THE

INTRODUCTION.

HE existing geographical and physical contours of the British Islands are the outcome of the long and varied geological history which this particular part of the world possesses. There are few other regions of the earth's surface which in so small an area exhibit so many different systems of rocks as are to be found in the British Islands. England alone includes portions of nearly all the rock-systems which are found on the continent of Europe.

Each of these rock-groups was formed during the maintenance of certain physical and geographical conditions, and the restoration of the particular conditions which prevailed during any one period will reveal one phase in the geographical evolution of Britain. Further, in the multiplicity of rock-groups we have proof that changes in the physical conditions of the area have been frequent, and it becomes evident that the history of this evolution is a long one; it is, in fact, a history of alternate upheaval and depression, of the repeated formation of islands and continents, and of their subsequent detrition and submergence. In this long succession of changes there have been many different arrangements of land and sea over the area where the British Isles now stand; every part of our country has been repeatedly depressed beneath the sea, though some parts have been submerged much more fre

quently than others, and conversely every part of the area which is now covered by the environing seas has more than once been part of the dry land. Sometimes nearly the whole area has formed part of continental land, and at other times it has been almost entirely submerged beneath the sea.

But we can look back to an epoch in the geographical history of Britain when the foundations of our islands had been laid, and when the older and more mountainous parts of the country had been brought into the relative positions which they now occupy. We may, in fact, regard the Paleozoic districts of the British Islands as portions of ancient lands which have been broken up and reduced to their present dimensions during successive periods of erosion and denudation; these tracts and the vanished lands of which they formed part have yielded the materials that compose the more recent (Neozoic) strata, and there were at least three periods in these later times when Neozoic strata filled up the gaps between the older blocks of ground and welded the British Islands into a continental whole. Erosion and submergence, however, again began the work of destruction and separation, with the ultimate result of reducing them once more to the state of islands and of giving them the outlines which they now present.

Before proceeding to describe the successive phases of this process of geographical evolution, some consideration of the evidence on which we have to rely in trying to restore the geography of any period seems to be desirable. The greater part of the strata with which the geologist has to deal are marine deposits, and it is only rarely that actual proof of the former existence of land in any district is found in the intercalation of purely freshwater deposits. Occasionally we meet with estuarine beds which indicate the close proximity of land; and of more frequent occur

rence are conglomerates and breccias recalling those which are formed along modern beaches. But in many cases the position of the land tracts during a given period can only be inferred from general considerations, such as the changes in the lithological characters of the sediments, the thinning or thickening of beds in certain directions, their entire absence in certain areas, their conformity or unconformity to the underlying rocks.

Shallow-water Beds.-In the first place, therefore, we must know what kinds of rock are likely to have been formed in shallow water. As a general rule, the coarser the grain of the rock the shallower was the water in which it was formed, and the nearer was the shore from whence the component materials were derived, or along which they were moved. Conglomerates consisting of stones which could only be moved by the beating of waves on a shore are, of course, decided evidence of the close neighbourhood of land to the spot where we now find them, and if they contain large angular or sub-angular boulders we may infer that they have been formed below a line of cliffs, and further, if such deposits form a mass of considerable thickness, it is probable that much of the material was brought down by floods and torrents from a range of hills above the coast-line.

We must remember, however, that such coarse deposits are not formed everywhere along a line of coast, and that sandstones may have been formed as close to a shore as conglomerates; nay, some sandstones have, doubtless, been formed above high water, and were originally sand-dunes, such as are common on our present coasts.

Sandstones formed from æolian drift, or blown sand, may generally be distinguished from water-borne sands by the character of their component quartz grains; those of the former being all much worn, rounded, and polished, while in the latter a large proportion of the grains are angular.

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Professor Daubrée1 and the late Mr. J. A. Phillips 2 have shown that the wearing down and rounding of angular quartz grains is an exceedingly slow operation, and that grain of quartz one-fiftieth of an inch in diameter requires an amount of abrasion equal to that which would result from its having travelled a distance of 3,000 miles in water before it becomes so rounded as to assume the form of a miniature pebble. In sand from the seashore of Cornwall Mr. Phillips found that the quartz grains between onetwentieth and one-fiftieth of an inch diameter were still angular, although they had been for years exposed to the beating of the waves. Only a few of the larger grains were partially rounded, and the same is the case with the sands of other shores.

The sands of the African and Arabian deserts, on the contrary, consist of completely rounded grains without any admixture of angular fragments, and the grains of blown sand from English dunes only differ in being rather less completely rounded. Most sandstones consist of more or less angular grains, but beds of consolidated æolian sand exist in some formations.

Pure quartzose sandstones of marine origin are always shallow-water deposits and are seldom found in deeper water than fifty or sixty fathoms, except where the bottom slopes very steeply from the coast; neither do they generally occur at a greater distance than twenty or thirty miles from land, except in shallow and land-surrounded seas like that which is sometimes called the German Ocean.

Outside a continent which has a fairly uniform coast-line trending for a long distance in one direction there is generally a regular succession of deposits from the coast-margin outwards, the coarseness of the materials decreasing with the distance from land, so that we pass from shingle or 1 "Géologie Expérimentale," p. 256 et seq.

2 44 Quart. Journ. Geol. Soc.," vol. xxxvii. p. 21.