The following table is from the statistics of Professor H. Wagner,[28] his metric measurements being transposed into British units:
Comparison of the Continents.
| Area total mil. sq. m. | Mean height, feet. | Area trunk, mil. sq. m. | Area penin- sulas, mil. sq. m. | Area islands, mil. sq. m. | Area limbs, mil. sq. m. | Area limbs, per cent. | |
| Old World | 35.8 | 2360 | |||||
| New World | 16.2 | 2230 | |||||
| Eurasia | 20.85 | 2620 | 15.42 | 4.09 | 1.34 | 5.43 | 26 |
| Africa | 11.46 | 2130 | 11.22 | .. | 0.24 | 0.24 | 2.1 |
| North America | 9.26 | 2300 | 6.92 | 0.78 | 1.56 | 2.34 | 25 |
| South America | 6.84 | 1970 | 6.76 | 0.02 | 0.06 | 0.08 | 1.1 |
| Australia | 3.43 | 1310 | 2.77 | 0.16 | 0.50 | 0.66 | 19 |
| Asia | 17.02 | 3120 | 12.93 | 3.05 | 1.04 | 4.09 | 24 |
| Europe | 3.83 | 980 | 2.49 | 1.04 | 0.30 | 1.34 | 35 |
The usual classification of islands is into continental and oceanic. The former class includes all those which rise from the continental shelf, or show evidence in the character of their rocks of having at one time been continuous with a neighbouring Islands. continent. The latter rise abruptly from the oceanic abysses. Oceanic islands are divided according to their geological character into volcanic islands and those of organic origin, including coral islands. More elaborate subdivisions according to structure, origin and position have been proposed.[29] In some cases a piece of land is only an island at high water, and by imperceptible gradation the form passes into a peninsula. The typical peninsula is connected with the mainland by a relatively narrow isthmus; the name is, however, extended to any limb projecting from the trunk of the mainland, even when, as in the Indian peninsula, it is connected by its widest part.
Small peninsulas are known as promontories or headlands, and the extremity as a cape. The opposite form, an inlet of the sea, is known when wide as a gulf, bay or bight, according to size and degree of inflection, or as a fjord or ria when Coasts. long and narrow. It is convenient to employ a specific name for a projection of a coast-line less pronounced than a peninsula, and for an inlet less pronounced than a bay or bight; outcurve and incurve may serve the turn. The varieties of coast-lines were reduced to an exact classification by Richthofen, who grouped them according to the height and slope of the land into cliff-coasts (Steilküsten)—narrow beach coasts with cliffs, wide beach coasts with cliffs, and low coasts, subdividing each group according as the coast-line runs parallel to or crosses the line of strike of the mountains, or is not related to mountain structure. A further subdivision depends on the character of the inter-relation of land and sea along the shore producing such types as a fjord-coast, ria-coast or lagoon-coast. This extremely elaborate subdivision may be reduced, as Wagner points out, to three types—the continental coast where the sea comes up to the solid rock-material of the land; the marine coast, which is formed entirely of soft material sorted out by the sea; and the composite coast, in which both forms are combined.
On large-scale maps it is necessary to show two coast-lines, one for the highest, the other for the lowest tide; but in small-scale maps a single line is usually wider than is required to represent the whole breadth of the inter-tidal zone. Coast-lines. The measurement of a coast-line is difficult, because the length will necessarily be greater when measured on a large-scale map where minute irregularities can be taken into account. It is usual to distinguish between the general coast-line measured from point to point of the headlands disregarding the smaller bays, and the detailed coast-line which takes account of every inflection shown by the map employed, and follows up river entrances to the point where tidal action ceases. The ratio between these two coast-lines represents the “coastal development” of any region.
While the forms of the sea-bed are not yet sufficiently well known to admit of exact classification, they are recognized to be as a rule distinct from the forms of the land, and the importance of using a distinctive terminology is felt. Efforts have Submarine forms. been made to arrive at a definite international agreement on this subject, and certain terms suggested by a committee were adopted by the Eighth International Geographical Congress at New York in 1904.[30] The forms of the ocean floor include the “shelf,” or shallow sea margin, the “depression,” a general term applied to all submarine hollows, and the “elevation.” A depression when of great extent is termed a “basin,” when it is of a more or less round form with approximately equal diameters, a “trough” when it is wide and elongated with gently sloping borders, and a “trench” when narrow and elongated with steeply sloping borders, one of which rises higher than the other. The extension of a trough or basin penetrating the land or an elevation is termed an “embayment” when wide, and a “gully” when long and narrow; and the deepest part of a depression is termed a “deep.” A depression of small extent when steep-sided is termed a “caldron,” and a long narrow depression crossing a part of the continental border is termed a “furrow.” An elevation of great extent which rises at a very gentle angle from a surrounding depression is termed a “rise,” one which is relatively narrow and steep-sided a “ridge,” and one which is approximately equal in length and breadth but steep-sided a “plateau,” whether it springs direct from a depression or from a rise. An elevation of small extent is distinguished as a “dome” when it is more than 100 fathoms from the surface, a “bank” when it is nearer the surface than 100 fathoms but deeper than 6 fathoms, and a “shoal” when it comes within 6 fathoms of the surface and so becomes a serious danger to shipping. The highest point of an elevation is termed a “height,” if it does not form an island or one of the minor forms.
The forms of the dry land are of infinite variety, and have been studied in great detail.[31] From the descriptive or topographical point of view, geometrical form alone should be considered; Land forms. but the origin and geological structure of land forms must in many cases be taken into account when dealing with the function they exercise in the control of mobile distributions. The geographers who have hitherto given most attention to the forms of the land have been trained as geologists, and consequently there is a general tendency to make origin or structure the basis of classification rather than form alone.
The fundamental form-elements may be reduced to the six proposed by Professor Penck as the basis of his double system of classification by form and origin.[32] These may be looked The six elementary land forms. upon as being all derived by various modifications or arrangements of the single form-unit, the slope or inclined plane surface. No one form occurs alone, but always grouped together with others in various ways to make up districts, regions and lands of distinctive characters. The form-elements are: