relations of the Metamorphic Rocks. 221 
<= st 
which, even now, are active in removing soluble matters from 
these rocks. The crystalline Lower Silurian rocks in Canada 
acid, and vegetation. If, however, it may be assumed that this 
action, other things being equal, has, on the whole, been propor- 
tionate to the newness of the formation, it is evident that the 
chemical and mineralogical composition of different systems of 
rocks must vary with their antiquity, and it now remains to fi 
in their comparative study a guide to their respective ages. 
_ It will be evident that silicious deposits, and chemical precip- 
itates, like the carbonates and silicates of lime and magnesia, 
may exist with similar characters in the geological formations 
of any age; not only forming beds apart, but mingled with the 
impermeable siliecaluitisicll sediments of mechanical origin. 
nasmuch as the chemical agencies giving rise to these com- 
pounds were then most active, they may be expected in greatest 
