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1. The plain or gently inclined uniform surface.
2. The scarp or steeply inclined slope; this is necessarily of small extent except in the direction of its length.
3. The valley, composed of two lateral parallel slopes inclined towards a narrow strip of plain at a lower level which itself slopes downwards in the direction of its length. Many varieties of this fundamental form may be distinguished.
4. The mount, composed of a surface falling away on every side from a particular place. This place may either be a point, as in a volcanic cone, or a line, as in a mountain range or ridge of hills.
5. The hollow or form produced by a land surface sloping inwards from all sides to a particular lowest place, the converse of a mount.
6. The cavern or space entirely surrounded by a land surface.

These forms never occur scattered haphazard over a region, but always in an orderly subordination depending on their mode of origin. The dominant forms result from crustal movements, the subsidiary from secondary reactions during the action of the primitive forms on mobile distributions.Geology and land forms. The geological structure and the mineral composition of the rocks are often the chief causes determining the character of the land forms of a region. Thus the scenery of a limestone country depends on the solubility and permeability of the rocks, leading to the typical Karst-formations of caverns, swallow-holes and underground stream courses, with the contingent phenomena of dry valleys and natural bridges. A sandy beach or desert owes its character to the mobility of its constituent sand-grains, which are readily drifted and piled up in the form of dunes. A region where volcanic activity has led to the embedding of dykes or bosses of hard rock amongst softer strata produces a plain broken by abrupt and isolated eminences.^[1]

It would be impracticable to go fully into the varieties of each specific form; but, partly as an example of modern geographical classification, partly because of the exceptional importance of mountains amongst the features of the land, one exception may be made. The classification of mountains Classification of mountains. into types has usually had regard rather to geological structure than to external form, so that some geologists would even apply the name of a mountain range to a region not distinguished by relief from the rest of the country if it bear geological evidence of having once been a true range. A mountain may be described (it cannot be defined) as an elevated region of irregular surface rising comparatively abruptly from lower ground. The actual elevation of a summit above sea-level does not necessarily affect its mountainous character; a gentle eminence, for instance, rising a few hundred feet above a tableland, even if at an elevation of say 15,000 ft., could only be called a hill.^[2] But it may be said that any abrupt slope of 2000 ft. or more in vertical height may justly be called a mountain, while abrupt slopes of lesser height may be called hills. Existing classifications, however, do not take account of any difference in kind between mountain and hills, although it is common in the German language to speak of Hügelland, Mittelgebirge and Hochgebirge with a definite significance.

The simple classification employed by Professor James Geikie^[3] into mountains of accumulation, mountains of elevation and mountains of circumdenudation, is not considered sufficiently thorough by German geographers, who, following Richthofen, generally adopt a classification dependent on six primary divisions, each of which is subdivided. The terms employed, especially for the subdivisions, cannot be easily translated into other languages, and the English equivalents in the following table are only put forward tentatively:—

Richthofens's Classification of Mountains^[4]

1. Tektonische Gebirge—Tectonic mountains.
   1. Bruchgebirge oder Schollengebirge—Block mountains.
      1. Einseitige Schollengebirge oder Schollenrandgebirge— Scarp or tilted block mountains.
         1. Tafelscholle—Table blocks.
         2. Abrasionsscholle—Abraded blocks.
         3. Transgressionsscholle—Blocks of unconformable strata.
      2. Flexurgebirge—Flexure mountains.
      3. Horstgebirge—Symmetrical block mountains.
   2. Faltungsgebirge—Fold mountains.
      1. Homöomorphe Faltungsgebirge—Homomorphic fold mountains.
      2. Heteromorphe Faltungsgebirge—Heteromorphic fold mountains.
2. Rumpfgebirge oder Abrasionsgebirge—Trunk or abraded mountains.
3. Ausbruchsgebirge—Eruptive mountains.
4. Aufschüttungsgebirge—Mountains of accumulation.
5. Flachböden—Plateaux.
   1. Abrasionsplatten—Abraded plateaux.
   2. Marines Flachland—Plain of marine erosion.
   3. Schichtungstafellan—Horizontally stratified tableland.
   4. Ubergusstafelland—Lava plain.
   5. Stromflachland—River plain.
   6. Flachböden der atmosphärischen Aufschüttung—Plains of aeolian formation.
6. Erosionsgebirge—Mountains of erosion.

From the morphological point of view it is more important to distinguish the associations of forms, such as the mountain mass or group of mountains radiating from a centre, with the valleys furrowing their flanks spreading towards every direction; the mountain chain or line of heights, forming aMountain forms. long narrow ridge or series of ridges separated by parallel valleys; the dissected plateau or highland, divided into mountains of circumdenudation by a system of deeply-cut valleys; and the isolated peak, usually a volcanic cone or a hard rock mass left projecting after the softer strata which embedded it have been worn away (Monadnock of Professor Davis).

The geographical distribution of mountains is intimately associated with the great structural lines of the continents of which they form the culminating region. Lofty lines of fold mountains form the “backbones” of North America in the Rocky Mountains and the west coast systems, of South AmericaDistribution of mountains. in the Cordillera of the Andes, of Europe in the Pyrenees, Alps, Carpathians and Caucasus, and of Asia in the mountains of Asia Minor, converging on the Pamirs and diverging thence in the Himalaya and the vast mountain systems of central and eastern Asia. The remarkable line of volcanoes around the whole coast of the Pacific and along the margin of the Caribbean and Mediterranean seas is one of the most conspicuous features of the globe.

If land forms may be compared to organs, the part they serve in the economy of the earth may, without straining the term, be characterized as functions. The first and simplest function of the land surface is that of guiding loose material to a lower level. The downward pull of gravityFunctions of land forms.

Land waste.

Glaciers. suffices to bring about the fall of such material, but the path it will follow and the distance it will travel before coming to rest depend upon the land form. The loose material may, and in an arid region does, consist only of portions of the higher parts of the surface detached by the expansion and contraction produced by heating and cooling due to radiation. Such broken material rolling down a uniform scarp would tend to reduce its steepness by the loss of material in the upper part and by the accumulation of a mound or scree against the lower part of the slope. But where the side is not a uniform scarp, but made up of a series of ridges and valleys, the tendency will be to distribute the detritus in an irregular manner, directing it away from one place and collecting it in great masses in another, so that in time the land form assumes a new appearance. Snow accumulating on the higher portions of the land, when compacted into ice and caused to flow downwards by gravity, gives rise, on account of its more coherent character, to continuous glaciers, which mould themselves to the slopes down which they are guided, different ice-streams converging to send forward a greater volume. Gradually coming to occupy definite beds, which are deepened and polished by the friction, they impress a characteristic appearance on the land, which guides them as they traverse it, and, although the ice melts at lower levels, vast quantities of clay and broken stones are brought down and deposited in terminal moraines where the glacier ends.

Rain is by far the most important of the inorganic mobile distributions upon which land forms exercise their function of guidance and control. The precipitation of rain from the aqueous vapour of the atmosphere is caused in part by vertical movements of the atmosphere involving heat changes and apparently Rain.independent of the surface upon which precipitation occurs; but in greater part it is dictated by the form and altitude of the land surface and the direction of the prevailing winds, which itself is largely influenced by the land. It is on the windward faces of the highest ground, or just beyond the summit of less dominant heights upon the leeward side, that most rain falls, and all that does not evaporate or percolate into the ground is conducted back to the sea by a route which depends only on the form of the land. More mobile and more searching than ice or rock rubbish, the trickling drops are guided by the deepest lines of the hillside in their incipient flow, and as these lines converge, the stream, gaining strength,River systems. proceeds in its torrential course to carve its channel deeper and entrench itself in permanent occupation. Thus the stream-bed, from which at first the water might be blown away into a new channel by a gale of wind, ultimately grows to be the strongest line of the landscape. As the main valley deepens, the tributary stream-beds are deepened also, and gradually cut their way headwards, enlarging the area whence they draw their supplies. Thus new land forms are created—valleys of curious complexity, for example—by

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1. On this subject see J. Geikie, Earth Sculpture (London, 1898); J. E. Marr, The Scientific Study of Scenery (London, 1900); Sir A. Geikie, The Scenery and Geology of Scotland (London, 2nd ed., 1887); Lord Avebury (Sir J. Lubbock), The Scenery of Switzerland (London, 1896) and The Scenery of England (London, 1902).
2. Some geographers distinguish a mountain from a hill by origin; thus Professor Seeley says “a mountain implies elevation and a hill implies denudation, but the external forms of both are often identical.” Report VI. Int. Geog. Congress (London, 1895), p. 751.
3. “Mountains,” in Scot. Geog. Mag. ii. (1896) p. 145.
4. Führer für Forschungsreisende, pp. 652-685.