MAP OF THE BRITISH ISLES AND PART OF THE NORTH-WEST OF EUROPE,
SHOWING THE GREAT AMOUNT OF SUPPOSED SUBMERGENCE OF LAND
BENEATH THE SEA DURING PART OF THE GLACIAL PERIOD.

The submergence of Scotland is to the extent of 2,000 feet, and of other parts of the British Isles, 1,300.

In the map, the dark shade expresses the land which alone remained above water. The area shaded by diagonal lines is that which cannot be shown to have been under water at the period of floating ice by the evidence of erratics, or by marine shells of northern species. How far the several parts of the submerged area were simultaneously or successively laid under water, in the course of the glacial period, cannot, in the present state of our knowledge, be determined.

CHAP. XIV.

IN BRITISH PHYSICAL GEOGRAPHY.

277

It appears from soundings made during various Admiralty surveys, that the gained land thus brought above the level of the sea, instead of presenting a system of hills and valleys corresponding with those usually characterising the interior of most of our island, would form a nearly level terrace, or gently inclined plane, sloping outwards like those terraces of denudation and deposition which I have elsewhere described as occurring on the coasts of Sicily and the Morea.*

It seems that, during former and perhaps repeated oscillations of level undergone by the British Isles, the sea has had time to cut back the cliffs for miles in many places, while in others the detritus derived from wasting cliffs drifted along the shores, together with the sediment brought down by rivers and swept by currents into submarine valleys, has exerted a levelling power, filling up such depressions as may have pre-existed. Owing to this twofold action few marked inequalities of level have been left on the sea-bottom, the silver-pits' off the mouth of the Humber offering a rare exception to the general rule, and even there the narrow depression is less than 300 feet in depth.

Beyond the 100 fathom line, the submarine slope surrounding the British coast is so much steeper that a second elevation of equal amount (or of 600 feet) would add but slightly to the area of gained land; in other words, the 100 and 200 fathom lines run very near each other.t

The naturalist would have been entitled to assume the former union, within the post-pliocene period, of all the British Isles with each other and with the continent, as expressed in the map, fig. 41, even if there had been no geological facts in favour of such a junction. For in no other way would he be able to account for the identity of the fauna and flora found throughout these lands. Had they been separated ever since

* Manual of Geology, p. 74.

† De la Beche, Geological Researches, p. 191.

MAP SHOWING WHAT PARTS OF THE BRITISH ISLANDS WOULD REMAIN ABOVE WATER AFTER A SUBSIDENCE OF THE AREA TO THE EXTENT OF 600 FEET.

The authorities to whom I am indebted for the information contained in this map are for

SCOTLAND.-A. Geikie, Esq., F.G.S. and T. F. Jamieson, Esq., of Ellon, Aberdeenshire.

ENGLAND. For the counties of Yorkshire, Lancashire, and DurhamCol. Sir Henry James, R.E,

Dorsetshire, Hampshire, and Isle of Wight - H. W. Bristow, Esq.

Gloucestershire, Somersetshire, and part of Devon R. Etheridge, Esq. Kent and Sussex-Frederick Drew, Esq.

IRELAND.-Reduced from a contour map constructed by Lieut. Larcom, R.E., in 1837, for the Railway Commissioners.

MAP OF PART OF THE NORTH-WEST OF EUROPE, INCLUDING THE BRITISH
ISLES, SHOWING THE EXTENT OF SEA WHICH WOULD BECOME LAND IF
THERE WERE A GENERAL RISE OF THE AREA TO THE EXTENT OF 600
FEET.

From Sir H. De la Beche's 'Geological Researches,' p. 190, 1834.

The darker shade expresses what is now land, the lighter shade the space intervening between the present coast line and the 100 fathom line, which would be converted by such a movement into land.

280

REVOLUTIONS IN PHYSICAL GEOGRAPHY.

CHAP. XIV.

the miocene period, like Madeira, Porto Santo, and the Desertas, constituting the small Madeiran Archipelago, we might have expected to discover a difference in the species of land-shells, not only when Ireland was compared to England, but when different islands of the Hebrides were contrasted one with another, and each of them with England. It would not, however, be necessary, in order to effect the complete fusion of the animals and plants which we witness, to assume that all parts of the area formed continuous land at one and the same moment of time, but merely that the several portions were so joined within the post-pliocene era as to allow the animals and plants to migrate freely in succession from one district to another.

Southernmost Extent of Erratics in England.

In reference to that portion of the south of England which is marked by diagonal lines in the map at p. 276, the theory of its having been an area of dry land during the period of great submergence and floating-ice does not depend merely on negative evidence, such as the absence of the northern drift or boulder clay on its surface; but we have also, in favour of the same conclusion, the remarkable fact of the presence of erratic blocks on the southern coast of Sussex, implying the existence there of an ancient coast-line at a period when the cold must have been at its height.

These blocks are to be seen in greatest number at Pagham and Selsea, fifteen miles south of Chichester, in lat. 50° 40'N.

They consist of fragments of granite, syenite, and greenstone, as well as of Devonian and Silurian rocks, some of them of large size. I measured one of granite at Pagham, twenty-seven feet in circumference. They are not of northern origin, but must have come from the coast of