90 RELICS OF PRIMEVAL LIFE 



thickness, another is 1,250 feet thick, and a third 

 750 feet ; making an aggregate of 3,500 feet.^ 

 These beds may be traced, with more or less inter- 

 ruption, for hundreds of miles. Whatever the 

 origin of such limestones, it is plain that they in- 

 dicate causes equal in extent, and comparable in 

 power and duration, with those which have produced 

 the greatest limestones of the later geological 

 periods. Now, in later formations, limestone is 

 usually an organic rock, accumulated by the slow 

 gathering from the sea-water, or its plants, of cal- 

 careous matter, by corals, foraminifera, or shell-fish, 

 and the deposition of their skeletons, either entire 

 or in fragments on 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 Lauren- 

 tian period. When, therefore, we find great and 

 conformable beds of limestone, such as those de- 

 scribed by Sir William Logan in the Laurentian of 

 Canada, we naturally imagine a quiet sea-bottom, 

 in which multitudes of animals of humble organi- 

 zation were accumulating limestone in their hard 

 parts, and depositing this in gradually increasing 



^ Logan : " Geology of Canada," p. 45. 



