182 FIRST YEAR COURSE IN GENERAL SCIENCE 



stone is colored gray or streaked with black from the carbon 

 in decayed seaweed and animal bodies. The chalk cliffs 

 of Dover, England, are a kind of limestone made from 

 shells so minute that they need no grinding to form a fine- 

 grained rock. Limestone is much used as a building-stone, 

 especially in the central and middle western states. 



204. Metamorphic Rocks. Crystalline rocks which were 

 made from sedimentary or igneous rocks are named meta- 

 morphic rocks. These rocks were made by great pressure or 

 heat in the presence of moisture. The heat, not sufficient 

 for melting, was caused by friction between layers of rock 

 when they were bent or pushed up in mountain making. 



Metamorphic means " changed in structure." Fragmental 

 limestone changes to crystalline marble, soft shale becomes 

 slate, and sandstone becomes schist. 



Two important classes of metamorphic rocks are gneisses 

 and schists: gneisses oftenest produced by metamorphism 

 of granite, and schists by metamorphism of sandstone and 

 shale. Gneisses and schists contain the same minerals as 

 granite, but in different proportions. Gneisses, containing 

 a large proportion of feldspar, split into thick layers; 

 schists, containing more mica, split into thinner layers. 

 Mica schist often contains crystals of the mineral garnet, 

 which is used as a gem. 



Some of the flagstones used in the eastern states are of 

 local schists, but the better flagstones are of a clayey sand- 

 stone from New York. Curbstones in many cities are cut 

 from gneiss. Much so-called "granite" is really gneiss. 



205. Folded Rocks. Metamorphic rocks in hills and 

 mountains are often composed of layers; in this respject they 

 resemble the sedimentary rocks from which they were evi- 

 dently made. They are not in the original position of 

 such rocks, but are tilted at various angles. This change 

 in position of the rocks came about in the following way. 

 As the heated interior of the earth cooled, it shrank; and 



