414 Introduction to the Study of Science 



cemented and compressed. These rocks and the materials 

 composing them are all of mechanical origin, derived from 

 igneous or other preexisting rocks or minerals by weathering, 

 or dissolved by water and transported and deposited in bodies 

 of water. The minerals are chiefly silica or quartz forming 

 sand beds, clay forming clay beds, and shells eventually form- 

 ing carbonate of lime beds. 



The limestone deposited by the water of effervescent springs, 

 and in caves as stalactites and stalagmites, is of chemical origin. 

 Limestone formed from the mineral remains of marine or- 

 ganisms, as shells and other secretions, the chalk formed from 

 deep sea ooze, and the coquina and reef limestone are all of 

 organic origin. 



Metamorphic rocks are formed from sedimentary and other 

 rocks by pressure, heat, and moisture. This group includes 

 marbles from limestone, slate from shale and quartzite from 

 sandstone that was made up principally of quartz granules. 

 Other rocks of this class are gneiss, which is often metamor- 

 phosed granite ; bituminous coal formed from lignite and peat ; 

 anthracite coal, metamorphosed from bituminous coal; and 

 graphite. 



The igneous rocks are not stratified and contain no fossil re- 

 mains. They are evidently the product of the molten mass 

 of minerals that issued from volcanic craters or fissures, or 

 cooled in the depths of the earth. Such rock is often thor- 

 oughly crystalline in structure, though it may be a glass rock 

 due to a rate of cooling and hardening too rapid to allow the 

 formation of crystals. The best known igneous rock is granite. 

 The glass rock known as obsidian contains a large amount of 

 silica. Lava, pumice, and basalt are familiar. Notable ex- 

 amples of igneous rock often mentioned are the columns of 

 the Giant's Causeway on the western coast of Ireland (basalt 

 rock), and the Palisades of the Hudson River. 



