362

PROMINENCE OF HARDER STRATA. [Ch. XIX.

strata, 2, 3, 4, 5, 6, represented in the map, fig. 355., and in thesection, fig. 356., may have been brought into their present position,the following hypothesis has been suggested: — Suppose the fiveformations to lie in horizontal stratification at the bottom of the sea;then let a movement from below press them upwards into the formof a flattened dome, and let the crown of this dome be afterwards cutolf, so that the incision should penetrate to the lowest of the fivegroups. The different beds would then be exposed on the surface,in the manner exhibited in the map, fig. 355.*

The quantity of denudation, or removal by water, of stratifiedmasses assumed to have once reached continuously from the Northto the South Downs is so enormous, that the reader may at first bestartled by the boldness of the hypothesis. But the difficulty willdisappear when once sufficient time is allowed for the gradual risingand sinking of the strata at many successive geological periods,during which the waves and currents of the ocean, and the power ofrain, rivers, and land-floods, might slowly accomplish operationswhich no sudden diluvial rush of waters could possibly effect.

Among other proofs of the action of water, it may be stated thatthe great longitudinal valleys follow the outcrop of the softer andmore incoherent beds, while ridges or lines of cliff usually occur atthose points where the strata are composed of harder stone. Thus,for example, the chalk with flints, together with the subjacent uppergreensand, which is often used for building, under the provincialname of “ firestone,” have been cut into a steep cliff on that side onwhich the sea encroached. This escarpment bounds a deep valley,excavated chiefly out of the soft argillaceous bed, termed gault(No. 3., map, p. 356.). In some places the upper greensand is in aloose and incoherent state, and there it has been as much denuded asthe gault; as, for example, near Beachy Head; but farther to thewestward it is of great thickness, and contains hard beds of bluechert and calcareous sandstone or firestone. Here, accordingly, wefind that it produces a corresponding influence on the scenery of thecountry; for it runs out like a step beyond the foot of the chalk-liills, and constitutes a lower terrace, varying in breadth from aquarter of a mile to three miles, and following the sinuosities of thechalk-escarpment.

Fig. 363.

d

a. Chalk with flints. b. Chalk without flints.

c. Upper greensand, or firestone. d. Gault.

* See illustrations of this theory, hy Sussex, &c., Geol. Trans., Second Series,Dr. Fitton, Geol. Sketch of Hastings. vol. ii. p. 98.f Sir R. Murchison, Geol. Sketch of