MATERIALS OF THE EARTH 35 



of the size of small grains, it is sand; if still finer, it is mud when 

 wet, and dust when dry. 



Deposition of sediment. When carried by any transporting 

 agency, such as wind or water, rock waste becomes sediment, and 

 sooner or later is deposited as such. Some of the material picked 

 up and transported by running water is left at the bases of the 

 slopes of mountains and hills from which it is washed, and some 

 of it is left on the flats through which streams flow; but much of it 

 is carried to the sea and left there. The coarser part of the sedi- 

 ment carried to the sea is left near the shore, and the finer parts 

 are taken farther out. This is seen along many coasts where the 

 gravel of the shore-line grades out into sand, and this into mud 

 as distance from the water's edge increases. Thus it comes about 

 that the coarser materials are more or less perfectly separated from 

 the finer. 



When the disintegration of the parent rock is by decay, the 

 fine products are usually of different composition from the coarser. 

 Thus the quartz grains of the granite are generally large enough 

 to be readily seen individually; and as the rock decays, this mineral, 

 already a simple compound, undergoes little change, and the grains 

 remain in the rock waste. By moving water, they are rounded 

 into the sand grains with which we are familiar. On the other 

 hand, the crystals of feldspar, which has a complex composition, 

 decompose into very fine particles (of kaolin or clay, A1 2 O 3 . 2SiO 2 - 

 2H 2 O) unlike the feldspar in composition, and containing but a few 

 of the elements of the feldspar. Thus it happens that the coarse 

 materials, such as quartz, are chemically unlike the finer materials, 

 such as the clay. Running water and waves assort the materials 

 on the basis of their size; but the result is often the separation of 

 materials which are chemically unlike. Thus beds of sand are 

 accumulated, quite separate from beds of clay. They are sep- 

 arated in deposition because they are unlike physically, but in 

 this case, physical unlikeness goes with chemical unlikeness. 



Sediments which result from the mechanical breaking up of the 

 rock are like the original rock in composition. When deposited 

 as sand or gravel, such sediment might have about the same com- 

 position as the rock from which it was derived, if there was no 



