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Sir Joseph Prestwich has tried to work the problem out. Now-a-days Kent and Sussex run eastwards in five more or less parallel ridges, not far short of 1,000 feet high, with deep valleys between. Formerly, however, no such valleys existed, and a great dome of chalk, some 2,500 feet high at its crown, perhaps, though others would say less, covered the whole country.

Professor Prestwich also points out that if the extrusion of lava were due to the elastic force of vapour of water there should be a distinct relation between the discharge of the lava and of the vapour; whereas the result of an examination of a number of well-recorded eruptions shows that the two operations are not related, and are, in fact, perfectly independent.

Prestwich, in his admirable Memoirs on the eocene deposits of England and France, is able to draw a close general parallelism between the successive stages in the two countries; but when he compares certain stages in England with those in France, although he finds in both a curious accordance in the numbers of the species belonging to the same genera, yet the species themselves differ in a manner very difficult to account for, considering the proximity of the two areas, unless, indeed, it be assumed that an isthmus separated two seas inhabited by distinct, but contemporaneous, faunas.

But the existence in Western America and other volcanic countries of fissures of eruption along which molten lava has been extruded without explosions of steam, shows that water is not an essential factor in the production of volcanic phenomena; and, as Professor Prestwich has clearly demonstrated, it is to be regarded as an element in volcanic explosions, rather than as a prime cause of volcanic action.