390 PART IV. VEGETABLE TAXONOMY. 



the middle and upper series of these rocks, at least, it existed 

 abundantly. Why the fossil forms have not been preserved is 

 readily understood, when we remember the perishableness of the 

 remains of most of the lower forms of plants, and the profound 

 metamorphism which the rocks of this era have undergone by 

 reason of heat and pressure. 



One reason for believing that life abounded in that far off 

 time, is the great quantities of graphite which the rocks contain. 

 It is clearly proved that the graphite of later formations is meta- 

 morphic coal, and this we know is carbonized organic matter. 

 There is no reason to doubt that the graphite of the Archaean 

 had the same origin. Moreover, the more modern coal and 

 graphite are shown to be largely the remains of plants and not 

 of animals, and this, it is reasonable to suppose, is true of the 

 graphite of the Archaean. 



Another reason is the accumulation of iron ore in beds. The 

 most productive iron-ore beds in the world occur in rocks of 

 this era, the Laurentian rocks of Canada, New York, northern 

 Wisconsin and northern Michigan. It is clearly evident from 

 an examination of these beds that they were deposited as sedi- 

 ment in water. Now, it is known that at the present time the iron 

 ore, which in the form of ferric oxide constitutes the red color- 

 ing matter of many soils, is gradually being reduced, by the 

 action of decaying organic matter, to a soluble form, and washed 

 out by means of percolating water and conveyed by streams to 

 lakes and marshes, and there deposited in beds, constituting 

 what is called bog iron ore. Organic matter is necessary to this 

 transfer, and it is difficult to see how in Archaean times, any more 

 than now, such accumulations could have taken place except by 

 the combined agency of organic matter and water. 



Still another reason is afforded by limestones which, in the 

 metamorphic form of marbles, are not uncommon in the upper 

 formations of this series. Most of the more modern stratified 

 limestones we know to be composed of calcareous skeletons, 

 chiefly of marine animals such as rhizopods, corals, shell-fish, 

 etc., but partly also of calcareous algae. The Archaean marbles 

 are also chiefly marine deposits, and are, no doubt, also the met- 

 amorphosed remains of animal and plant skeletons. 



While, therefore, positive proofs are lacking, the indirect 



