CHAPTER XXII 
LIFE DURING THE ARCHEAN AND PALEOZOIC TIMES 
Archean Rocks. — In the Archean, conditions existed which can- 
not now be determined, for the rocks of that period, whatever their 
origin was, are now so altered and metamorphosed that they have 
very little to tell us of the story of the earth. These rocks are 
either igneous or metamorphic. Among the latter there is a 
noticeable difference between two groups, one, the lower, being 
extremely metamorphosed; the other, or upper series, being only 
partially changed. This difference in the rocks has given rise to 
the division of the Archean into two groups, the Huronian and 
the Laurentian. 
Among the characteristic rocks of the Laurentian period are 
eneisses, schists, and certain plutonic igneous rocks, particularly 
the granites. There are also beds of iron, and other minerals and 
rocks in less abundance than those which are considered as char- 
acteristic. These ancient strata are complexly intermingled and 
folded, so that with all the study which has been given them, 
very little has been determined concerning their structure, and 
practically nothing as regards their origin. 
In the upper series, or the Huronian, rocks are arranged in 
more distinct beds, and are much less folded and changed, so that 
in many cases distinct stratification may be seen, and the original 
nature of the rocks can be determined. Thus among the Huro- 
nian strata are found the common forms of lava, and more or less 
distinctly altered beds of sandstone, limestone, and shale, which 
by metamorphism have been transformed to quartzite, marble, 
and slate. 
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