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Show 294 EOCENE PERIOD. [Ch, XXI. lines, which form ridges and troughs running nearly parallel to each other. Much of the picturesque character of the scenery of this dis-trict arises from the depth of the narrow valleys and ridges to which the sharp bends and fractures of the strata have given 1·ise; but it is also in part to be attributed to the excavating power exerted by water, especially on the intcrstratificd argil-laceous beds. From the above description it will appear that, in the tract intervening between the North and South Downs, there are a series of parallel valleys and ridges; the valleys appearing evidently to have been formed principally by the removal of softer materials, while the ridges are due to the resistance offered by firmer beds to the destroying action of water. Rise and denudation of the strata gradual.-Let us then consider how far these phenomena agree with the changes which we should naturally expect to occur during the gradual rise of the secondary strata. Suppose the line of the most violent movements to have coincided with what is now the central ridge of the Weald Valley ; in that case, the first land which emerged must have been situated where the Forest ridge is now placed. Here a number of reefs may have existed, and islands of chalk, which may have been gradually devoured by the ocean in the same manner as Heligoland and other European isles have disappeared in modern times, as related in our first volume ''If.. Suppose the ridge or dome first elevated to have been so rent and shattered on its summit as to give more easy access to the waves, until at length the masses represented by the fainter l!nes in the annexed diagram were removed. Two strips of land 1mght No.69. * Page 289, and Second Edition, page 330. Ob.XXI.] ALLUVIUM OP WEALD VALLEY. 295 the opposite coasts of France and England, composed of chalk, present ranges of white cliffs facing each other. A powerful current might then rush, like that which now ebbs and flows through the straits of Dover, and might scoop out a channel in the gault. We must bear in mind that the intermittent action of earthquakes would accompany this denuding process, fissuring rocks, throwing down cliffs, and bringing up, from time to time, new stratified masses, and thus greatly accelerating the rate of waste. If the lower bed of chalk on one side of the channel should be harder than on the other, it would cause an under terrace, as represented in the above diagram, resembling that presented by the upper green-sand in parts of Sussex and Hampshire. When at length the gault was entirely swept away from the central parts of the channel, the lower green-sand (3, diagram No. 70,) would be laid bare, and portions of it would No.70. Tile dotted tine represents the sea-level, become land during the continuance of the upheaving earth~ quakes. Meanwhile the chalk cliffs would recede farther from one another, whereby four parallel strips of land, or perhaps rows of islands, would be caused. The edges of the argillaceous strata, No. 2, are still exposed to erosion by the waves, and a portion of the clay, No. 4, is already removed. This clay, as it gradually rises, will be swept ofF from part of the subjacent group, No. 5, which will then be laid bare, and may afterwards become land by subsequent elevation. Why no r~ins of challc on central dist·rict.-By this theory of the successive emergence and denudation of the groups 1 2 3 ' ) ) '4, 5, we may account for an alluvial phenomenon which seems inexp1icable on any other hypothesis. The summits of the chalk downs are covered everywhere with flint gravel, which |