346 PRACTICAL LESSONS IN SCIENCE. 



ever the current is checked. When a mountain torrent, with its 

 burden of sediment, flows out upon a plain, it drops the greater 

 part in a fan-shaped mass, the apex pointing up the stream ; lit- 

 tle streams flowing into ponds deposit sediments in the same 

 shape ; and the delta of the Mississippi is an enormous mass of 

 sediment of the same form, and deposited in the same way, the 

 current slackened by the waters of the Gulf. 



A river seldom deposits any sediments along its mountain 

 course, but when it enters its middle course the current is lessened 

 and the sediments deposited sometimes raise the bed of the river 

 above the level of the adjacent land. The same thing occurs fre- 

 quently in rivers flowing through low plains or deltas. Some 

 rivers in Northern Italy illustrate the first and the Mississippi 

 river the second. During floods rivers often spread out over 

 adjacent low lands which are covered with vegetation ; the rough 

 ground and the vegetation check the current and a layer of sedi- 

 ment results. Turbid rivers flowing into lakes drop their sedi- 

 ments, the water at the outlet being clear. In this way lakes 

 are sometimes obliterated, entirely filled by the sediments of their 

 inlets. The course of a stream consists of a system of irregular 

 curves or bends, with occasional stretches of nearly straight 

 channel. On the convex side of a curve the current is strong 

 and erosive, while on the concave side it is weaker and depos- 

 iting sediments. In this way streams change their channels, 

 wandering about their valleys, eroding old sediments and depos- 

 iting new, so that a bit of gravel may form a portion of scores 

 of different gravel beds before it reaches the ocean. The whole 

 matter of the geological work of streams can be studied in almost 

 every phase along any little brook or creek in the country. 



Marine erosion is very extensive, but is not as rapid or exten- 

 sive as river erosion. The waves with their burden of sand or 

 gravel gradually undermine the rocks so that they fall within 

 reach of the waves ; the smaller fragments are rolled backward 

 and forward by the waves, hurled against the larger ones till at 

 length they are broken up, and the fragments abrading each 

 other become pebbles that slide up and down the beach with 



