regularly along different joints, and we may have the overhanging cliff benched back and replaced by the sloping face b-b.

Massive crystalline igneous rocks yield forms of cliff that offer strong contrasts to cliffs excavated in bedded strata. Owing to inequalities in their composition, texture, and structure, and to the frequent irregularity of their joints, they are prone to assume particularly rugged, broken, and bizarre forms, amongst which we may look in vain for any trace of the rectangular outlines so commonly present in the case of bedded rocks. The faces of the cliffs are very rarely approximately even, but vary indefinitely, the harder and more sparingly jointed portions projecting, it may be, to form buttresses and bastions, while the softer and more shattered portions are eaten away and replaced by coves and gullies. Now and again, however, when the joints are more regular, as in the columnar structure of many basalts, etc., and the approximately rectangular joints of certain granites, mural cliffs may appear. The crystalline schists, again, exhibit every variety of feature. But inasmuch as their bedding is usually more or less highly inclined or contorted, and their jointing is irregular, they do not often show the rectangular forms that are characteristic of cliffs hewn out of sedimentary strata. Their coast-lines are usually as steep and rugged as those of massive crystalline rocks, but they present greater variety of forms, the alternation of different kinds of schist and the highly inclined, curved, or contorted bedding, and ir

regular joints often giving rise to most complex and peculiar features. Rugged stacks and skerries are very commonly present when either massive crystalline rocks or schists form the coast-line.

Of the formation of caves by marine action we have already spoken. Caves are not confined to any one kind of rock or rock-structure, and naturally vary in form and extent with the character and the arrangement of the masses in which they are excavated. When the rocks at the base of a sea-cliff are of unequal durability the undermining action of the waves and breakers must result either in the formation of caves or in the irregular retreat of the sea-wall. Much will depend on the character of the rocks above the reach of the tide. Should these be massive and not traversed by many joints, the conditions will be favourable for the formation of large caves. It is obvious, however, that if well marked joints be plentifully present the rocks cannot be undermined to any extent before collapse takes place.

We may now very shortly consider the appearances presented by flat or gently shelving, regular coastlines. As a rule these are softly sinuous, showing a succession of broad, evenly curved bays separated usually by low capes and headlands. Shores of this character are often bordered by banks of beachgravels and sand-dunes, behind which not infrequently appear salt-water or brackish-water lagoons. In the absence of the latter we may have a coastal plain traversed by parallel series of old beach-gravels and

sand-dunes. Such coastal plains obviously owe their origin to the action of streams and rivers, and are typically represented by those great deltas which we have referred to in an earlier chapter as examples of plains of accumulation. But the material carried by rivers to the sea does not always accumulate opposite their mouths. Tidal currents often prevent the rapid growth of deltas by sweeping much of the material away and depositing it alongshore, so as to form gradually a far-extended coastal - plain. The low plains that fringe the Atlantic shores of the Southern States of North America consist in this way of the sediment brought down by numerous streams and rivers, collected and redistributed by the sea. Indeed, of coastal-plains generally it may be said that they are either directly or indirectly of fluviatile origin. The delta of a great river is the direct product of river-action. Immense quantities of alluvial matter, however, are swept down to sea, and accumulate upon the bottom at no great distance from the shore. Should a negative movement of sea-level take place, a narrower or broader belt of sea-floor then becomes dry land, the new coastal plain having been built up chiefly of sediment washed down by streams and rivers. Coastal plains are thus not infrequently the result of crustal movements. As showing the dependence of coastal plains upon the activity of rivers, Professor Penck has pointed out that such plains are invariably absent from coasts to which no considerable streams and rivers descend.

In fine, as regards regular coast-lines, we see that they are not fixed, but oscillating, retreating in some places, advancing elsewhere. Cliffs, stacks, and skerries show us where the land is losing, and coastal plains where it is gaining. Much sediment washed down from the land comes to rest in quiet bays, and these in time tend to be filled up. We note also how detritus derived from cliffs and rocky headlands. is apt to be swept by tidal currents into the same quiet receptacles. Thus, while cliffs retreat, the flat shores of adjacent bays often advance, until a definite relation between the steep and low coasts has been established. When at last the coast-line presents, in the words of Reclus, "a series of regular and rhythmical curves," it may become relatively stable. But by the continuous descent of sediment from the land and its accumulation along low shores, and by the gradual retreat of cliffs elsewhere, complete stability is impossible.

Indented or Irregular Coasts. When we consider the surface of the earth's crust as a whole we recognise two great areas, an oceanic depressed region and a continental elevated region, or, shortly, an oceanic basin and a continental plateau. The larger landmasses are all situated upon, but are nowhere co-extensive with, this plateau, considerable portions of which are under the sea-level. In regions where existing coast-lines approach the margin of the continental plateau, they are apt to run for long distances in one determinate direction, and, whether the coastal

land be high or not, to show a gentle sinuosity. Their course is seldom interrupted by bold headlands or peninsulas, or by long intruding inlets, while fringing or marginal islands rarely occur. Where, on the other hand, the coast-line retires to a great distance from the edge of the oceanic basin, its continuity is constantly interrupted, and fringing islands usually abound. Thus the coast-lines of West Africa owe their freedom from deep indentations, their continuous direction, and general absence of fringing islands, to their approximate coincidence with the steep boundary-slopes of the continental plateau. Conversely, the irregularities characteristic of the coast-lines of North-west Europe, and the corresponding latitudes of North America, are determined by the superficial configuration of the same plateau, which in those regions is relatively more depressed. In a word, coast-lines are profusely indented or not according as they recede from or approach the edge of the continental plateau. Hence all highly indented coast-lines are evidence that the land is sinking, or has recently sunk, the directions of the coastline depending on the form or configuration of the submerged land. If the region be devoid of rivervalleys, as most desert areas are, the coast-line will show no prominent indentations. If, on the other hand, it be well watered and mountainous, its shores will be interrupted by more or less numerous narrow inlets running often far into the land, while peninsulas and fringing islands will probably abound. The fiord