2 66 MATERIALS AND THEIR ARRANGEMENT 



moved by any agency which shifts materials about on the surface 

 of the earth. If the products of rock disintegration are coarse, they 

 may become gravel after being rounded by streams or waves. If 

 the material is finer, say 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. 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 sediment 

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

 farther out. Thus along many coasts the gravel of the shore-line 

 grades out into sand, and this into mud as distance from the water's 

 edge increases. The coarser materials are thus separated more or 

 less perfectly from the finer. 



When the disintegration of the parent rock results from decay, the 

 rock-waste is unlike the parent rock in composition, because some of 

 the original material has been dissolved and carried away in solution. 

 Not only this, but the fine products of decay may differ' from the 

 coarser in composition. Thus the quartz grains of granite are 

 generally large enough to be readily seen individually; and as the 

 granite 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 have a 

 complex composition, decompose into very fine particles of kaolin 

 (p. 257) or clay, unlike the feldspar in composition, and containing 

 but a few of the elements of feldspar. Thus it happens that the 

 coarser products of decay, such as quartz, are chemically unlike 

 the finer, such as clay, and the two are partly separated when they 

 are deposited. In this case, the composition of the rocks formed 

 from the sediments may be very different from that of the rock 

 from which the sediments were derived. On the other hand, when 

 rock-waste resulting from the mechanical breaking of rock is depos- 

 ited, the sediment has about the same composition as the rock from 

 which it came. Sediment which contains feldspar derived from 

 granitic rock is called arkose. Arkose represents incomplete decom- 

 position of the parent rock. 



