METAMORPHISM. 765 



6. Chronological Relations of Metamorphic Rocks. 



It is generally admitted that metamorphic rocks are not of any par- 

 ticular geological age. But it is still queried whether particular kinds 

 do not characterize periods or ages. 



Some peculiarities of Archaean formations are stated on page 152, 

 namely, their comprising beds of crystalline hematite, and magnetite 

 that are often twenty-five to over a hundred feet thick ; and showing 

 the unusual amount of iron of the era also in the wide distribution of 

 hornblende-bearing rocks, such as syenyte and syenytic gneiss. Be- 

 sides, metamorphic rocks containing corundum, chrysolite, and zircon 

 are far more common among the older Archaean terranes than in sub- 

 sequent formations. The older Archaean gneisses, which also are 

 widely distributed, are usually thick-bedded and much jointed ; and 

 the bedding is often faint and obscured by the joints ; yet in other 

 cases the bedding is brought out with bold contrasts of color between 

 the black hornblende beds or mica layers and the whitish feldspathic. 

 Coarsely crystallized Labradorite rocks, such as noryte, are also a fea- 

 ture of some of the older Archaean regions, — rocks that have the same 

 composition as the dolerytes among igneous kinds. But thin fissile 

 mica schists and evenly schistose and easily-cleaving gneisses are not 

 common. 



Thus there are some characters that occasionally give aid in the de- 

 termination of the Archaean age of a metamorphic rock. But, in 

 general, Archaean gneisses and granites cannot be distinguished from 

 those of later time ; and there are Archaean quartzytes, diorytes, horn- 

 blende schists, chlorite schists, which are identical in all respects with 

 others of subsequent origin. This is what should be expected, since 

 the Archaean rocks are the chief source of material for the later rocks. 

 Their trituration produced sediments ; metamorphism of the sediments 

 reproduced the crystalline rocks. There has never been a year since 

 the first granite and gneiss began to exist, in which great beds of 

 quartz-sand were not in progress, through weathering and erosion, as 

 great of granitic sands (that is, sands consisting of feldspar and quartz, 

 with more or less mica), and greater beds of mud ; and the mud direct 

 from such a source would largely have been made of triturated or 

 granulated, instead of decomposed, feldspar. Daubree has shown that 

 the trituration of feldspar by the ocean's waters removes but a bare 

 trace of the alkalies, — far less than when the trituration is done by 

 fresh waters. 



As these older Archaean rocks are various in their distribution, so 

 the different kinds of sediments originated from them would be as 

 various ; and it is impossible that the same kind should at any one time 



