324 PRACTICAL LESSONS IN SCIENCE. 



varies widely in appearance but is usually recognized without 

 difficulty. 



Under the action of atmospheric influences the rocks are gradu- 

 ally pulverized, reduced to boulders, pebbles, gravel, sand and 

 clay. We may find all kinds of rocks represented among the 

 boulders and pebbles, while the gravel and sand are largely of 

 quartz. The clay, dust, etc., making up the soil are from the 

 feldspar, mica and other easily disintegrated materials. Much 

 of the clay, sand and dust are eventually spread out over the sea 

 bed as sediments. Great quantities of rocks are broken down 

 and pulverized by the action of the waves along the sea coast 

 and added to the sediments of the sea. 



Pebbles or gravel, and sometimes boulders, may be cemented 

 together into conglomerate rock, and it is called quartz, lime- 

 stone or granite conglomerate, according to the nature of the 

 material. Conglomerates differ also in the nature of the cement 

 or paste, which may be of quartz, lime, clay or even iron. 



The sand may be cemented into sandstone, which may vary 

 much as the conglomerate varies. The typical sandstone, how- 

 ever, is composed of quartz grains united by a paste of quartz. 



The clays are compacted into shales. They are easily cut with 

 a knife, break into irregular blocks and vary greatly in purity. 



The finer sands and coarser clays mingle in varying propor- 

 tions so as to form clayey sandstones or sandy shales. 



In deeper water, away from the shore, calcareous sediments 

 are formed from materials furnished mainly by different forms of 

 life; these at length become limestone rocks. Peat and soft coal 

 are sediments. 



The different kinds of sandstone, clay and limestone rocks are 

 formed from sediments, and are known as sedimentary rocks. 



Metamorphic rocks are sedimentary rocks that have been 

 changed by heat, pressure and other agencies into a crystalline 

 form. The sandstones become quartzites ; the mixed sands and 

 clays become gneiss, granites, etc. Limestone becomes crystal- 

 line limestone and marble, and the clays and shales become slate, 

 which is but slightly crystalline. Peat and soft coal become 



