788 THE POPULAR SCIENCE MONTHLY. 



earth. J>ut, in the midtlle portion of the Laurentian, rocks are found 

 which indicate that there were already land and water, and that the 

 waters and possibly the land were already tenanted by living beings. 

 The great beds of limestone which exist in this part of the system 

 furnish one indication of this. In the later geological formations the 

 limestones are mostly organic — that is, they consist of accumulated 

 remains of shells, corals, and other hard parts of marine animals, which 

 are composed of calcium carbonate, which the animals obtain directly 

 from their food, and indirectly from the calcareous matter dissolved 

 in the sea-water. In like manner great beds of iron-ore exist in the 

 Laurentian ; but in later formations the determining cause of the ac- 

 cumulation of such beds is tlie partial deoxidation and solution of the 

 peroxide of iron by the agency of organic matter. Besides this, cer- 

 tain forms known as Eozoon Canadense have been recognized in the 

 Laurentian limestones, which indicate the presence at least of one of 

 the lower types of marine animals. Where animal life is, we may 

 fairly infer the existence of vegetable life as well, since the plant is 

 the only producer of food for the animal. But we are not left merely 

 to this inference. Great quantities of carbon or charcoal in the form 

 of the substance known as graphite or plumbago exist in the Lauren- 

 tian. Now, in more recent formations we have deposits of coal and 

 bituminous matter, and we know that these have arisen from the ac- 

 cumulation and slow putrefaction of masses of vegetable matter. 

 Further, in places where igneous action has affected the beds, we find 

 that ordinary coal has been changed into anthracite and graphite, that 

 bituminous shales have been converted into graphitic shales, and that 

 cracks filled with soft bituminous matter have ultimately become 

 changed into veins of graphite. When, therefore, we find in the Lau- 

 rentian thick beds of graphite and beds of limestone charged with de- 

 tached grains and crystals of this substance, and graphitic gneisses 

 and schists and veins of graphite traversing the beds, we recognize the 

 same phenomena that are apparent in later formations containing 

 vegetable debris. 



The carbon thus occurring in the Laurentian is not to be regarded 

 as exceptional or rare, but is widely distributed and of large amount. 

 In Canada more especially the deposits are very considerable. 



The graphite of the Laurentian of Canada occurs both in beds and 

 in veins, and in such a manner as to show that its origin and dcj)Osi- 

 tion are contem])oraneous with those of the containing rock. 



The quantity of graphite in the Lower Laurentian series is enor- 

 mous. Some years ago, in the township of Buckingham, on the Ottawa 

 River, I examined a band of limestone believed to be a continuation 

 of that described by Sir W. K. Logan as the (^reen Lake limestone. 

 It was estimated to amount, with some thin interstratified bands of 

 gneiss, to a thickness of six hundred feet or more, and was found to 

 be filled with disseminated crystals of graphite and veins of the rain- 



