THE FOUNDATIONS OF TIIF CONTINENTS 89 



give US evidence that they also are made up of 

 calcareous skeletons of marine animals or frac^mcnts 

 of these. Now when we find in the Grenvillian 

 series, the first oceanic group of beds known to us, 

 great and widely extended limestones, thousands of 

 feet in thickness, and rivalling in magnitude those 

 of any succeeding period, we naturally infer that 

 marine life was at work. No doubt the primitive sea 

 contained more lime and magnesia than the present 

 ocean holds in solution ; but while this might locally 

 favour the accumulation of inorganic limestones, it 

 cannot account for so great and extensive deposits. 

 On the other hand, a sea rich in lime would have 

 afforded the greatest facilities for the growth of 

 those marine plants which accumulate lime, and 

 through these for the nutrition of animals forming 

 calcareous shells or corals. Thus we have pre- 

 sumptive evidence that there must have been in 

 the Upper Laurentian sea something corresponding 

 to our coral reefs and shell-beds, whatever this 

 something may have been. 



These limestones, however, demand more par- 

 ticular notice (Fig.- 19). 



One of the beds measured by the officers of the 

 Geological Survey is stated to be i,5CX) feet in 



