142 GEOLOGY. 



It is not improbable that the Archean is really much more com- 

 plicated than this rather simple statement implies. Not unlikely, 

 it consists of a much more extended succession of formations, reaching 

 indefinitely downward, a succession such as might naturally arise 

 during the processes of evolution sketched in the previous chapter. 

 But at the present stage of investigation it is perhaps sufficient for 

 a general view to recognize two great classes of formations as making 

 up the definable Archean, viz., (1) a great schist series or group of 

 series, and (2) a great granitoid series or group of series. 



(1) The great schist series. — The schists of this series are the meta- 

 morphosed products of the lava flows, volcanic tuffs, and the included 

 sedimentaries. The transformation involved both katamorphism and 

 anamorphism; i.e., the lava-flows were mashed down into the schistose 

 structure (metamorphism) and the elastics, while also pressed into 

 the schistose form, were built up molecular ly into a crystalline struc- 

 ture (anamorphism). In some portions, however, the original massive 

 condition of the lavas is retained. In composition these schists and 

 massive igneous rocks vary greatly, but since the most abundant 

 material is of the neutral and basic types, the commoner metamorphic 

 rocks are hornblende schists, greenstone schists, mica schists, etc. 

 The local formations of the Lake Superior region, the Mona schists, 

 the Kitchi schists, the Quinnissec schists, and the schists of the Keewatin, 

 are regarded as members of this group. How far these and similar local 

 series are equivalent, and how far they represent successive formations, 

 it is not now possible to say. 



(2) The great granitoid series (Laurentian). — Among the most con- 

 spicuous features of the regions of Archean rock, in their present eroded 

 condition, are the great masses of granite and gneiss that commonly 

 appear as embossments protruding through the schists, or as the country 

 rock, over great areas. Like the schists, they are affected more or 

 less by later intrusions. Until recently, these granites and gneisses 

 were rather commonly regarded as the oldest known rocks. Accord- 

 ing to this view they constituted a basal formation, and so were styled 

 " primitive" or " fundamental. " The granites in particular were 

 once quite generally regarded as portions of the original crust of the 

 supposed molten globe. The associated gneisses and other foliated 

 rocks, however, were sometimes regarded as the highly metamorphosed 

 products of very early sedimentaries, supposed to have been derived 



