THE COMPOSITION OF THE EARTH 35 



The oldest known rocks are igneous rocks, or metamorphic 

 rocks which have been produced from them. Since all 

 other rocks have been formed directly or indirectly from 

 igneous rocks, the latter have been called the mother rocks. 

 Igneous rocks, or their altered products, are thought to un- 

 derlie all other kinds of rocks, and to make up a very large 

 proportion of the earth's mass. 



SEDIMENTARY ROCKS 



The formation of sediments. It has been seen (p. 16) 

 that the greater part of the land surface is covered with loose 

 rock material formed from the solid rock. This rock waste 

 varies greatly in size, ranging from fine clay, through sand and 

 gravel, to large pieces of rock. It is a matter of common ob- 

 servation that the finer material is shifted frequently from 

 place to place. Winds blow dust in quantity from roadways 

 and the bare surfaces of fields. Rain sometimes washes large 

 amounts of earth down the sides of freshly plowed hills. 

 Streams are commonly made muddy in rainy weather by the 

 fine silt which they carry, and they drag and roll coarser ma- 

 terial, such as sand and gravel, along their channels. Since 

 water always flows down slope, the material it carries is also 

 moving to lower levels. And because all the water which 

 does not sink into the ground, evaporate, or stop in some lake 

 runs to the sea, it follows that much of the rock waste it moves 

 is carried into the ocean. 



If water from any stream is evaporated, a mineral residue 

 remains. This means that rivers are carrying mineral matter 

 to the sea in solution as well as in solid pieces. The Thames 

 River of England carries over a ton of dissolved matter to the 

 sea each minute on the average. It has, indeed, been declared 

 that the one great mission of running water is to get the land 

 into the sea. The dissolved material is likely to remain in 

 solution in the sea water for a longer or shorter period, some 

 of it indefinitely. Nearly all of the sediment which is carried 

 to the sea in the solid form soon settles to the bottom, 



