PLAN OF THE EARTH Although the vertical distance between the highest point on the surface of the lithosphere and the lowest point on this sur face amounts to about 12 miles the surface is moulded into such low relief, having respect to the terrestrial circumference of about 25,000 miles, that a correct model eight inches in diameter would appear almost smooth. The surface is nevertheless impressed with a distinct system of depressions and elevations showing a rough symmetry in its plan. Round the north pole there is a region of depression within the parallel of 8o° N. and round the south pole an area of elevation within the Antarctic circle. All the rest of the surface is occupied by two great areas interlocking with each other, one of elevation, the other of depression. The de pressed area encircles the globe about 65° S. and stretches north ward in three great bays or world-hollows corresponding to the Pacific, the Indian and the Atlantic basins. The floors of the world-hollows are gently undulated in their configuration for the most part but they exhibit considerable diversities of level, the deepest depressions occurring as a rule near the margins while gentle rises form a central low ridge along the narrow Atlantic hollow, and more or less parallel bands across the wide Indian and Pacific hollows. The elevated area surrounds the north polar de pression as a continuous ring and stretches southward with an easterly twist in three tapering world-ridges culminating respec tively in the continents of North and South America, Europe and Africa and Asia and Australia. From the depressed area the lower portion of the elevated area slopes upward with a gradient so steep compared with any prevailing on the floor of the depressed area as to form a sharp transition between the two; but near the top of the world-ridges there is a notable flattening into a broad gently shelving plain which leads to a culminating region of abrupt elevations and high plateaux.
The two world areas are separated by the line of mean-sphere level which lies in such a position that if all the elevations project ing above it were shaved off parallel with the surface of the spheroid the material so cut off would suffice to fill all the hol lows lying below it. The average level of the depressed area is, so far as can be guessed in our present ignorance of its detailed configuration, rather more than two miles below the average level of the elevated area.
The effort to visualize the plan of the lithosphere with reference to the line of mean sphere level is rendered difficult because it ignores the familiar divison of the earth's surface into land and sea. The areal ratio between land and sea of course depends on the volume of the hydrosphere. If that were just enough to fill the world-hollows up to mean sphere level, the outlines of the restricted oceans would be similar to what they are at present; the outlines of the continents, although enlarged and smoothed, would be easily recognizable, and the areas of land and sea would be practically equal. The volume of the hydrosphere is, however, such that the water covers the steep slopes of the world-ridges and reaches half way over the shelving plain so that the present coast line is in a position where a given rise or fall in level would make the greatest possible difference in the area of sea and land. Only 28 per cent of the surface of the lithosphere projects as land through the hydrosphere into the atmosphere. The volume of the hydrosphere is not fixed; a considerable portion of it exists as water flowing over or resting in hollows in the land, a considerable portion is withdrawn in the form of permanent ice in the polar regions and on high mountains, and a portion is diffused through the atmosphere in the form of vapour. Thus, apart from move ments of upheaval or subsidence in the solid crust the level at which the water stands against the land may vary from time to time under the action of climatic changes.
The surface of the sea furnishes a convenient datum level from which to reckon heights and depths. Speaking precisely, the sur face of the sea, even when calm and at its mean tidal range, is not that of a spheroid concentric with the earth from which absolute measures of distance from that centre can be made; it is a surface of equilibrium, distorted from that of the spheroid by the gravi tational attraction of the lithosphere which varies with the form of the crust and density of the rocks. Except in geodetic measure ments no allowance is made for distortion of surface.
As new and much more exact measurements of the depths of the oceans are in progress it seems undesirable to repeat the approximations to the volumes of the heights and hollows of the lithosphere and the boundaries between its various areas based upon them by John Murray, H. R. Mill and Hermann Wagner. It may suffice to say that the mean level of the lithosphere lies somewhere between i,000 and 2,000 fathoms below sea-level. Murray's term abysmal area or Wagner's oceanic plateau may be applied to the whole of the depression below mean sphere level.
Wagner's continental slope to the steep rise from mean sphere level to the outer margin of his continental plateau that includes the shelving plain of which the seaward portion retains Mill's name of continental shelf and the landward portion the lowland plains, while all above the level of 600 or 700 feet above sea-level may be termed the culminating continental area. The four con tours known as mean sphere level, the edge of the continental shelf, the coast line and the line of 65o feet above the sea (which is the approximate mean Ievel of the surface of the terraqueous globe) have a definite physical meaning and are not a mere arbitrary choice. While physically the outer edge of the conti nental shelf is situated at the abrupt change of gradient between the continental slope and the shelving plain, which is not always at the same depth, it is usually found so near 600 feet that gen erally speaking it is well defined by the contour line of i oo fathoms or of 200 metres, now shown on all bathymetrical maps. Wider or narrower, this shelf surrounds all the continents and in its wider parts often bears groups of islands which are to be classed as parts of the continents although separated from the nearest mainland by shallow seas.
Homology of the Continents.—The three great world-ridges which radiate southward and eastward from the north polar region bear each a pair of continents, either separate or united, all of which exhibit a certain homology or approximation to a common type. Each pair consists of a north-western and a south-eastern partner separated by a group of islands and south-pointing penin sulas. The outline of a typical continent approaches a triangle, widest in the north. The central portion is a low plain dipping southward and eastward in the southern part and northward and eastward in the northern part. On the east there is a broad high land area of ancient rocks the eroded remnant of a folded moun tain system, and beyond that on the east is a broad coastal plain. On the west of the central plain there is a high continuous chain of mountains folded up out of geologically recent rocks and en closing an elevated plateau, while beyond on the west there is a very narrow coastal plain. On the western world-ridge this type is well exemplified by South America, and North America diverges but little from it except by being broader in the south where it merges into the West Indian archipelago. On the eastern world ridge Asia and Australia diverge considerably from the type both in outline and orientation, but there is the contrast of mountain and plateau with a low central plain and, to some extent, lower heights may be traced on the other side, while the Malay Archi pelago uniting the two partners is on a far larger scale than the West Indies. The continents of the central world-ridge, Europe and Africa, are much farther from the type. Like the other northern partners of the pairs, Europe is the more diversified into peninsulas and gulfs while Africa like the other southern partners has a smoother outline practically without minor sinuosities ; but here plateaux take the place of folded mountains except for the Atlas system in the north. The exact configuration of Antarctica crowning the isolated southern elevation has still to be discovered, but high mountains bordering the coast of Victoria Land certainly buttress a great plateau to the west. Greenland though united to the American World Ridge may from some of its aspects be also claimed as a continent in itself. New Guinea, Borneo and Sumatra as the largest members of the Malay Archipelago belong to the continental system of Asia and Australia.
Islands.—Two classes of islands are recognised : the first, termed continental, and usually found on the continental shelf, are incidents of sea-level and a comparatively slight rise of the shelv ing plain or a fall of the ocean would unite them with the neigh bouring mainland. A few other islands, of which Madagascar is the largest, are classed as continental on account of their rocks being geologically characteristic of the large land masses. In con trast with these are oceanic islands which never formed part of a continent but rise more or less abruptly from the floor of the oceans. They are classified by their origin into volcanic islands consisting of erupted lava or ashes and coral islands composed of limestone derived from the calcareous supports of polyps produced by their physiological action on sea water.
Coasts.—The line along which the sea meets the land is from every point of view the most important which is shown on a map. Coast lines have been classified in many ways with reference both to their form and their origin. One distinction is between the coasts of regions where the land has long been stationary as re gards sea level, and those of regions where depression or upheaval is taking place. On stationary coasts the effect of wave-erosion is to form broad shelving beaches of sand or gravel where the land is low or the rocks soft and to produce narrow beaches where the land is high or the rocks hard, the beach in each case being cut out of the land by wave action and erosion in its upper part and built up from the sea bed by the deposit of detritus or sediment in its lower part. On sinking coasts the sea has invaded the lower valleys of the land forming wide funnel-shaped estuaries for the rivers where the land is low and narrow winding fjords or rigs where the land is high. On rising coasts the shore-line, being newer, is less indented than in other types, and shelf-like raised beaches may be found backed by the old wave eroded cliffs. The minuter classification of coast-lines on this basis is complicated by alter nations of elevation and depression or tilting of the land masses. Land Forms.—Richthofen and Penck, the founders of geo morphology, have elaborated the classification of land-forms with reference both to origin and configuration and many volumes have been written on the subject. Fundamentally the typical form of the land is a slope or inclined plane which may stand at any angle from the horizontal to the vertical. Such slopes may be grouped together in many ways, giving rise to a great variety of form elements amongst which Penck recognizes six as primary. These are (r) the Plain or gently inclined uniform surface, (2) the Scarp or steeply inclined slope, (3) the Valley consisting of two lateral parallel or slightly diverging slopes inclined towards a nar row strip or plain at a lower level which itself dips downwards in the direction of its length. (4) the Mount or slope falling away on every side from a particular place which may be a small area as in the case of a volcanic cone, or a line as in the case of a ridge or a mountain chain. (5) the Hollow or converse of a mount, being a slope digging inwards on all sides to a point or a relatively small plain, and (6) the Cavern or space entirely surrounded by a land surface. The character of a region depends on the size and grouping of these form elements and any particular landscape may be a complex of several of them. Only a plain can extend as a single unit for hundreds of miles in every direction and at any ele vation above sea-level. All the other form elements are restricted to comparatively small areas or extend in length only as relatively narrow strips. The varieties of mountains and valleys necessitate a very complex classification if full account is to be taken of them. Mountains.—From the morphological point of view it is most important to distinguish the association of forms as the mountain mass or group of mountains radiating from a centre, with valleys furrowing their flanks and spreading towards every direction ; the mountain-chain or line of heights forming a 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 wide and 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. The distribution of mountains is closely related to the great structural lines of the continents of which they form the culminating region. Lofty mountains folded up from comparatively recent geological strata form the backbones of North America (the Rocky Moun tains and the western Coast Ranges), South America (the Andes), Europe (the Pyrenees, Alps, Carpathians and Caucasus), and of Asia in the mountains of Asia Minor converging on the Pamirs and diverging thence in the Himalayas and the vast ranges of central and eastern Asia north of the Tibetan plateau.
Functions of Land Forms.—The simplest function of land f orms in the economy of the earth as a machine worked by solar energy is the guiding of loose material descending from a higher to a lower level under the pull of gravity. The slope and state of the surface of the land determine the path by which rock f rag ments, broken off by alternate expansion and contraction due to heating and cooling by radiation, will slip or roll and the distance to which they can be carried. On a uniform scarp detritus falling from above comes to rest near the bottom thus gradually reducing the slope by forming a talus or scree at the base. Where the surface is broken by valleys and plains the heaping up of rolling stones and sand add greatly to the variety of the scenery. So far the sun, gravity and the land forms could work if there were no atmosphere but the process of erosion is enormously accelerated when solar energy acts through the agency of air and water.