We may say, then, that on shores of elevation the presence of sand reefs and lagoons indicates the stage of youth, while the absence of these features and the vigorous and unimpeded attack by the sea upon the mainland indicate the stage of maturity. Where much waste is brought in by rivers the maturity of such a coast may be long delayed. The waste from the land keeps the sea shallow offshore and constantly renews the sand reef. The energy of the waves is consumed in handling shore drift, and no energy is left for an effective attack upon the land. Indeed, with an excessive amount of waste brought down by streams the land may be built out and encroach temporarily upon the sea; and not until long denudation has lowered the land, and thus decreased the amount of waste from it, may the waves be able to cut through the sand reef and thus the coast reach maturity. SHORES OF DEPRESSION Where a coastal region is undergoing submergence the shore line moves landward. The horizontal plane of the sea now intersects an old land surface roughened by subaerial denudation. The shore line is irregular and indented in proportion to the relief of the land and the amount of the submergence which the land has suffered. It follows up partially submerged valleys, forming bays, and bends round the divides, leaving them to project as promontories and peninsulas. The outlines of shores of depression are as varied as are the forms of the land partially submerged. We give a few typical illustrations. The characteristics of the coast of Maine are due chiefly to the fact that a mountainous region of hard rocks, once worn to a peneplain, and after a subsequent elevation deeply dissected by north-south valleys, has subsided, the depression amounting on its southern margin to as much as six hundred feet below sea level. Drowned valleys penetrate the land in long, narrow bays, and rugged divides project in long, narrow land arms prolonged seaward by islands representing the high portions of their extremities. Of this exceedingly ragged shore there are said to be two thousand miles from the New Brunswick boundary as far west as Portland,--a straight-line distance of but two hundred miles. Since the time of its greatest depression the land is known to have risen some three hundred feet; for the bays have been shortened, and the waste with which their floors were strewn is now in part laid bare as clay plains about the bay heads and in narrow selvages about the peninsulas and islands. The coast of Dalmatia, on the Adriatic Sea, is characterized by long land arms and chains of long and narrow islands, all parallel to the trend of the coast. A region of parallel mountain ranges has been depressed, and the longitudinal valleys which lie between them are occupied by arms of the sea.