30 THE DAWN OF LIFE. 



cases, in the first instance, 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, how- 

 ever, suppose either that the graphitic matter of the Lauren- 

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

 it has consisted of diflfused 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 elsewhere in Canada, are so 

 pure and regular that one might fairly compare them with the 

 graphitic coal of Khode Island. These instances, however, 

 are exceptional, and the greater part of the disseminated and 

 vein graphite might rather be compared in its mode of occur- 

 rence to the bituminous matter in bituminous shales and 

 limestones. 



" We may compare the disseminated graphite to that which 

 we find in those districts of Canada in which Silurian and 

 Devonian bituminous shales and limestones have been meta- 

 morphosed and converted into graphitic rocks not dissimilar 

 to those in the less altered portions of the Laurentian.* In 

 like manner it seems probable that the numerous reticulating 

 veins of graphite may have been formed by the segregation 

 of bituminous matter into fissures and planes of least resist- 

 ance, in the manner in which such veins occur in modern 

 bituminous limestones and shales. Such bituminous veins 

 occur in the Lower Carboniferous limestone and shale of Dor- 

 chester and Hillsborough, New Brunswick, with an arrange- 

 ment very similar to that of the veins of graphite ; and in the 

 Quebec rocks of Point Levi, veins attaining to a thickness of 

 more than a foot, are filled with a coaly matter having a trans- 

 verse columnar structure, and regarded by Logan and Hunt 

 as an altered bitumen. These palaeozoic analogies would lead 

 us to infer that the larger part of the Laurentian graphite falls 

 under the second class of deposits above mentioned, and that, 

 if of vegetable origin, the organic matter must have been 



* Granby, Melbourne, Owl's Head, etc. , Geology of Canada, 1863, 

 p. 599. 



