4.). Lastly, the sea would penetrate to the sand (No. 5.), and then the state of things indicated in the dark lines of the upper section (fig. 253.), would be consummated. [Illustration: Fig. 261. The Coomb, near Lewes.] It was stated that there are many lines of flexure and dislocation, running east and west, or parallel to the central axis of the Wealden. They are numerous in the district of the Hastings sand, and sometimes occur in the chalk itself. One of the latter kind has given rise to the ravine called the Coomb, near Lewes, and was first traced out by Dr. Mantell, in whose company I examined it. This coomb is seen on the eastern side of the valley of the Ouse, in the suburbs of the town of Lewes. The steep declivities on each side are covered with green turf, as is the bottom, which is perfectly dry. No outward signs of disturbance are visible; and the connection of the hollow with subterranean movements would not have been suspected by the geologist, had not the evidence of great convulsions been clearly exposed in the escarpment of the valley of the Ouse, and the numerous chalk pits worked at the termination of the Coomb. By the aid of these we discover that the ravine coincides precisely with a line of fault, on one side of which the chalk with flints (_a_, fig. 262.), appears at the summit of the hill, while it is thrown down to the bottom on the other. Mr. Martin, in his work on the geology of Western Sussex, published in 1828, threw much light on the structure of the Wealden by tracing out continuously for miles the direction of many anticlinal lines and cross fractures; and the same course of investigation has since been followed out in greater detail by Mr. Hopkins. The mathematician last-mentioned has shown that the observed direction of the lines of flexure and dislocation in the Weald district coincide with those which might have been anticipated theoretically on mechanical principles, if we assume certain simple conditions under which the strata were lifted up by an expansive subterranean force. He finds by calculation that if this force was applied so as to act uniformly upwards within an elliptic area, the longitudinal fissures thereby produced would nearly coincide with the outlines of the ellipse, forming cracks, which are portions of smaller concentric ellipses, parallel to the margin of the larger one. These longitudinal fissures would also be intercepted by others running at right angles to them, and both lines of fracture may have been produced at the same time.[251-A] In this illustration it is supposed that the expansive force acted simultaneously and with equal intensity at every point within the upheaved area, and not with greater energy along the central axis or region of principal elevation. [Illustration: Fig. 262. Fault in the cliff hills near Lewes. Mantell. _a._ Chalk with flints. _b._ Lower chalk.[251-B]]