CONSTRUCTION OF THE EARTH. 



127 



fore called fossiliferous rocks. Metamorphic rocks are 

 stratified rocks which have been changed by the action of 

 heat, and perhaps some other auxiliary agencies. In this 

 alteration, any fossils that the rocks originally contained 

 are obliterated, the material which composed them hav- 

 ing been altered with the rest of the rock in the arrange- 

 ment of its particles. Thus limestone containing shells 

 and corals has been often converted into granular lime- 

 stone or marble, a crystalline texture being thus given to 

 the whole. The term metamorphism means transforma- 

 tion, and comes from two Greek words, meta, which is 

 th'e same as trans in the Latin, and morphe, form. A 

 very decisive proof that heat is the principal cause of 

 metamorphism we have in the fact that great artificial 

 heat, if long continued, changes the structure of stones. 

 Sandstone used in furnaces has sometimes been known 

 to become crystalline that is, has been metamorphosed. 

 The unstratified rocks are not divided into parallel lay- 

 ers, but they are commonly a shapeless mass, as seen in 

 Fig. 49 (p. 128). They were not deposited from sedi- 

 ment, but were formed under the influence of heat, and 

 were thrust up from within the crust of the earth. They, 

 of course, never have any fossils in them. They general- 



