the Wealden. They consist of many hundred species of extinct genera of bivalves and univalves, and of corals and other zoophytes; of crustaceans, star fishes, crinoideans, sea-urchins, and

numerous fishes. The subdivisions of the formation are characterised by peculiar fossils; but throughout the entire system there is a general resemblance in the predominating types of animal and vegetable forms, showing that the various strata were deposited in the basin of the same ocean, and under like physical conditions. From the variable composition of the cretaceous strata much diversity has been imparted to the geographical features of the tracts they occupy. The picturesque character of the bold escarpments produced by the disrupted masses of the upper chalk is displayed in the beautiful sketches of Freshwater Bay in "Barber's Illustrations of the Isle of Wight," p. 92, and of Scratchell's Bay and the Needles (ibid. p. 97). The charming effect occasioned by the degradation of the cliffs of the lower greensand deposits, is exemplified in the views of Blackgang Chine (ibid. p. 83), and Shanklin Chine (ibid. pp. 64, 66); and the lovely scenery to which the faults and slips of the firestone and galt strata have given rise, is shown by that part of the southern coast known as "THE UNDERCLIFF," and delineated in the sketches of Bonchurch, Ventnor Cove (ibid. p. 77), and Puckaster Cove (ibid. p. 80); while the fine chain of Downs, rising in many places to an altitude of 800 or 900 feet, exhibits the usual features of the smooth rounded masses intersected by gentle valleys, so characteristic of the mountain ranges of the white chalk strata. St. Boniface Down (ibid. p. 73), and the sketch of the Down behind Carisbrook Castle (ibid. p. 47), are examples of this scenery.

III. The EOCENE, or Ancient Tertiary System (ante, p. 55).—The strata above the chalk extend over the northern half of the Island. They are subdivided into two groups; the lowermost consists chiefly of clays, with alternations of layers of sand, gravel, loam, &c., abounding in marine shells, crustaceans, fishes, &c., of extinct species, and distinct from any that have been observed in the

344 DESCRIPTION OF THE GEOLOGICAL SCENERY.

cretaceous strata on which these beds are superposed. This group is commonly designated the London Clay, because the strata and fossils correspond in character with those on which the metropolis is situated. It extends from White Cliff Bay on the eastern extremity of the Island, along the northern flank of the Chalk Downs to the well-known cliffs of Alum Bay.

The upper tertiary series consists of sands, clays, and bands of good building limestone, abounding in freshwater shells and vegetables, many of the layers are composed of various kinds of lacustrine snails and aquatic plants, consolidated by an infiltration of carbonate of lime. In these deposits bones and teeth of extinct mammalia allied to the tapir, peccary, (ante, p. 115,) &c., and bones of crocodiles and turtles, are occasionally found. These strata form the northern coast of the Island; their extent is indicated on the map by the unshaded pink tint.

The scenery of the tertiary strata, though less bold than that of the tracts formed by the chalk, is in many localities eminently beautiful; the gentle valleys and sequestered dells and coves dispersed here and there over the northern district and along its shores, imparting a charming variety to the landscape. Brading, Bembridge, Ryde, Cowes, her Majesty's Marine Residence, and Yarmouth, are situated on these deposits.