The general conclusion, then, to which we are led by a review of the greater geographical changes through which the European continent has passed is simply this—that the substructure upon which all our sedimentary strata repose is of primeval antiquity. Our dry lands are built up of rocks which have been accumulated over the surface of a great wrinkle of the earth’s crust. There have been endless movements of elevation and depression, causing minor deformations, as it were, of that wrinkle, and inducing constant changes in the distribution of land and water; but no part of the continental ridge has ever been depressed to an abysmal depth. The ridge has endured through all geological time. We can see also that the land has been evolved according to a definite plan. Certain marked features begin to appear very early in Palæozoic times, and become more and more pronounced as the ages roll on. All the countless oscillations of level, all the myriad changes in the distribution of land and water, all the earthquake disturbances and volcanic eruptions—in a word, all the complex mutations to which the geological record bears witness—have had for their end the completion of one grand design.
A study of the geological structure of Europe—an examination of the manner in which the highly folded and disturbed strata are developed—throws no small light upon the origin of the larger or dominant features of our Continent. The most highly convoluted rocks are those of Archæan and Palæozoic age, and these are developed chiefly in the north-western and western parts of the Continent. Highly contorted strata likewise appear in all the mountain-chains of central Europe—some of the rocks being of Palæozoic, while others are of Mesozoic and of Cainozoic age. Leaving these mountains for the moment out of account, we find that it is along the western and north-western sea-board where we encounter the widest regions of highly-disturbed rocks. The Highlands of Scandinavia and Britain are composed, for the most part, of highly-flexed and convoluted rocks, which speak to titanic movements of the crust; and similar much-crushed and tilted rock-masses occur in north-west France, in Portugal, and in western Spain. But when we follow the highly-folded Palæozoic strata of Scandinavia into the low-grounds of the great plains, they gradually flatten out, until in Russia they occur in undisturbed horizontal positions. Over thousands of square miles in that country the Palæozoic rocks are just as little altered and disturbed as strata pertaining to Mesozoic and Cainozoic times.
These facts can have but one meaning. Could we smooth out all the puckerings, creases, foldings, and flexures which characterise the Archæan and Palæozoic rocks of western and north-western Europe, it is certain that these strata would stretch for many miles out into the Atlantic. Obviously they have been compressed and crumpled up by some force acting upon them from the west. Now, if it be true that the basin of the Atlantic is of primeval origin, then it is obvious that the sinking down of the crust within that area would exert enormous pressure upon the borders of our continental area. As cooling and contracting of the nucleus continued, subsidence would go on under the oceanic basin, depression taking place either slowly and gradually, during protracted periods, or now and again more or less suddenly. But whether gradually or suddenly effected, the result of the subsidence would be the same upon the borders of our Continent; the strata along the whole western and north-western margins of the European ridge would necessarily be flexed and disturbed. Away to the east, however, the strata, not being subject to the like pressure, would be left in their original horizontal positions.
Now it can be shown that the mountains of Scandinavia and the British Islands are much older than the Alps, the Pyrenees, and many other conspicuous ranges in central and southern Europe. Our mountains and those of Scandinavia are the mere wrecks of their former selves. Originally they may have rivaled—they probably exceeded—the Alps in height and extent. It is most likely, indeed, that the areas of Palæozoic rocks in France, Portugal, and Spain also attained mountainous elevations. But the principal upheaval of the western margins of our Continent was practically completed before the close of the Palæozoic period, and since that time those elevated regions have been subjected to prodigious erosion, the later formations being in large measure composed of their débris. I do not, of course, wish it to be understood that there has been no upheaval affecting the west of Europe since Palæozoic times. The tilted position of many of our Mesozoic strata clearly proves the contrary. But undoubtedly the main disturbances which produced the folding, fracturing, and contortion of the Palæozoic strata of western Europe took place before the close of the Palæozoic period. The mountains of Britain and Scandinavia are amongst the oldest in Europe.
When we come to inquire into the origin of the mountains of central Europe we have little difficulty in detecting the chief factors in their formation. An examination of the Pyrenees, the Alps, and other hill-ranges having the same general trend shows us that they consist of flexed and convoluted rocks. They are, in short, mountains of elevation, ridged up by tangential thrusts. Of this we need not have the slightest doubt. If, for example, we approach the Alps from the low-grounds of France, we observe the strata as we come towards the Jura beginning to undulate—the undulations becoming more and more marked, and passing into sharp folds and plications, until, in the Alps, the beds become twisted, convoluted, and bent back upon themselves in the wildest confusion. Now, speaking in general terms, we may say that similar facts confront us in connection with every true mountain-range in central Europe. Let it be noted, further, that all those ranges have the same trend, which we may take to be approximately east and west, or nearly at right angles to the trend of the Palæozoic high-grounds of western and north-western Europe. Looked at broadly, our continental ridge may be said to be traversed from west to east by two wide depressions or troughs, separated by the intervening belt of higher grounds just referred to. The former of these troughs corresponds to the great central plain, which passes through the south of England, north-east France, the Low Countries, and Denmark, whence it sweeps east through Germany, and expands into the wide low-grounds of Russia. The southern trough or depression embraces the maritime tracts of the Mediterranean and the regions which that sea covers. Such, then, are the dominant features of our Continent, to which all others are of subordinate importance. Now it cannot be doubted that the two great troughs are belts of subsidence in the continental ridge itself. And their existence explains the origin of the mountain-ranges which separate them. We know that the northern trough is of extreme antiquity; it is older, at all events, than the Silurian period. Even at that distant date its southern limits were marked out by ridges of Archæan rocks, which seem to have formed islands in what is now middle Germany, and probably also in Switzerland and central France. The appearance of those Archæan rocks in central Europe was doubtless due to a ridging up of the crust induced by those parallel movements of subsidence which produced the northern and southern troughs. The northern trough was probably always the shallower depression of the two, for we have evidence to show that, again and again in Mesozoic and later times, the seas which overflowed what are now the central plains of Europe were of less considerable depth than that which occupied the Mediterranean trough. As time rolled on, therefore, the northern trough eventually became silted up; but so low even now is the level of that trough that a relatively slight depression would cause the sea to inundate most extensive regions in middle Europe.
In Cainozoic times, as we have seen, the last great elevation of the Alps was effected—an elevation which can hardly have been due to any other cause than the more or less abrupt depression of the earth’s crust under the Mediterranean basin. The area of that sea is now much less considerable than it was in Tertiary times—a change due in part to silting up, but chiefly perhaps to the sinking down of its bed to profounder depths.
Thus we may conclude that from a very early period—a period ante-dating the formation of our oldest fossiliferous strata—the physical structure of our Continent had already been planned. The dominant features of the primeval continental ridge are those which have endured through all geological time. They are the lines along which the beautiful lands in which we dwell have been constructed. Tilted and convoluted, broken and crushed by myriad earth-movements—scarred, furrowed, worn and degraded by the frosts, the rains, the rivers, and the seas of countless ages—the rocks of our Continent are yet eloquent of design. Where the ignorant sees nothing save confusion and discord, the thoughtful student beholds everywhere the evidence of a well-ordered evolution. Such is the conclusion to which we are led by all geological research.
SKETCH-MAPS ILLUSTRATING THE GEOGRAPHICAL EVOLUTION OF CONTINENTAL AREAS
By PROFESSOR JAMES GEIKIE, LL.D., D.C.L., F.R.S.
PLATE V