The drowned valleys. The conditions of embayed waters of no great depth, and bottoms that will be fertile when drained, are normally found about the mouths of the larger rivers. The reason for this is that a recent geological accident, the newest of all having a world-wide effect, consisted in a general rise of the sea to the extent of some hundred feet, due to the upward movement of a portion of the deep-sea floor. The gain of the sea on the land led to the flooding of the valleys of the greater rivers for a long distance upward from their ancient mouths; forming such great reentrants of the sea as we have well preserved in the admirable examples of the Chesapeake and Delaware Bays. In many cases these drowned valleys have been so far filled in with delta deposits, as in the case of the Mississippi, that the alluvial plain again projects out into the sea as at its mouth and at the Nile; more commonly there is an embayment, as in the case of Mobile Bay. In any event this inundated valley is certain to have more or less extensive areas of shallow water which, as in Holland, may be drained and turned to cultivated fields.

The work of the mangrove trees. Besides the land won from the sea by the plants which develop the marine marshes in the higher latitude, we find in the tropics a group of trees known as mangroves, which have an even more swift and effective method of capturing land in shallow embayments. These trees are fitted to grow in salt-water silt, submerged it may be by some feet at high tide. They have long runnerlike branches which, as they grow, extend outward and downward into the water of the bays until they touch the bottom, where they take root and form new crowns and stems which in like manner send their runners further seaward. In this way a mangrove swamp will speedily close over a shallow bay even if it be some miles in width, covering it with a dense low forest. While the trees are thus marching outward, their seed, long cylinders in form, with grapples at their lower end, catch on the bottom as they drift away from the plant that bore them, rapidly grow to the surface of the water, and found new plantations. Beneath the very dense growth of the mangroves the scouring action of the tides and waves is arrested and a rapid deposit of plant and animal remains takes place, so that what was sea bottom is soon lifted to the state of a fresh-water swamp. As there are numerous varieties of mangroves in the tropical regions, some of which, as in Florida, extend their range to several degrees further toward the poles, the area they occupy and the land they have won from the sea are alike great. There is no basis for a reckoning as to the extent of their work, but it is evident that in the aggregate these fields must amount to some tens of thousand square miles, all of which have been brought by these remarkable plants into the state where the engineer may easily complete the work of converting them to the uses of man.

Area reclaimable from the sea. Although the basis for computation is imperfect, it may fairly be reckoned that in this debatable ground of the shore zone now occupied by mud flats, marshes, and mangrove swamps, there is a reserve of land awaiting such work of improvement as has been done in Holland, amounting to an aggregate area of not less than 200,000 square miles of land which with a fully peopled earth will be brought into tillage. As this land is of rare fertility and enduring to the tax of cropping beyond that of any upland fields, it has a prospective value as a human asset far beyond an equal area of ordinary ground. They are likely, in time, to afford the food for several hundred million people.

Area reclaimable from rivers. Turning now to the areas of the continents which are occupied by the fresh waters, as in swamps and lakes, we find a more extensive set of fields for reclamation than on the seashore belt - and a much greater variety of problems for the work of the drainage engineer. First we will consider the clearly limited group of areas which lie along the great rivers, where the annual floods render the land untillable. The higher parts of these alluvial plains where the annual inundations are such as to prevent tillage are easily dealt with by ordinary dyking, and have been thus improved in all the great valleys of long-occupied countries. Yet there remains along the larger streams of Africa, the Americas, and Northern Asia aggregating several hundred thousand square miles of naturally fertile land still unwon to use. A rough reckoning of these areas which gives only approximate results, indicates that the possible winning in the ultimate state of culture will amount to not less than 300,000 square miles with a tillage value for the area quite as great as that which may be had from the gains made on the seashores, or the possible subsistence of many million. If it should prove possible to till the middle and lower reaches of the great rivers which flow toward the Arctic Ocean, the Mackenzie in North America, and the several streams that traverse Siberia, the aggregate area of useful alluvial land may be much greater than is indicated by this reckoning.

How glacial lakes were formed. The true morasses, those inundated fields lying outside the alluvial fields, are much more abundant than the winnable flooded ground beside the rivers. The most common of this group are the bogs formed in the lakes which gathered in the shallow pits that were shaped by the irregular disposition of the drift left on the surface of those areas occupied by the ice in the last glacial period. When that covering melted away these basins so placed as to hold water were almost incredibly numerous. Thus, in New England, when the earth was cleared of the glaciers, the number of them varying in size from areas of an acre to those one hundred square miles in extent were to be numbered by, the tens of thousands. The writer has estimated that not less than ten per cent of this district was thus covered with tarns or lakes. Taking the glaciated parts of the world as a whole, the disturbance of the drainage induced by the ice invasion probably brought about something like this proportion of inundated lands where in the earlier times the brooks and rivers had in their usual manner provided a complete drainage.

Growth of peat bogs. As soon as the glacial sheet had disappeared and the basins held in its debris were filled by water, a process of closing them began, a process which has been continued to our own day. Along the shores of each of those lakes where the waves did not have too much power to admit of such growth, a species of moss known as sphagnum, the form familiar in almost any swamp, found a foothold. The microscopic spores of this plant are readily borne by the wind for many miles from their parent stations, so that as fast as the pools were formed, the growth began, and as the ice sheet retreated the mosses were always ready to set about their peculiar work. Their task is, indeed, one of the most extensive and important of those performed by vegetable life. It is as follows: