CONTINENTS, ORIGIN OF. Throughout the greater part of the I9th century geologists in general believed that our present continents and oceans were only temporary features of the globe. In the last quarter of the century, however, the idea of the permanence of the ocean basins began to gain ground. It cannot be said that there is any universal agreement upon the question even yet. Many geologists are now inclined to ascribe a high antiquity to the Pacific ocean but to look upon the Atlantic and Indian oceans as comparatively modern. Haug and others still suspect the former existence of a Pacific continent. If continents and oceans are not permanent, the present distribution of land and sea is only an episode in the history of the globe and has no more significance than the distribution in the past. It was not geologists, therefore, who first endeavored to formulate general theories to account for the present shapes of continents and oceans. Those who made such attempts ignored the changes in the past and their speculations were received with little favour by geologists. The most suggestive of these speculations was the tetrahedral theory of Lowthian Green, who assumed that the earth is cooling, the interior contracting more 'rapidly than the exterior, and under the force of gravity the outer crust collapses. Fair bairn's experiments on the crushing of wrought-iron tubes led him to believe that the collapsing sphere will tend to approach a tetrahedral form. The corners of the tetrahedron will rise above the water, forming triangular masses of land ; the faces of the tetrahedron will remain covered and will form the oceans. He places one corner at the South Pole and the other three in the northern hemisphere. The corner at the South Pole is the Antarc tic continent and the opposite face of the tetrahedron is covered by the Arctic ocean. The triangular masses of land formed by the other three corners are represented respectively by North and South America, Europe and Africa, Asia and Australia, all of them wide toward the north and tapering toward the south. The two last are united in the north, but the Caspian depression is below sea-level and the plain of the Obi but little above it. Be tween these three land-masses lie the Atlantic, Indian and Pacific oceans, all narrowing toward the north and, in the south, where the tetrahedral edges are lowest, uniting into a continuous belt around the globe.
The actual form which Lowthian Green believes the earth to have reached is the hexakistetrahedron with all the faces so much rounded that the departure from the spheroidal form is slight. Lowthian Green's theory was entirely neglected in England for many years, but was more favourably received in France. De Lapparent seems to have been the first to recognize it as a probable hypothesis and at a later date both Michel Levy and Marcel Ber trand adopted it in a modified form. Since the closing years of the last century a tetrahedral theory in some shape or other has been accepted by many writers in England, Germany and America; but once more it is generally discredited. On physical grounds objection has been raised that the tetrahedron is not a figure of equilibrium for a rotating earth, and even a slight approximation to it cannot be retained.
In the Northern Hemisphere there are three areas in which the Cambrian beds remain unfolded. These are : ) Laurentia, which includes most of Canada east of the Rockies and probably stretched to the western islands of Scotland, (2) the Baltic shield and Russian platform, (3) Angaraland, which includes a great part, but not the whole, of eastern Siberia.
In the Southern Hemisphere also there are areas in which the oldest fossiliferous beds known show no folding. The most exten sive is Gondwanaland, which includes the greater part of South America east of the Andes, most of Africa between the Atlas and the mountains of the Cape, Arabia, Syria and the peninsula of India. The upper Cretaceous sea extended over a considerable area and even marine Jurassic beds are found in places, but much the larger part was land throughout the Mesozoic era. A similar re mark applies to a great part of Australia. Between the Mesozoic land-areas of the north and those of the south lies a belt in which the Mesozoic beds are for the most part marine. This is the Tethys of Suess.
Between the rigid masses of the north and those of the south the more yielding crust beneath the sea of Tethys was crushed, and the great series of mountain ranges which run from west to east across the Old World was elevated. Tethys was much reduced in size and is now represented chiefly by the Mediterranean sea. About the same time Laurentia and Gondwana-land broke up, large portions sinking beneath the sea, and thus the present Atlan tic came into existence. The Pacific appears to have been sea throughout the Mesozoic era at least, but nevertheless it has functioned as a rigid area.
Wegener's view is that the sheets of sial are not only separate and floating in the sima but also moving laterally, and that their positions relatively to one another have altered in the past and are altering still. According to him if we take the edge of the continental shelf as the edge of the sial sheets and allow for the effects of Tertiary folding, the pieces of sial can be fitted together into one continuous sheet covering about a third of the globe. This, he thinks, was the condition at the close of the Palaeozoic era. He does not follow the edge of the continental shelf with any precision, and the allowance that he makes for the Tertiary folding is purely fanciful.
It is in the Southern Hemisphere that the geological evidence for a former union of the continental masses is strongest, and it is perhaps significant that the nearest approach to a fit is that between Africa and South America. The close similarity between the rocks and fossils of these two areas has led most geologists to assume a former land connection. Both regions are character ized by the occurrence of the fossil Glossopteris flora, which differs considerably from the contemporaneous flora of Europe. The same flora has also been found in India and Australia. All these are brought together in Wegener's scheme and the similarity in rocks and fossils is at once explained. But the Glossopteris flora is also found in Siberia and northern Russia, which in his reconstruction become farther away from the presumed home of Glossopteris than they are at present.
Suppose that at a certain period the whole of the sial is solid and the sima also is solid down to a considerable depth. Heat can only escape by the slow process of conduction. Beneath the sial, according to Joly, there will be no escape from the sima, because the base of the sial itself, owing to its own radio-activity, must be nearly at the melting point of sima. Since the escape of heat is so slow the temperature rises and the sima melts from below up wards. Tidal movements in the molten sima acting upon the downward projections of the sial move the whole crust so that the local accumulations of heat originally formed beneath the sial come to lie under the thin layer of solid sima beneath the ocean. This is quickly thinned still further, and molten sima escapes through fractures. The loss of heat now becomes more rapid and an era of cooling and consolidation begins.
We must suppose, then, that there are periods during which the sima layer melts more or less completely and periods during which it solidifies down to a considerable depth. It has been shown experimentally that basalt, and most other rocks that have been tested, expand on melting and decrease in density. As the sima melts it expands, and therefore the general level of the earth's surface is raised. But because the density of the sima is decreased the masses of sial which are floating in it sink more deeply into the layer. Their surfaces, owing to the general rise, become farther from the earth's centre, but, relatively to the surface of the sima they are depressed. In this way the widespread transgressions characteristic of certain geological periods are ex plained. As the sima cools again and becomes denser the surface falls but the sial masses rise relatively to the sima. When the sima begins to cool, the crust upon it, if not already complete, is soon completed by the consolidation of molten material in its fissures. Together with the sial masses it now covers the whole globe. But the sima continues to contract further and the cover ing becomes too large. And now begins a period of folding and other earth movements to enable the crust to descend with the sima.
The great merit of the theory, from the geological point of view, is that it offers an explanation of the most remarkable features in the history of the globe, the periodicity of earth-folding and sea transgressions and their widespread character. However, in view of our ignorance of what goes on in by far the greater part of the in terior of the globe it remains improbable that any theory founded on examination of a thin external skin can be complete.
(P. LA.)