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 



