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.