18 THE DAWN OF LIFE. 



stones of the later geological periods. Now, in later 

 formations, limestone is usually an organic rock, accu- 

 mulated by the slow gathering from the sea-water, or 

 its plants, of calcareous matter, by corals, f oraminifera, 

 or shell-fish, and the deposition of their skeletons, 

 either .entire or in fragments, in the sea-bottom. The 

 most friable chalk and the most crystalline limestones 

 have alike been formed in this way. We know of no 

 reason why it should be different in the Laurentian 

 period. When, therefore, we find great and con- 

 formable beds of limestone, such as those described by 

 Sir William Logan in the Laurentian of Canada, we 

 naturally imagine a quiet sea-bottom, in which multi- 

 tudes of animals of humble organization were accumu- 

 lating limestone in their hard parts, and depositing 

 this in gradually increasing thickness from age to age. 

 Any attempts to account otherwise for these thick and 

 greatly extended beds, regularly interstratified with 

 other deposits, have so far been failures, and have 

 arisen either from a want of comprehension of the 

 nature and magnitude of the appearances to be ex- 

 plained, or from the error of mistaking the true 

 bedded limestones for veins of calcareous spar. 



The Laurentian rocks contain great quantities of 

 carbon, in the form of graphite or plumbago. This 

 does not occur wholly, or even principally, in veins or 

 fissures, but in the substance of the limestone and 

 gneiss, and in regular layers. So abundant is it, that 

 I have estimated the amount of carbon in one division 

 of the Lower Laurentian of the Ottawa district at an 



