THE STORY OF THE EARTH. 



It is on evidence of this kind that geological 

 time is divided into stages, ages, and epochs, 

 which record a series of transitions and SU( 

 sions, which the life of a limited part of the globe 

 has undergone. Sometimes the diffusion of world- 

 wide species appears to have been as remarkable 

 in the seas of old geological periods, as any geo- 

 iiical extension of living species which is 

 known at the present day. 



CHAPTER X. 



THE AR< BEAN ROCKS. 



THE most ancient rocks are termed Archean. 

 They consist chiefly of crystalline schists, and 

 other crystalline substances, such as quartzite, 

 limestone, graphite. Formerly they were gener- 

 ally regarded as metamorphic. At the present 



day some writers do not believe that they 



Stallized out of ancient strata, which were ac- 



i la ted in water. Nevertheless they show in 



many localities, and especially in the 1 anient ian 



rock- i t Canada, two constituents which may in- 



a stratified origin. One is the presence of 



ystalline limestone, which is not known 



m nature, e\< ept by metamorphism of 



limestone built up by organisms which lived in 



• - e ] aurentian r< >cks contain 



. mount of graphite, which has been stated to 



ik, if l! ; ii together, 



uantit) i d, No 



Upon the earth for th< n <>i w huh 



ept the nieta- 



