There are good grounds for believing that the great mountain-axis of Southern Europe, with its continuation in Asia, had no existence during the earlier geological periods. Indeed, it has been proved that all the higher among the existing mountain-chains of the globe have been almost entirely formed in Tertiary times. The reason of this remarkable fact is not far to seek. So rapid is the work of denudation in the higher regions of the atmosphere, that the elevated crags and pinnacles are being broken up by the action of moisture and frost at an exceedingly rapid rate. This fact is attested by the existence of those enormous masses of angular rock-fragments which are found lodged on every vantage-ground among the mountain-summits, as well as by the continually descending materials which are borne by glaciers and mountain-torrents to the valleys below. Where such a rate of disintegration as this is maintained, no elevated mountain-crests could exist through long geological periods. It is true we find in all parts of the globe relics of many mountain-chains which were formed before the Tertiary period; but these have by long-continued denudation been worn down to 'mere stumps.' Of such worn-down and degraded mountain-ranges we have examples in the Scandinavian chains, and some of the low mountain-regions of Central Europe and North America. Let us now proceed to illustrate this subject by briefly sketching the history of that series of operations by which the great mountain-chains of the Alpine system have been formed. The first stage of that grand series of operations appears from recent geological researches to have consisted in the opening of a number of fissures running along a line near to that at which, in a long subsequent period, the elevation of the mountain-masses took place. This betrayal of the existence of a line of weakness in this part of the earth's crust occurred in the Permian period, and from that time onward a series of wonderful movements and changes have been going forward, which have resulted in the production of the Alpine chains as we now see them. [Sidenote: VOLCANIC FISSURES OF PERMIAN PERIOD.]