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Continent

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CONTINENT. A definition of continent based on the origin and development of land masses is not possible in the present state of our knowledge, and about all that can be done is to define a continent as a very large body of land. Africa, North America, South America, Europe, Asia and Australia are such land areas, but since Europe and Asia form a single land mass, many physical geographers speak of the two as one continent Eurasia. There is also probably a land area of continental size in the Antarctic regions.

Origin of The solid earth, the lithosphere, is largely covered by water, the hydrosphere; the proportion of land to water on the surface of the globe being about 1 to 2.63, malcing the land surface about 27.5 per cent of the total. The extreme height of the land above sea-level is about five miles (Mount Ever est 29,000 feet), and the extreme depth of the ocean somewhat more; the extreme relief of the lithosphere is therefore over 10 miles. Ac cording to Lapparent the mean elevation of the continents is as follows: Europe 958 feet; Australia 1,118 feet; South America 1,702 feet; North America, 1,953 feet; Africa 2,007 feet; Asia 2,883 feet The mean height of all land is estimated at a little over 2,000 feet, and the mean depth of the ocean at 12,000 to 15,000 feet. Therefore if the solid earth, the lithosphere, were free from inequalities, the hydrosphere would cover it, perhaps to a depth of over two miles. Inequalities of surface have, however, existed since the earliest geological time of which we have Icnowledge. The oldest rock formations show traces of sedimentary origin, and therefore represent in part the waste of land areas from the action of streams and the waves of the ocean. So far as North America is concerned the position of the continent had been determined in Archman time. What de termined the position of this and other con tinents is, however, still an unsolved problem, though there are several theories, each with some show of reasonableness.

The generally accepted theory is that when the crust of the molten earth had solidified and cooled enough to allow the condensation of aqueous vapor, it cooled and contracted un equally, some parts cooling and contracting toward the centre more rapidly than others.

Thus were formed areas of depression and ele vation, the seas filling the former, and the higher parts of the latter projecting above the water as land. The water would hasten cooling under the depressions, and thus the general tendency would be to increase the area and elevation of the land and increase the depth, but decrease the area of the sea. Those who hold that the earth was never molten believe that the oceanic segments are heavier than the continental (a fact substantiated by actual in vestigation), and that as a result, the continental blocks are forced aside and up by the settling of the heavier oceanic blocks.

Distribution and Form of The distribution and form of continents show some curious features, and on these features theories of the origin of continents have been based. North and South Aznerica, for instance, are roughly triangular in shape, with the apex of the tnangle at the south. Eurasia and Africa together form another roughly triangular land area tapering to the south, the Cape of Good Hope being the apex of the triangle. Owing to the lands tapering to the south, the northern hemisphere contains more land than the south ern, and it is possible to divide the globe, by taking a north pole in the English channel, into two hemispheres, one nearly all land, and the other nearly all water.

Continents consist typically of a great in terior basin bordered by mountain ranges, this form being shown by the continents of some what regular outline, the irregular Eurasian continent being an exception. In each continent the greatest mountain system faces the greatest ocean. In North and South America these mountains are on the west, facing the Pacific, with the smaller Appalachian Mountains, the Venezuelan Mountains and Brazilian highlands facing the Atlantic. In Africa the greatest range is on the east, facing the Indian Ocean, in Australia the greatest range faces the Pacific, and in Asia the Himalayas face the Indian Ocean. Omitting the mountains of Eurasia, the mountain chains of the world have a northwest southeast or northeast-southwest direction, and on this account the continents taper toward the south.

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