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and mountains are gradually demolished, and the basins and depressions in the surface of the great continental plateau become slowly filled with their detritus. Thus inland seas and lakes tend to vanish, inlets and estuaries are silted up, and the land in places advances seaward. To the action of rain and rivers that of the sea is added, so that by the combined operation of all epigene agents the irregularities of coast-lines tend to become reduced. This is best seen in regions where the seas are comparatively shallow—where the coast-lines are withdrawn for some considerable distance from the edge of the great oceanic depression. In such shallow seas sedimentation and erosion proceed apace. But when the coastlines are not far removed from that depression, they are necessarily washed by deeper waters, and become modified chiefly by erosion.
"Should they preserve that position for a prolonged period of time, they will eventually be cut back by the sea. In this way a shelf or terrace will be formed, narrow in some places, broader in others, according to the resistance offered by the varying character of the rocks. But no inlets or fiords can result from such action. At most the harder and less readily demolished rocks will form headlands, while shallow bays will be scooped out of the more yielding masses. In short, between the narrower and broader parts of the eroded shelf or terrace a certain proportion will tend to be preserved. As the shelf is widened sedimentation will become more and more effective, and in places may come to protect the land from further encroachment by the sea. All long-established coast-lines thus acquire a characteristically sinuous form."
"To sum up, then," as we have elsewhere remarked, "the chief agents concerned in the development of coast-lines are crustal movements, sedimentation, and marine erosion. All the main trends are the result of elevation and depression. Considerable geographical changes, however, have been brought about by the silting-up of those shallow and sheltered seas which in certain regions overflow wide areas of the continental plateau. Throughout all the ages, indeed, epigene agents have striven to reduce the superf1cial inequalities of that plateau by levelling heights and filling up depressions, and thus, as it were, flattening out the land-surface and causing it to extend. The erosive action of the sea, from our present point of view, is of comparatively little importance. It merely adds a few finishing touches to the work performed by the other agents of change."
But if it be true that all the main trends of our coastlines are the result of crustal movements, it is no less true that many of the indentations that break the continuity of an otherwise regular coast-line are often due to the same cause. The general trend of the coast-line of South America, for example, from Pernambuco to the mouth of the River Plate, coincides with the direction of the continental plateau, and may be said, therefore, to have been determined by crustal movements. The shores, however, have been greatly modified by sedimentation, and to a less extent by erosion, while the numerous indentations and islets at and near Rio Janeiro are evidence of recent depression. In a word, it holds true for all the coast-lines of the globe that not only their general direction, but their more or less numerous indentations, are the result of crustal movements. Estuaries, fiords, and inlets generally are merely the seaward prolongations of valleys and other hollows of the land. The indentations due to marine erosion are relatively so insignificant, that they can be rarely expressed upon a map of small scale. It is the form of the land that determines the character of a coast-line. An indented coast-line is the result of depression; a smooth, flat shore with no indentations is more usually, although not always, due to elevation or sedimentation. But a featureless desert-land, smoothed out by aeolian erosion and accumulation, would necessarily be bounded by an even coast-line, whether that coast-line were the result of upheaval or depression. Finally, the coast-lines of regions which have remained for a long time undisturbed by crustal movements tend, as we have seen, to assume a special form. Erosion and sedimentation in this case combine to produce "a series of regular and rhythmical curves."
We ha»e made no reference to the interesting fact that plants and animals play a certain part in the formation of coast-lines in some regions. This is only conspicuous, however, in tropical and subtropical latitudes. The mangrove-tree, for example, which flourishes along the margins of low, shelving shores, forms dense belts of jungle, which continue to extend seaward until the depth becomes too great. Some of these jungles attain a width of ten or even of twenty miles, and are in places rapidly extending. Professor Shaler is inclined to think that on the coast of Florida the trees may advance over the sea-floor at the rate of twenty to thirty feet in a century. The closely set roots and rootlets bring about the deposition of sediment, and flotsam and jetsam of all kinds become entangled, so that eventually a low mole is formed along the swampy shore, which bars the escape of rain-water towards the sea, and thus marshes capable of supporting fresh-water plants and various bushes and trees come into existence.
In other warm seas coral plays a not unimportant part in the formation of new lands. Fringing-reefs, barrier-reefs, and atolls are of great interest from many points of view, but into the history of their formation we need not enter. It is enough to recognise the fact that shore-lines now and again owe their very existence to organic action.
PLAINS OF ACCUMULATION AND OF EROSION—PLATEAUX OF
OR EROSION VALLEYS—BASINS—COAST-LINES.
WE have now passed in rapid review the more salient and notable features of the land-surface, and have discussed the several causes of their origin. The present chapter may therefore be devoted to the classification of those features, and will serve as a general summary of the results arrived at.
The leading features to be recognised are plains, plateaux, hills and mountains, valleys, basins, and other hollows and depressions of the surface, and, lastly, coast-lines.
i. Plains. These are areas of approximately flat or gently undulating land. It is needless to say, however, that plains almost invariably have a general slope in one or more directions. This, however, is so gentle, as a rule, that it is hardly perceptible. They are confined to lowlands; but now and again,