THE ARCHEOZOIC ERA 441 



(1) The schist series is made up chiefly of the metamorphosed 

 products of lava flows and volcanic tuffs. In composition these 

 schists vary greatly; but the dominant types are hornblende 

 schists, greenstone schists, mica schists, etc. Associated with the 

 metamorphosed surface lavas and pyroclastic formations, there 

 are some massive igneous rocks and occasional beds of metamor- 

 phosed conglomerate, sandstone, shale and limestone, and beds of 

 iron ore, all of which imply the contemporaneous activity of water. 

 In a few places, as at Vermilion, Minn., the iron ore is in workable 

 quantities. 



(2) Among the most conspicuous features of the Archean rocks, 

 in their present eroded condition, are the great masses of granite 

 and gneiss that protrude through the schists. Until recently, 

 these granites and gneisses were commonly regarded as the oldest 

 known rocks, and were styled " primitive" or ''fundamental"; 

 but it is now known that many of them, at least, are intrusions into 

 the schist series, and therefore younger than the latter. The 

 gneisses are regarded as metamorphosed granites. 



In the formation of both the surface flows and the intrusions, 

 the ascending lavas must have occupied numerous fissures or con- 

 duits connected with the interior, and hence there came to be 

 numerous dikes and other intrusions, traversing the older parts of 

 the Archean. It is also to be borne in mind that all younger intru- 

 sions and extrusions of lava must have passed through the 

 Archean, leaving dikes and other bodies of igneous rock in it. 

 These later intrusions are of course not strictly a part of the 

 Archean system, but they are not always separable, and their 

 presence adds to the complexity of the Archean as a whole. 



Diastrophism and metamorphism. The most satisfactory ex- 

 planation of the prevalent foliated structure of the Archean (Fig. 

 340) seems to be that which refers it to the movements of the 

 outer part of the earth, in Archeozoic and later time. Intrusions of 

 igneous rock probably helped to metamorphose the rocks by fur- 

 nishing heat and by developing pressure in the zone where they 

 were intruded. Not only this, but the transfer of so much material 

 from below developed lateral pressure by causing the outer parts 

 to settle down to take the place of the lava transferred upward. 



