FRESH-WATER STREAMS. 643 



centuries only of work. Lyell mentions the case of the Simeto, in 

 Sicily, which had been dammed up by an eruption of lavas in 1603. 

 In two and a half 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. He also describes a gorge made in a 

 deep bed of decomposed rock, three and a half miles west of Milledge- 

 ville, Georgia, that was at first a mud-crack a yard deep, in which the 

 rains found a chance to make a rill, but which, in twenty years, was : 

 300 yards long, 20 to 180 feet wide and 55 feet deep; and Liais 

 describes a similar gorge, of twice the length, in Brazil, made in forty 

 years. These erosions of soft material show what may be done in 

 hard rocks, when time for the work is given. The most of the valleys 

 of the world have been formed entirely by running water. Subterra- 

 nean movements have sometimes made fissures that have determined 

 the direction of the water ; but this has not been ordinarily the case. 

 At Tahiti, where the valleys are one to three thousand feet deep, they 

 all terminate before reaching the sea, showing that they have been 

 formed while the land has stood, as now, above the ocean, and there- 

 fore that they are due to fresh-water streams. 



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

 ous 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 1,080 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. The rushing waters make 

 the large loose masses to rock, and this wears the surface beneath, and 

 gradually deepens it ; and then the whirl begins which carries around 

 stones and pebbles, and hastens the wear. Or, where the waters are 

 made to whirl by the position of the rocks of the bottom, the whirled 

 stones are at once set about the work of excavation. The " Basin," in 

 the Franconia Notch (White Mountains), 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 White River, in the Green Mountains, and elsewhere. One 

 of those orf the White River is fifteen feet deep and eighteen in 

 diameter ; another, twelve feet deep and twenty-six in diameter. 



4. 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 features 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 



