344 



PHYSICAL GEOLOGY 



Classification of Metamorphic Rocks 



Quartzite. — A quartzite is a metamorphic sandstone. It is a com- 

 pact rock composed of grains of quartz sand cemented by material 

 of the same kind, that is, by silica. It can usually be distinguished 

 from sandstone by its appearance when broken. The broken sur- 

 face of sandstone usually has a more or less granular feel and appear- 

 ance, since the fracture takes place in the weak cement leaving the 

 grains outstanding. In quartzite, on the other hand, since the grains 

 and cement are of the same material, the fracture takes place in 

 cement and grains alike. It is often difficult to state whether a 

 quartzite owes its character to heat and pressure or to cementation 

 by underground water. A quartz schist is a quartzite in which a 

 foliated structure has been developed, the planes of foliation being 

 covered with white mica. 



Marble. — Commercially, any calcareous rock which will take 

 a polish is called a marble, but in a more technical sense a marble 

 is a metamorphic limestone. It is distinguished from limestone by 

 its granular appearance (texture) and, unlike most metamorphic 

 rocks, if pure is not schistose. When limestone is heated where the 

 pressure is slight, it is converted into quicklime by the escape of 

 carbon dioxide ; but when heated under pressure, which prevents 

 the escape of the gas, it crystallizes into marble. The clouded 

 shadings and " veins " of marble are produced by the crystallization 

 of impurities, with the resulting formation of colored minerals. 



Slate. — This rock may be considered as a hardened shale or mud 

 in which a tendency to break along parallel planes — not bedding 



planes — is developed. This con- 

 dition is called slaty cleavage and 

 by its means the rock splits 

 readily into broad, thin sheets. 

 The cause of slaty cleavage is 

 to be found in the great lateral 

 pressure to which such fine- 

 grained sediments as clay or 

 volcanic ash are sub- 

 one direction, they are 

 A rock is affected by 



337. — Slaty cleavage in folded rocks. 

 (Modified after Pirsson.) 



(rarely) 

 jected, especially if, when compressed in 

 able to expand to some extent in others. 



such compression in three ways : (1) any particles capable of 

 compression are flattened and correspondingly lengthened at right 



