Page:Philosophical Transactions of the Royal Society A - Volume 184.djvu/1096

Rh assumption, but by the agreement of the consequences which it involves with the facts, and by the extent to which it satisfies the various conditions of the problem.

Another objection may be raised on the grounds that the thickness of the Earth's crust is such as to render the movements spoken of improbable; but there again the physical data upon which that thickness has been estimated depend upon observations of such extreme delicacy that they scarcely yet afford a sure basis for calculation, whilst the geological facts are antagonistic to the great thickness and rigidity contended for. No conclusion can, at all events, be correct which does not satisfy the geological as well as the physical conditions.

[Whatever may be the present rigidity of the crust of the Earth, the facts I have briefly alluded to in the preceding pages respecting the presence of Raised Reaches at many different levels and at great heights on the shores of the Mediterranean, show how great its mobility must have been throughout Quaternary times, and up to the date of the Rubble-drift. The latter merely marks the last stage of a long series of earth movements of variable intensity and duration.—J. P., June, 1893.]

Though some geologists may not admit that the presence of marine shells at the height of 1300 feet, on Moel Tryfaen, is a proof of upheaval of the sea-bed, none deny that the terraces and shell beds in Norway and Sweden, at heights of from 200 to 600 feet, have been caused by elevation of the land during the same Quaternary times with which we are now dealing. The main question for consideration in connection with these great earth movements is the rate at which the upheavals have been effected. Here again, for reasons before stated, I cannot hold that the present affords a just criterion for the past; but as these are questions which I have discussed elsewhere, I merely refer to them here that the geological argument may not be overruled by postulates that are not founded on a more certain basis.

Other, and to my mind more serious, objections may, however, be raised, which will require careful investigation, such, for instance, as that there is nothing to show, in the absence of marine sediments, the occupation of the land by the sea waters. But, if that occupation were of short duration, it would not have been possible for ordinary sedimentation to have taken place; it could only have been such sedimentation as might have fallen in a limited time from the turbid superincumbent waters—like that which now forms the beds of brick-earth or Loess in the Channel Islands and in certain continental areas. Nor would there, for the same reason, have been time for the migration and establishment of a marine fauna on the submerged area. I can only conclude from these and other circumstances that the submergence of the land was of short duration, in which case this objection would no longer hold.

The questions, also, which arise in connection with the effect that such a wide-spread submergence would have upon both the land and the marine life of the period, present many difficulties. How far both the land and sea faunas would suffer, and in