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Show 304 DESCRIPTION OF TilE PLATES AND MAP. formation of the valley. The great lava currents of 1819 and 1811, described in the first volume (p. 367), are seen pouring down fl'Om the higher parts of the valley, overrunning the forests of the great plain, and rising up in the foreground on the left with a rugged surface, on which small hillocks and depressions are seen, such as often characterize a lava-current immediately after its consolidation. The small cone, No. 7, was formed in 1811, and was still smoking when I saw it in 18~8. Immediately in front of it is seen another cone, formed during the same eruption. The other small volcano to the left, from which vapour is issuing, was formed, I believe, in 1819. This sketch, which forms part of a panoramic drawing which I made in November 18~8, is merely intended to assist the reader, in comprehending some geological details into which we shall hereafter enter, on the structure of the older portion of Etna, but it will give no idea of the extraordinary geological interest, still less of the picturesque grandeur of this magnificent scene of desolation. · Nor is the view sufficiently extensive to exhibit the entire form of the vast amphitheatre, part only of the northern, and scarcely any of the southern boundary of which is included. MAP Shewing the extent of Surface in Europe which has been covered by Water since the Deposition of the olde·r Tertia·ry Strata. (St-rata of the Paris and London Basins, q.c.) (Constructed chiefly from M. Amie Boue's Geological Map of Europe.] This map will enable the reader to perceive at a glance the great extent of change in the physical geography of Europe, which can be proved to have taken place since some of the older tertiary strata were deposited. 'rhe most ancient part of the period to which the map refers cannot be deemed DESCRIPTION OF THE PLATES AND MAP. 305 very remote, considered geologically, because the deposits of the Paris and London basins, of Auvergne, and many other districts belonging to the older tertiary epoch, are newer than the greater part of the sedimentary rocks of which the crust of the globe is composed. The species, moreover, of marine and fresh-water testacea, of which the remains are found in these formations, are not entirely distinct from such as now live; a proportion of about three in a hundred of the fossil species having been identified with species now living. Yet, notwithstanding the comparatively recent epoch to which the retrospect is carried, the variations in the distribution of land and sea depicted on the map, form only a part of those which must have taken place during the period under consideration. Some approximation has merely been made to a correct estimate of the amount of sea converted into land in that part of Europe best known to geologists, but we cannot determine how much land has become sea during the same period; and there may have been repeated interchanges of land and water in the same places, mutations of which no account is taken in the map, and respecting the amount of which little accurate information can ever be obtained by geologists. The proofs of submergence, during some part of the tertiary period, throughout the districts distinguished by ruled lines, are of a most unequivocal character; for the area thus described is now covered by deposits, containing the remains of aquatic animals belonging to tertiary species. We have, indeed, extended the sea in two or three instances beyond these limits, because other geological data have been obtained for inferring the submergence of these tracts subsequently to the commencement of the deposition of the tertiary strata. Thus we shall explain, in the next volume, our reasons for concluding that part of the chalk of England, (the north and south downs, for example, together with some other adjoining secondary tracts,) continued beneath the sea until the older tertiary beds had begun to accumulate. It is possible also that a considerable part of Caernarvon- VoL. II. X |
