94 RELICS OF PRIMEVAL LIFE 



continuation of that described by Sir W. E. Logan 

 as the Green Lake Limestone. It was estimated 

 to amount, with some thin interstratified bands of 

 gneiss, to a thickness of 600 feet or more, and was 

 found to be filled with disseminated crystals of 

 graphite and veins of the mineral to such an extent 

 as to constitute in some places one-fourth of the 

 whole ; and making every allowance for the poorer 

 portions, this band cannot contain in all a less 

 vertical thickness of pure graphite than from twenty 

 to thirty feet. In the adjoining township of Locha- 

 ber Sir W. E. Logan notices a band from twenty- 

 five to thirty feet thick, reticulated w^ith graphite 

 veins to such an extent as to be mined with profit 

 for the mineral. At another place in the same 

 district a bed of graphite from ten to twelve feet 

 thick, and yielding twenty per cent, of the pure 

 material, is worked. When it is considered that 

 graphite occurs in similar abundance at several 

 other horizons, in beds of limestone which have 

 been ascertained by Sir W. E. Logan to have an 

 aggregate thickness of 3,500 feet, it is scarcely an 

 exaggeration to maintain that the quantity of car- 

 bon in the Laurentian is equal to that in similar 

 areas of the Carboniferous system. It is also to 



