THE ARCH^AN AGE. 
13 
sometimes more than a hundred feet thick. To this for¬ 
mation belong the iron mines in northern Wisconsin 
and Michigan, Missouri, New Jersey, and Sweden. It 
might be called the Age of Iron. In Canada these rocks 
also contain beds of graphite, some of it almost pure. 
The rocks consist chiefly of granite, and are much 
used as paving-blocks. Besides, they furnish excellent 
building-stone and material for columns and monu¬ 
ments, as they can be highly polished. 
Pig. 7.—7, Old strata distorted by disturbances of the earth’s 
crust. 2 , Newer strata deposited upon them. 
The boulders, so common in the Northern States, 
are excellent examples of the Archaean rocks. They 
are within easy reach of most readers, and should be 
carefully examined. 
All rocks which are stratified—that is, lying in 
layers upon each other, like courses of masonry—are 
called sedimentary (Latin, sedere, to settle). They 
have been laid by water. The rock material settled 
to the bottom, and hence such rocks were formed in 
a horizontal position, or nearly so. 
Besides sedimentary rocks we find others which 
have once been in a liquid form; they are known as 
volcanic rocks. We frequently find them forced up 
through fissures in other rocks, or poured out of vol¬ 
canoes and spread over the adjacent surface. 
Furthermore, rocks are constantly undergoing 
