24 NATURE STUDY REVIEW [9:1— Jan., 1913 



000 tons of suspended matter are carried to the sea every year. 

 Another esthnate, but only for the Mississippi valley, is one foot 

 in 3,500 years. Whatever the rate may be, the constant wearing 

 away of the land supplies material for beds of sediments now 

 being deposited in the sea. 



As a definition for a rock we may use the following : a rock 

 is a mineral or a mass of minerals that forms an essential part 

 of the earth's crust. By this definition many things that are not 

 commonly regarded as rocks, become rocks, such as the soil, sand 

 hills, and clay deposits. The important feature is that the rock 

 shall be an essential part of the crust. (By crust we mean merely 

 the outer part of the earth and do not in any way refer to its 

 origin.) The size of a rock cannot be arbitrarily fixed. Gypsum, 

 salt, and limestone (which consists almost entirely of calcite) are 

 examples of rocks that are composed of one mineral. The greater 

 part of the rocks of the earth's crust is composed of more than 

 one mineral. All the common minerals that are found in rocks 

 are included in the table of common minerals in the previous 

 paper on this subject, hence any one should be able to determine 

 the minerals in the more common rocks, and knowing these, the 

 rock name can be easily found. Familiarity with the common 

 rock-forming minerals should be acquired by all before begin- 

 ning to work with the rocks. 



There are three principal classes of rocks. They are as fol- 

 lows: igneous, sedimentary and metamorphic. Ninety-five per 

 cent of the outer ten miles of the earth is estimated to be igneous 

 rock and five per cent is sedimentary. The metamorphic rocks are 

 also included in this estimate. Of the sediments four per cent is 

 shale, three-fourths of a per cent sandstone and one-fourth lime- 

 stone.^ The igneous rocks are those that have been formed by 

 the solidification of molten masses from within the earth. They 

 are the primary rocks from which all others have been derived, 

 in fact they were once called "primary rocks." When the igneous 

 mass solidifies below the surface it is called an intrusive rock. 

 Since the process of cooling under these conditions would be 

 slow, the minerals that compose it would have an opportunity to 

 grow large, hence intrusive rocks are generally coarse grained. 

 The intrusive rocks have various modes of occurrence. When 

 they fill long narrow fissures or joints cutting across the other 

 rocks they are called dikes. When they are in thin layers between 

 beds of other rocks they are known as sills. Large irregular 



iClarke, Bulletin 491, p. 32, U. S. Ceol. Survey. 



