with bones of enormous land-reptiles, and terrestrial plants and trees, the whole group having evidently been a vast delta.

The essential characters of these formations, and of the subdivisions into which they are separated, are concisely expressed in the annexed table.

CHRONOLOGICAL SEQUENCE.

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ORDER OF SUPERPOSITION. -Such is the chronological sequence of the formations of the south-east of England; in other words, these several groups of marine and freshwater sediments have been deposited in the following order :

:

1st, The Wealden: the delta of a river, composed of the spoils of a vast continent, of which no other vestiges remain. These strata rest upon a marine formation called the Oolite, but which does not come within the scope of our present observations.

2ndly, The Chalk: the bed of an ocean of great extent, deposited upon the Wealden.

3dly, The Tertiary Eocene Formation: accumulated in depressions or basins on the surface of the Chalk.*

Now, as each of these formations is many hundred feet in thickness, it follows that the inferior strata would for ever have been concealed from observation, had the original order of superposition remained undisturbed. But not only portions of each of the upper series appear on the surface, but even the lowermost deposits form an extensive tract of country which is diversified by hills and valleys, and constitutes one of the richest agricultural provinces of the British Isles. This distribution of the strata has originated from the displacements occasioned by those disturbing forces which elevated above the waters the ancient ocean and river beds, and converted them into dry land. The nature of those changes we now proceed to consider.

LONDON EOCINE FORMATION.-The London Tertiary or Eocene deposits form the area on which the metropolis is situated, as well as the surrounding country to a distance varying from ten to twenty or thirty miles. Around these strata the Chalk presents a distinct boundary of hills on

* With the view of simplifying the subject, the Drift, or superficial alluvial deposits, will not be especially alluded to in these excursions.

the south, west, and north; but on the east the range is broken, and though this chasm the Thames winds its way to the sea.

From this geological character of the metropolitan district it results, that all the lines of railway that proceed from London traverse, for the first ten or twenty miles, beds of clay, loam, and loosely aggregated sand and gravel; hence the numerous slips that have taken place in the embankments at New Cross, Wandsworth, Hanwell, &c.; and in all these lines it is obvious, from the nature of the soil and the steepness of the cuttings, that similar subsidences will again occur.

CHALK FORMATION.-The next geological feature observable along the London railways is the Chalk, which is invariably traversed by steep cuttings and tunnels; as, for example, in the South-eastern line, from Croydon to Merstham; and in the Great Western, from Maidenhead to beyond Wallingford. After passing through the chalk, the lower subdivisions of the cretaceous formation, viz., the Marl, Firestone, Galt, and Greensand, successively appear; and these are followed by the Oolite on the Great Western and Birmingham lines, and by the Wealden on the SouthEastern; but on the South-Western, by which we shall travel to Southampton, the chalk and the tertiary strata only are exposed.

The chalk, as is well known, constitutes the prevalent geological feature of this part of England. The upper or white chalk rises into chains of hills, called downs, which are remarkable for their smooth and undulated outline, and form a well-defined geographical boundary. The southern range extends from Hampshire, through Sussex to the seacoast, terminating in the bold promontory of Beechy Head; the northern passes from Hampshire through Surrey and Kent to the British Channel, and ends in the line of cliffs near Dover.