The fourth stage in the great work of mountain-building in the case of the Alps consisted in the operation of the denuding forces, the disintegrating action of rain and frost, the transporting action of rivers and glaciers, by which the Alpine peaks were gradually sculptured out of the indurated and altered masses constituting the geanticlinal. The action of this fourth stage went on to a great extent side by side with that of the third stage. So soon as the earth-movements had brought the submerged sedimentary masses of the geosynclinal under the action of the surface tides and currents of the ocean, marine denudation would commence; and, as the work of elevation went on, the rock-masses would gradually be brought within the reach of those more silently-working but far more effective agents which are ever operating in the higher regions of the atmosphere. It is impossible to say what would have been the height of the Alpine chain if the work of denudation had not to a great extent kept pace with that of elevation. Only the harder and more crystalline masses have for the most part escaped destruction, and stand up in high craggy summits; while flanking hills, like the well-known Rigi, are Been to be composed of conglomerates thousands of feet in thickness, composed of their disintegrated materials. It is a remarkable fact, as showing how enormous was the work of elevation daring the formation of the geanticlinal, that some of the youngest and least consolidated rocks of the Nummulitic period are still found at a height of 11,000 feet in the Alps, and of 16,000 feet in the Himalaya. From what has been said, it will be seen that mountain-chains may be regarded as cicatrised wounds in the earth's solid crust. A line of weakness first betrays itself at a certain part of the earth's surface by fissures, from which volcanic outbursts take place; and thus the position of the future mountain-chain is determined. Next, subsidence during many millions of years permits of the accumulation of the raw materials out of which the mountain-range is to be formed; subsequent earth-movements cause these raw materials to be elaborated into the hardest and most crystalline rock-masses, and place them in elevated and favourable positions; and lastly, denudation sculptures from these hardened rock-masses all the varied mountain forms. Thus the work of mountain-making is not, as was formerly supposed by geologists, the result of a simple upheaving force, but is the outcome of a long and complicated series of operations. [Sidenote: ORIGIN OF OTHER MOUNTAIN-CHAINS.]