SOME COMMON ROCKS 259 



"sets," around and between the fragments, much as lime- 

 stone holds sand together in mortar. 



The most common rocks are sandstone and limestone. 

 In sandstone, fragments of sand (quartz) are cemented by 

 iron oxide or silicic (pronounced sil-is'-ik) acid. Lime- 

 stone usually consists of masses, sometimes very small, 

 having the shape of shells and the hard parts of water 

 animals. These are compact, and cemented together by 

 calcium carbonate. Animal and plant remains, as we 

 find them in rocks, are called fossils. 



When the cementing materials of a rock are removed, 

 the rock falls into its fragments. Sandstones are often 

 found that can be crumbled in the hand, owing to the 

 loss of cementing material. 



Shale is a soft rock that splits easily into thin leaves. 

 It is made of flattened particles of clay, with cementing 

 material. 



Granite consists of four minerals : quartz, feldspar, mica, 

 and hornblende. Quartz looks like glass, but is harder. 

 It crystallizes in hexagonal forms (cf. Fig. 75, 95). In 

 granite it forms small, shiny masses. The feldspar of 

 granite is usually pink or white. The mica forms shiny 

 masses that can be split off in very thin leaves (cf. 250) . 

 The hornblende usually exists as black crystals. 



The most easily attacked of the ingredients of granite 

 is feldspar; the carbonic acid of rain water decays it. So 

 the granite's quartz becomes sand, and its feldspar be- 

 comes clay. 



Conglomerate is a gravel of various materials cemented 

 together. 



We have artificial rocks as well as natural ones. In making articles 



