THE COMPOSITION OF THE EARTH 41 



(4) These sediments are cemented to form conglomerate, 

 sandstone, and shale, the principal classes of fragmental 

 rocks. (5) Limestones are formed from organic remains, 

 and sometimes by precipitation from solution. (6) These 

 sedimentary rocks may themselves be exposed and may decay, 

 and the resulting waste may be carried to the sea, or other 

 lodgment areas, to form new sediments and rocks, which may 

 in turn experience a similar fate. In this manner many gen- 

 erations of sedimentary rocks have been formed, and later 

 more or less wholly destroyed. Since all sedimentary rocks 

 are formed from still older rocks, they are sometimes called 

 secondary rocks. 



METAMORPHIC ROCKS 



Metamorphic means changed, and metamorphic rocks are 

 those which, originally igneous or sedimentary, have been 

 altered in composition, or in texture, or in both, since they 

 were made. Metamorphism may result in the weakening and 

 decay of rocks, or it may strengthen and consolidate them. 



When rocks formed at or near the surface are buried deeply 

 beneath later beds, they encounter conditions very different 

 from those under which they were made. They are under the 

 great pressure of the rocks above, and may also be subjected to 

 lateral compression. They are affected by higher temper- 

 atures, and are acted upon by ground waters that are made 

 powerful chemically by heat and pressure. In consequence of 

 these things, their composition may be altered, their minerals 

 changing into other minerals whose chemical composition is 

 more stable under the new conditions. They may become more 

 thoroughly consolidated, harder, and more crystalline. They 

 may develop also a sheeted or banded structure (Fig. 18), 

 which is distinct from the stratification of sedimentary rocks. 

 Similarly, when igneous rocks that were formed by the solidi- 

 fication of lavas at great depths are exposed at the surface 

 through erosion, they find entirely new conditions. They 

 are subjected to the influences of temperature changes, of the 



