Geologically Scotland belongs in great part to Scandinavia; and the long line of Scandinavian Mountains is a continuation of the Highlands, and so is of the same age.
Mountain-chains and hill-ranges have been upheaved at various geological periods; and some are very old, while others are much younger.
Turning to the southeast of England, we find the ranges of chalk hills forming the North and South Downs (see page [237]). As explained previously, these owe their existence to the upheaval and subsequent denudation of the low arch, or anticline, of the Weald. They are called "escarpments," because they are like lines of cliffs that are being gradually cut back. Now, it is clear that these hills are much newer than either of those we have just considered. Look at the table on page 324, and you will see that the Cretaceous rocks (chalk, etc.) belong to the Mesozoic era. The chalk was the last rock formed during the Cretaceous Period.
So the Wealden arch must have been heaved up after the chalk was formed; that is, ages and ages later than the date of the Pennine range or the Scotch Highlands. From other evidences it has been shown that this anticline was heaved up in the early part of the Cainozoic Era, perhaps during the Miocene Period.
Let us now take the case of the Alps. And here we have an instructive example of a great mountain system formed by repeated movements during a long succession of geological periods. We cannot say that they were entirely raised up at any one time in the world's past history. In the centre of this great range we find a series of igneous and metamorphic rocks, such as granite, gneiss, and crystalline schists. Some of these may belong to the very oldest period,—namely, the Archæan; others are probably Palæozoic and Cainozoic deposits greatly altered by heat and pressure.
The ground from Savoy to Austria began to be an area of disturbance and upheaval towards the close of the Palæozoic Era, if not before; so that crystalline schists and Carboniferous strata were raised up to form elevated land around which Permian conglomerates and shingle-beds were formed,—as on the seashore at the present day.
During the early part of the Mesozoic Era local fractures and certain up and down movements occurred. After this there was a long period of subsidence, during which a series of strata known as Oölites and Cretaceous were deposited on the floor of an old sea.
Towards the close of this long era, a fresh upheaval took place along the present line of the Alps,—an upheaval that was prolonged into the Eocene Period. It was during this latter period that a very extensive formation known as the "Nummulitic limestone" was formed in a sea that covered a large part of Europe and Asia. We have already referred (see chap. v., pp. [169]-[171]) to the way in which limestones have been formed. Nummulites are little shells that were formed by tiny shell-fish.
But after this, the greatest upheaval and disturbance took place,—an upheaval to which the Alps as we now see them are chiefly due. By this means the older Cainozoic strata, once lying horizontally on the floor of the sea, were raised up, together with older rocks, to form dry land, and not only raised up, but crumpled, dislocated, and in some cases turned upside down.
So intense was the compression to which the Eocene rocks were subjected that they were converted into a hard and even crystalline state. It seems almost incredible that these highly altered rocks which look so ancient are of the same date as our London clay and the soft Eocene deposits of the south of England; but in our country the movement that raised up those strata was of the most feeble and gentle kind compared to the violent disturbances that took place in Switzerland.