Page:Quarterly Journal of the Geological Society of London, vol. 31.djvu/96

50 50 J. PRESTWICH ON THE QUATERNARY PHENOMENA included in some part of the island the lower beds of the Middle Series, of which both the fossils and lithological fragments are so abundant at the Bill. The iron-sandstone grit and the fragments of Sarsen-stone of Tertiary age may be derived indirectly from the drift bed of the Admiralty Quarries, which then also was, no doubt, of greater extent. I therefore infer that, probably in consequence of some major dis- turbance at a distance, the Isle of Portland, together with the whole line of coast from the Land's End to the Straits of Dover, as well as the opposite coast of France, was gradually submerged, and that the sea, the boundaries of which are marked by the raised beaches on both coasts, encroached upon the adjacent land ; that this submer- gence was only temporary ; that the emergence, at first gradual, was marked by short oscillations, which, according to their relative force and duration, swept down the soil with its land shells and softer beds, alternately with the coarser materials and the bones of animals drowned by the inundation, spreading first one and then the other in irregular beds and lenticular masses ; while the final emergence, more sudden and consequently of greater effect, swept down the overlying coarser debris. By such agency, I conceive, were the Middle Purbecks denuded and swept seaward to the Bill of Port- land ; while at Chesilton the coarse debris with the large blocks of chert and stone were transported in another direction, large and small together, and spread out far beyond the foot of the imme- diate escarpment. Or, again, it is possible that a succession of waves caused by earth- quake-movements may have swept at short intervals over the adja- cent land. In any case, as the "angular debris" comes down to the sea-level at Chesilton, again at Sangatte, and elsewhere, I think it clear that, whatever the disturbance, one effect was to raise suddenly the old beaches which fringed the then coast to their present height above sea-level. A review of the extent of the submergence is beyond the limits of the present paper ; but from the fact that at Chesilton the great mass of the debris and the large blocks of Portland flint come from beds now from 350 to 450 feet above the sea in that part of the island, and from their great size and the force necessary to remove them, we may assume not only that the highest summit of Portland was submerged* but also that there must have been above it a column of water of some height and power. Such a comparatively sudden change of the relative level of land and sea could not have been effected without great disturbance ; and we have some evidence of that in the partly open fissures by which Portland is traversed, and especially by those near the Bill, where I have shown that, after the formation of the old Eaised Beach, rents were formed in that beach and in the subjacent rocks, into which the more recent loam and angular debris must have been let down while apparently in a semifluid state or under water. Of the relative age of the old " Land-wash " (which is the name I would suggest for this deposit), and of the several small drift beds