the United States of America

Rivers and Harbours.

In a very general way the major divides follow the great highlands, hence the rivers entering the Atlantic between the St. Lawrence and the Gulf of Mexico are of moderate size and of little direct value to navigation. Their indirect value is great, however, because all their valleys were carved when the continent stood at a higher level. Subsidence of the land drowned the lower courses of streams, making estuaries and harbours as at New York, Baltimore and all the other Atlantic seaports. The Hudson was drowned to Albany and the Delaware to Trenton, permitting ocean vessels to reach Philadelphia. Chesa peake bay is the largest indentation thus made. One of its branches, the Potomac, is at sea-level as far up as Washington. The St. Lawrence system embraces the five Great Lakes on the Canadian border. The divide limiting the drainage basin of these lakes on the west and south is not far from their shores, hence all tributary streams are short. But the mouths of these streams have likewise been drowned by the tilting of the basin in such a manner that the south-western shores of these lakes are being progressively submerged while the north-eastern shores are rising. Milwaukee, Chicago, Toledo and some other ports owed their original harbours to this cause. The extraordinary harbour of Duluth lies behind a sand bar which has cut off the western end of Lake Superior.

Five-twelfths of the United States (I1 million sq.m.), mainly in the Interior plains, drain to the Mississippi and its branches.

These afford thousands of miles of navigable water, chiefly on the main stream and its tributaries from the humid east. The Ohio is the most navigated stream in America. New Orleans is a river city 100 m. from the Gulf ; it is a seaport only in the sense that sea-going vessels can reach it. Mobile and some other Gulf ports are on drowned valleys altered and supplemented by artificial works. A small estuary at Galveston is fronted by a broad sand reef on which the city stands.

The Mississippi river has received much attention on account of navigation and flood prevention. More than three-fourths of its water comes from three main streams, the Missouri, the Upper Mississippi (above the mouth of the Missouri) and the Ohio. Their several contributions are approximately as f ollows: The Missouri drains the largest basin but has the least rainfall, hence its volume is least. Each year the Mississippi carries to the sea 340,500,000 tons of solid matter in suspension and 136,400,000 tons in soltition. Erosion would thus reduce the level of the entire drainage basin by one foot in 6,000 years.

West of the continental divide on the Rocky mountains are two long rivers, the Snake-Columbia in the north and the Colorado in the south, which traverse the full width of the Intermontane plateaux. Other streams are short and the total amount of water

reaching the Pacific is small in proportion to the great area drained. In this part of the United States, chiefly in the States of California, Nevada, Utah and Arizona, and in the adjacent part of Mexico, is a vast area too arid to drain to the sea, compris ing 3.2% of the area of the continent or nearly a quarter of a million square miles. The Pacific shore, like the Atlantic, has suffered submergence, drowning the lower courses of most streams. Thus was made the harbour of Portland, Oreg., on Columbia river, oo m. from the sea. The local subsidence that made San Fran cisco bay came at a point where the chief rivers of California break through the Coast ranges to the Pacific. Puget sound, with Seattle, Tacoma and other harbours, is similar to San Francisco bay. San Diego, near the Mexican border, owes its harbour to the protection of a magnificent sand reef.

Atlantic and Gulf Coastal Plain.—South of New York, the continent slopes almost imperceptibly to the shore, beyond which the sea bottom continues the very gentle slope many miles, and then descends less gradually to the oceanic abyss. The line where this relatively rapid descent begins is the real edge of the conti nent and this margin of shallow sea bottom is the continental shelf. Within this margin is deposited most of the sand and mud carried down from the land. A large part of the ocean's life is concentrated on this shallow bottom where the water is relatively warm and food is abundant.

Repeatedly in late geologic time this border of the continent has stood lower, and also higher, than at present, thus broadening and narrowing the continental shelf alternately. The part of the shelf above water at any one time is coastal plain. All that is shown on the map (p. 72o) as coastal plain has been continental shelf at a time so recent that its structure remains unchanged. The materials of its underlying strata are the same as those now accumulating beneath the adjacent shallow sea. Some of the sand, clay end ooze has been poorly consolidated into sandstone, shale or limestone. All beds dip faintly seaward, the older and lower ones passing seaward beneath the younger beds, so that the several formations appear at the surface in strips parallel to the coast, the lower beds outcropping farthest inland. As the beds offer unequal resistance to erosion and are unlike in their soil making properties, the striped pattern of the geologic map is apt to be reproduced in the topography of the coastal plain.