Page:Quarterly Journal of the Geological Society of London, vol. 26.djvu/750

548 Solent has become an arm of the sea by the severance of the Isle of Wight from the mainland.

(e) The last movement appears, however, to have been one of subsidence. Submerged forests have been observed in Poole Harbour, off Bournemouth, off Southsea Castle, off Pagham, and at Portsmouth. Sir Charles Lyell adopts the conclusion of the Bishop of Gibraltar, that the Bournemouth forest was submerged by the washing out of a sandy substratum without a general subsidence of the land, and explains the occurrence of the similar deposits on the north side of Poole Harbour in the same manner. The section at Portsmouth, which was described by Sir H. James, appears, however, to afford good evidence of a downward movement. An old terrestrial surface with rooted trees which, when living, must have stood at least 30 feet higher, was exposed in the dockyard in 1847, and has been again brought to light, but at a higher level, in the works for the extension of the dockyard now in progress. The origin of Poole Harbour and Christchurch Harbour, and of Portsmouth, Langston, and Chichester Harbours, is probably in a great measure due to this subsidence, which, according to Mr. Godwin-Austen, there is some reason to believe has gone on till within the last few centuries.

(f) The denudation which the surface of the country has undergone since the deposition of the gravel on the plains is as remarkable for its partial character as for its extent. Large areas of the plains at the highest levels appear to have remained quite unchanged, while close by deep valleys have been scooped out, and high escarpments have been formed by what cannot have been any other than subaërial causes, and while large tracts of land have been gradually removed by the action of the sea on the neighbouring coast-line. The nature of the geological formation appears to have had no part in influencing the destruction or the preservation of the surface. Barton Clay, Bagshot Sand, and Headon Marls alike underlie the gravel of the New-Forest tableland, and have alike been removed over the area drained by the tributaries of the Boldre river, of which Lyndhurst is the centre. The gravel covering itself is probably the protective agent, and it may have been thinner or more loamy where the old surface has been destroyed.

The chief points to which attention has been called in the foregoing paper are as follows:—

1. That the gravel plains of the New Forest and the neighbourhood are portions of a tableland rising slightly to the northward and attaining an elevation of 420 feet: and that the remains of similar tablelands exist on the east side of Southampton Water,