THE ARCHEOZOIC ERA 319 



Almost all the Archaean rocks have been buried to great 

 depths and subsequently uncovered by erosion. The Archaean 

 rocks are therefore commonly schists and gneisses. Certain 

 portions of later systems of rock are likewise schistose, but 

 in the Archaean alone are the schists and gneisses almost 

 universal (Fig. 319). 



Metamorphic rocks are of course always derived from other 

 rocks of igneous or sedimentary origin (p. 41). It is often 

 impossible to determine the original character of some of the 

 Archaean rocks, while that of others may be discovered by 

 study. It is a singular fact that among the oldest rocks 

 which have yet been found in the Archaean system are green 

 schists which were once surface volcanic materials, such as 

 lava flows and cinders. These must have been cast out during 

 successive eruptions upon a still older surface, but as yet that 

 surface has not been identified. It is known that some of 

 the gneisses were once granites, parts of batholiths which 

 were intruded into these green schists, and subsequently 

 metamorphosed. 



The Archaean system is not composed entirely of igneous 

 rocks, as it was formerly thought to be. In the Lake Superior 

 region it contains small bodies of iron ore, metamorphosed 

 conglomerate, and slate. In China and Finland even lime- 

 stone has been found in the oldest rocks, which seem to be of 

 Archseozoic age. 



Conditions in Archaean time. Evidently we can hope to 

 learn but little of Archaean times from such a disordered and 

 obscure record as this, but the very remoteness of that era 

 lends interest to any bit of information about it. Nothing 

 is clearer than that there were many successive volcanic 

 eruptions and intrusions, probably more than have occurred 

 in any later era. That the weathering and erosion of the 

 lands were already in progress is suggested by the presence of 

 slates and other sedimentary rocks, for slates were once clay 

 and clay is made from many kinds of rocks by chemical decay 

 and the sorting action of water. 



