304 GENERAL SCIENCE 



carry sediment and also various salts in solution. The 

 evaporation of the water from the surface of the lake 

 increases the density of these solutions. Great Salt Lake 

 has about 15 per cent of various salts in solution, and the 

 Dead Sea has even more. 



Work of Rivers. All streams carry sediment down 

 their valleys. In flood times the amount of sediment is 

 large enough to make the water muddy in quite small 

 streams, while in many of the largest rivers the water 

 is always muddy. Besides carrying sediment in suspen- 

 sion, streams roll sand, gravel, and even large stones along 

 their beds. Much of this coarse material picked up in 

 flood time by a stream is carried for a distance and dropped 

 as the current of the stream becomes slower. This coarse 

 material will be picked up again when the next high water 

 occurs. A large part of the suspended sediment in such 

 rivers as the Mississippi River is carried to the ocean and 

 dropped near the shore. It is estimated that the Missis- 

 sippi River carries an average of more than a million tons 

 of suspended material into the Gulf of Mexico every day. 

 In addition to this it carries a great deal of material in 

 solution. 



All the rivers of the world are constantly at work carry- 

 ing materials from the land to the. sea. The sediment 

 carried by a stream, including that rolled along its bed, is 

 called its load. 



Erosion. The wearing away of the materials which 

 form the earth's crust is called erosion. It is due mainly 

 to the mechanical action and the dissolving power of the 

 surface water. Even the hardest rocks are decomposed 

 by these agencies. In regions which are sufficiently cold 

 the action of the frost also aids in wearing away the 

 surface rock. 



