The processes which develop coastal indentations, together with the antecedent subaërial and the subsequent wave gradation, account for most of the islands which affect indented coasts. Some of them are high and some low for reasons which will be readily understood. The long narrow belts of land constituting irregularities parallel to the general trend of the coast (Figs. 319 and 320) are usually the result of deposition in shallow water. They are usually sand or coral reefs, built up above water-level by waves. The deposits at the debouchures of streams give rise to projecting deltas. Most small irregularities of angular form, especially if high (Pl. XX), indicate wave-erosion, and their details of form are determined by the structure of the rock along shore, while most irregularities of curved outline involve something of shore-deposition, if not due wholly to it. Glaciation, or glaciation and subsidence, may also give rise to peninsulas, capes, and islands of curved outlines (Pl. XXIV, coast of Maine). Curving outlines may, however, be developed by erosion alone in weak rock structures. This is illustrated by the weak rock structures (clay, sand, etc.) of most of the Atlantic coastal plain. Thus inspection of the horizontal configuration of coasts will often indicate the processes which have been dominant there in recent times. On the other hand, the interpretations of many coastal irregularities, such as Hudson Bay, Puget Sound, the Gulf of California, the Baltic Sea, etc., are not to be read from the map. In such cases, diastrophism and gradation have usually coöperated, but the relative importance of the two processes can only be determined by detailed study in the field. When it is remembered that the tendency of shore-erosion is to reduce great irregularities of horizontal configuration, though not to obliterate small ones if the coast be heterogeneous in composition (p. 353), and that the tendency of shore-deposition is also to regularity, it is clear that the great irregularities of coast-lines are due neither to shore-erosion nor to shore-deposition, though minor ones may be due to either. THE WORK OF OCEAN-CURRENTS.