Local disturbances caused the existence of a shallow water region in the east during the deposition of the Middle and Upper Eocene deposits, and accordingly the well-marked marine deposits which form the representatives of these divisions in Hampshire are replaced by the Bagshot beds of the London Basin, consisting chiefly of coarse mechanical sediments with a poor marine fauna, but even in the west shallow water prevailed at times during the accumulation of various plant-bearing strata. The Middle Eocene beds only are found in the Bovey Tracey outlier, though the Upper Eocene beds may originally have been laid down in that area, and subsequently denuded. The fourth area displays a very different succession of Eocene strata, and one of extreme interest. Mechanical sediments and plant-bearing clays and lignites alternate with a vast accumulation of basaltic lavas, indicating the outbreak of the volcanic forces in the British area, after a period of quiescence which lasted through the greater part of Mesozoic times. The region in which these lavas were poured out was probably a land area during the greater part of the period of volcanic activity, but the horizontal lie of the lava flows and their wide extent indicate the existence of a flat tract of country, gradually raised into a plateau by the accumulation of sheet over sheet of basalt. How far this plateau extended it is impossible to say. The distribution of the lavas at the present day is somewhat limited in our isles, but there is no sign of dying out at the present margins of the accumulations, and they have probably escaped denudation in these regions, as maintained by Professor Judd, on account of the faults which have depressed them, while the portions which were not depressed have been removed by denudation. Two views as to the origin of the lavas have been put forward: according to Prof. Judd, they were poured forth from gigantic volcanoes, while Sir A. Geikie maintains that they represent portions of massive or fissure eruptions, the molten rock having welled out from great cracks in the earth, which are now filled by once molten rock in the form of dykes. As these dykes extend far away from the present volcanic plateau, one actually extending to the Yorkshire coast, we may well believe, whatever was the origin of the sheets of lava, that they were formerly spread far away from their present terminations[104]. Without entering here into a discussion of the exact nature of extrusion of these igneous sheets, it will suffice to say that all the evidence points to the formation of extensive plateaux, which must have presented a fairly uniform surface, similar to that which is still found characterising the volcanic districts of the western territories of North America.