FKESH-WATER STREAMS. 651 



Still other irregularities result from changes in the river-channel. The transfer of 

 material, from one side of a stream to the other, ends often in making a long bend, and 

 finally in cutting off the bend and turning it into an- island, and ultimately into a part 

 of the mainland, by the rilling up of the old channel. 



The islands in the large rivers are also very unstable. In the Mississippi, as Hum- 

 phreys & Abbot observe, they often begin in the lodging of drift-wood on a sand-bar; 

 this causes the accumulation of detritus; a growth of willow succeeds; the height of the 

 alluvium still increases, until finally the island reaches the level of high water, or rises 

 even above it, and becomes covered with a growth of cotton-wood, willow, etc. By a 

 similar process, the island may be united to the mainland; or, " by a slight change of 

 direction of the current, the underlying sand-bar is washed away, the new-made land 

 caves into the river, and the island disappears." 



Alluvial Fans. When a flooded .stream descends along a steep ravine, the detritus 

 carried down is piled up at the foot of the slope over the plain, making a section of a 

 very low cone, usually 3° or less to 8° or 10°, called by Drew, from their shape, Alluvial 

 Jans. The streams producing these " fans" are small ones, having more transporting 

 than denuding power. The material is bedded, but concentrically, or parallel with the 

 curved surface. When such "fans" are afterward cut through by the little stream, 

 and then partly worn away by the floods of the river in the valley which they border, 

 and then formed anew at an outer and lower level, and so on, the bedding becomes 

 quite complex in its directions and abrupt transitions; and there are parts of successive 

 '" fans " at different levels. (Q. Jour. Geol. Soc, xxix. 441, 1873). 



2. Delta Formations. — The larger part of the detritus of a river is 

 carried to the ocean (or lake) into which it empties ; and it goes to 

 form, about the mouth of the stream, more or less extensive flats. 

 Such flats, when large and intersected by a network of water-channels, 

 are called deltas ; they reach a large size, only where the tides are 

 quite small or are altogether wanting. They are formed from the 

 conjoined action of the river and the ocean, and are sometimes called 

 jluvio-marine formations. Great streams, like the Amazon, carry 

 their muddy waters hundreds of miles into the ocean ; but far the 

 greater part of the detritus, even in the case of the largest rivers, is 

 beaten back by the waves on soundings, and by the shore currents, 

 and either falls in the shallow waters, or is thrown upon the coast 

 near by. In floods, the river-water of the Mississippi is distinguish- 

 able in the Gulf, at the distance of twenty or twenty-five miles from 

 the bar ; in low water, at the distance of only five or ten miles. 

 (Humphreys & Abbot.) 



The eastern North American coast, from Texas to Florida, and 

 from Florida to New Jersey, is nearly a continuous range of fluvio- 

 marine formations. 



Only a single example — that of the Mississippi delta — need here be referred to. 

 The annexed map (Fig. 1093) presents its general features. It commences below the 

 mouth of Red River, where the Atehafalaya " bayou " begins, — - the first of the many 

 side-channels that open through the great flats to the Gulf. The whole area is about 

 12,300 square miles; and about one-third is a sea-marsh, only two-thirds lying above 

 the level of the Gulf. Professor E. W. Hilgard has shown that, about, New Orleans, 

 the modern alluvium has a depth of only thirty-one to tifty-six feet, there existing be- 

 low this the alluvial clay, etc., of the Port Hudson group (p. 548). 



