THE ARCHEOZOIC ERA 321 



The presence of limestone may mean that shell-bearing 

 animals were already in existence, but the fact that some 

 limestones are even to-day formed by the direct precipitation 

 of lime carbonate from water leaves the question in doubt. 

 Beds of graphite, which are thought to be simply metamor- 

 phosed coal seams, indicate that plants lived in Archaean time. 

 No fossils have been found in the Archaean rocks and so we 

 know nothing of the real character of the life which may have 

 existed then. All we can be safe in imagining about the plants 

 and animals of the era is that they were on the average much 

 simpler and lower in the scale of life than those which exist 

 to-day. 



Length of the Archaeozoic era. It is difficult to imagine 

 the vast length of time embraced in the Archseozoic era. Since 

 the bottom of the Archaean rocks has never been reached it 

 is clear that we know nothing of the earlier part of that era. 

 Yet even the knowable part gives us a fragmentary story of 

 many/ successive volcanic disturbances and several distinct 

 periods of folding and metamorphism. From the clearer 

 record of later times we know that such changes take place 

 slowly and are separated by periods of quiet, often to be 

 measured in millions or tens of millions of years. Considera- 

 tions such as these have led to the conjecture that the Ar- 

 chaeozoic era may have been longer than all the later periods 

 together. 



QUESTIONS 



1 . In some places the Archaean system is found to contain both 

 gneiss (once granite) and schistose basalt. Which of the two 

 would you consider the older (1) if the gneiss contained rough frag- 

 ments of the basalt within itself and if the main mass of basalt were 

 cut by branching layers of gneiss, or (2) if the gneiss were crossed 

 by layers of basalt continuous with the main mass of the latter ? 



2. Why should we not expect to find fossils in schist even though 

 the original mud from which it was derived was filled with shells ? 



3. An old name for the Archaean system is "Basement Complex." 

 Why is this a good descriptive phrase ? 



