3i8 ARCHEOZOIC ERA 



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

 products of lava flows and volcanic tuffs. In composition they vary 

 greatly, but the dominant types are hornblende schists, other green- 

 stone schists, and mica schists. Associated with the metamorphosed 

 surface lavas and pyroclastic formations, there are some massive 

 igneous rocks, and occasional beds of metamorphosed conglomerate, 

 sandstone, shale and limestone, and beds of iron ore, all of which 

 imply the contemporaneous activity of water. 



(2) The granitoid series. One of the conspicuous features of 

 the Archean system, in its present eroded condition, is the great 

 masses of granite and gneiss that protrude through the schists. 

 Formerly, these granites and gneisses were regarded as the oldest 

 rocks, and were styled "primitive" or "fundamental"; but it is 

 now known that many of them, at least, are intrusions into the 

 schists, and therefore younger than the latter. The gneisses are 

 regarded as metamorphosed granites. 



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

 the ascending lavas must have occupied numerous fissures or con- 

 duits connected with the interior; hence there are numerous dikes 

 and other intrusions, traversing the older parts of the Archean. 

 It is to be borne in mind also that all younger intrusions and extru- 

 sions of lava must have passed through the Archean, leaving in- 

 trusions of diverse sorts (p. 228). These later intrusions are not 

 strictly a part of the Archean, 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. 

 294) is that which refers it to the movements of the outer part of 

 the earth in Archeozoic and later time. Intrusions of igneous rock 

 probably aided metamorphism (i) by furnishing heat, and (2) by de- 

 veloping pressure. The pressure was developed in two ways, (a) by 

 the intrusion itself, which developed pressure when it was intruded, 

 Sand (6) the shifting of so much lava from below upward must have 

 ,'caused the outer parts of the earth to settle down to take the place 

 of the material transferred upward. 



That the rocks should have been much metamorphosed under 

 these conditions is natural. By crushing and shearing, massive 

 igneous rocks were given a foliated or schistose structure, and it is 

 in the rocks of this era especially that metamorphism of this type 

 prevails. It is now believed that the larger part of existing gneisses, 



