THE LAURENTIAN ROCKS. 19 



aggregate thickness of not less than twenty to thirty 

 feetj an amount comparable with that in the true coal 

 formation itself. Now we know of no agency existing 

 in present or in past geological time capable of 

 deoxidizing carbonic acid, and fixing its carbon as an 

 ingredient in permanent rocks, except vegetable life. 

 Unless, therefore, we suppose that there existed in the 

 Laurentian age a vast abundance of vegetation, either 

 in the sea or on the land, we have no means of 

 explaining the Laurentian graphite. 



The Laurentian formation contains great beds of 

 oxide of iron, sometimes seventy feet in thickness. 

 Here again we have an evidence of organic action ; for 

 it is the deoxidizing power of vegetable matter which 

 has in all the later formations been the efficient cause 

 in producing bedded deposits of iron. This is the 

 case in modern bog and lake ores, in the clay iron- 

 stones of the coal measures, and apparently also in the 

 great ore beds of the Silurian rocks. May not similar 

 causes have been at work in the Laurentian period ? 



Any one of these reasons might, in itself, be held 

 insufficient to prove so great and, at first sight, un- 

 likely a conclusion as that of the existence of abundant 

 animal and vegetable life in the Laurentian ; but the 

 concurrence of the whole in a series of deposits un- 

 questionably marine, forms a chain of evidence so 

 powerful, that it might command belief even if no 

 fragment of any organic and living form or structure 

 had ever been recognised in these ancient rocks. 



Such was the condition of the matter until the 



