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 æolian 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 have 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 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.

CHAPTER XVI

CLASSIFICATION OF LAND-FORMS

PLAINS OF

ACCUMULATION AND OF EROSION-PLATEAUX OF ACCUMULATION AND OF EROSION-HILLS AND MOUNTAINS ; ORIGINAL OR TECTONIC, AND SUBSEQUENT OR RELICT MOUNTAINS—VALLEYS; ORIGINAL OR TECTONIC, AND SUBSEQUENT OR EROSION VALLEYS-BASINS-COAST-LINES.

WE

E 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.

1. 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,

in the case of very extensive areas, the surface of a plain rises inland so imperceptibly that it may attain an elevation eventually of several thousand feet. This, however, is exceptional. Elevated flat lands. are usually termed plateaux. Two kinds of plains are recognised, viz., plains of accumulation and plains of erosion. A plain of accumulation is built up of approximately horizontal deposits, so that the external surface is a more or less exact expression of the internal geological structure. All such plains tend to become modified by epigene action. If the plain be at or near a base-level of erosion, rain and running water have little effect upon it, but under certain conditions the surface may be considerably modified by the action of the wind. If the plain be traversed by a great river, or margined by the sea or by an extensive lake, sand-dunes may invade it more or less abundantly. Many coastal plains, indeed, have been formed partly by aqueous sedimentation and partly by the activity of the wind blowing sand before it from the exposed beaches. The higher a plain rises above its base-level the more it is subjected to aqueous erosion, and the more irregular and undulating does its surface become, the nature of the materials of which it is composed having no small influence in determining the character and extent of the denudation. Other things being equal, a plain consisting chiefly of impervious argillaceous deposits is more readily washed down than one built up largely of sand, shingle, gravel, and other more

or less porous materials. Many plains of accumulation are among the richest and most fertile tracts in the world, while others (and these are usually the most extensive) are relatively infertile, not a few being more or less destitute of vegetable covering. Among European plains of accumulation may be mentioned the French Landes, the far-extending flats of the Low Countries, and the grassy Steppes of Hungary and Russia. The arid wastes of the AraloCaspian depression and the broad Tundras of Siberia, the Prairies of North America, and the Llanos and Pampas of South America, are all more or less plains of accumulation-their approximately flat or gently undulating surface is due directly either to aqueous sedimentation or to wind-action, or to both.

A

Not infrequently, however, the superficial accumulations of such tracts are of no great thickness, but spread over and conceal old plains of erosion. plain of erosion is distinguished by the fact that its surface does not necessarily coincide with the underground structure. It is only when the plain has resulted from the levelling of a series of horizontal strata that external form and internal structure can agree. In the great majority of cases no such coincidence occurs. The plains in question may consist either of horizontal or slightly inclined and gently undulating, or highly folded and contorted, strata, or they may be composed largely or wholly of igneous or of schistose rocks. They are the base-levels to which old land-surfaces have been reduced; they re