WATER. 641 



centuries it had excavated a channel fifty to several hundred feet 

 deep, and in some parts forty to fifty feet wide, although the rock 

 is a hard solid basalt. The larger part of the valleys of the world 

 are formed entirely by running water. At Tahiti, where they are 

 one to three thousand feet deep, they all terminate before reach- 

 ing the sea, showing that they have been formed while the- land has 

 stood, as now, above the ocean. 



The windings of the stream in large alluvial flats are most nume- 

 rous where the current is exceedingly slow ; for slight obstacles 

 change the course, throwing the current from one side to the 

 other. Between the mouth of the Ohio and the Gulf of Mexico 

 (head of the Passes), the length of the Mississippi is 1080 miles, and 

 the actual distance in a straight line about 500 miles. 



Pot-holes are incident to the process of erosion when the waters 

 flow in rapids over a bed of hard rocks. Any obstacle causes the 

 waters to move in a whirl and carry around pebbles or stones, and, 

 by this grinding process, circular pits or basins are worn in the 

 solid rock. The " Basin" in the Franconia Notch (White Moun- 

 tains) is a pot-hole in granite, fifteen feet deep and twenty and 

 twenty-five feet in its two diameters. There are many pot-holes at 

 Bellows Falls, on the Connecticut ; others on the White Eiver, in 

 the Green Mountains, and elsewhere. One of those on the White 

 River is fifteen feet deep and eighteen in diameter ; another, twelve 

 feet deep and twenty-six in diameter. 



3. Flood-plain. — The facts connected with the flood-plains derive 

 a special importance from their bearing on the subject of terraces. 



The breadth of the flood-plain of a stream depends (1) on the general fea- 

 tures of a country, and (2) on the stream's capability of encroaching laterally 

 on the hills either side. In some cases this breadth is ten to twenty miles, and 

 even fifty miles along such rivers as the Sacramento. In the case of these broad^ 

 plains, the valley is seldom one of erosion simply, but generally a synclinal trough. 

 When a stream crosses a series of synclinal valleys, the flood-plain generally 

 expands as it enters each, and contracts at the passage from one to the other. 



The surface of a flood-plain is only approximately flat. (1) The margin 

 along a stream is often higher than the part back of it; (2) some portions 

 are frequently within the reach of only the very highest freshets; (3) others 

 are quite low, and are sometimes occupied by ponds of water or lagoons 

 fed from the river by percolation through the soil. The variation of height 

 from these sources is often equal to two-thirds of the whole average height 

 of the flood-plain above the river. The surface is sometimes changed much 

 in height during freshets, by the wearing away of one part and the increase 

 of others. 



The height and pitch of the flood-plain are essentially that of the stream at 

 flood-height, and will, therefore, be affected by the oauses mentioned on page 



42 



