ROCKS OF THE EARTH'S CRUST 189 



very hot. If the cooling has taken place slowly, we have a crys- 

 talline structure, of which granite is an example. Where the 

 cooling took place very rapidly, the same material produced 

 what is called natural glass, or obsidian, having no structure 

 whatever. Where lava is blown up by the expansion of 

 steam, pumice and volcanic ash are formed. 



Metamorphic Rocks. Where rocks have been subjected 

 to the action of earth heat, or great pressure, or both, certain 

 changes have taken place in them. We call this metamor- 

 phism. Thus sandstones may become a solid mass of quartz, 

 and shale change to slate. Coal, in some cases, has been 

 changed to graphite, which is pure carbon. Under the effects 

 of metamorphism, some minerals have been recrystallized, 

 according to some other plan, and other rocks have had their 

 nature entirely altered. 



Chemically Formed Rocks. Nearly all of this kind of 

 rock is formed beneath the surface of the earth, although it 

 does appear around geysers and hot springs. Some of the 

 minerals through which water flows dissolve when the water 

 is hot, and may impart carbon dioxide to the stream. As the 

 water cools or loses its carbon dioxide, the dissolved material 

 is deposited. This forms stalactites in caverns, calcareous 

 tufa around the hot springs, and silica around the geysers of 

 Yellowstone Park. 



^olian Rocks. There is one other class of rocks which, 

 because they have been caused by winds, are called ceolian. 

 The wind, picking up sand and pebbles, has blown them into 

 hollows, and as the centuries went by has piled tons upon tons, 

 which, on account of the pressure, and the presence of some 

 cementing material, have finally formed solid rocks. Sand 

 dunes may be considered under this head, although they are 

 not compact. 



