12 THE GEOLOGICAL HISTORY OF PLANTS. 



scarcely an exaggeration to maintain that the quantity of 

 carbon in the Laurentian is equal to that in similar areas 

 of the Carboniferous system. It is also to be observed 

 that an immense area in Canada appears to be occupied 

 by these graphitic and Eozoon limestones, and that rich 

 graphitic deposits exist in the continuation of this sys- 

 tem in the State of New York, while in rocks believed to 

 be of this age near St. John, New Brunswick, there is a 

 very thick bed of graphitic limestone, and associated with 

 it three regular beds of graphite, having an aggregate 

 thickness of about tive feet.* 



It may fairly be assumed that in the present world, 

 and in those geological periods with whose organic re- 

 mains we are more familiar than with those of the Lau- 

 rentian, there is no other source of unoxidized carbon in 

 rocks than that furnished by organic matter, and that 

 this has obtained its carbon in all cases, in the first in- 

 stance, from the deoxidation of carbonic acid by living 

 plants. No other source of carbon can, I believe, be 

 imagined in the Laurentian period. We may, however, 

 suppose either that the graphitic matter of the Laurentian 

 has been accumulated in beds like those of coal, or that 

 it has consisted of difEused bituminous matter similar to 

 that in more modern bituminous shales and bituminous 

 and oil-bearing limestones. The beds of graphite near 

 St. John, some of those in the gneiss at Ticonderoga in 

 New York, and at Lochaber and Buckingham, and else- 

 where in Canada, are so pure and regular that one might 

 fairly compare them with the graphitic coal of Ehode 

 Island. These instances, however, are exceptional, and 

 the greater part of the disseminated and vein graphite 

 might rather be likened in its mode of occurrence to the 

 bituminous matter in bituminous shales and limestones. 



* Matthew in " Quarterly Journal of the Geological Society," vol. 

 xxi., p. 423. "Acadian Geology," p. 662. 



