THE MESOZOIC AGES. 197 



may be estimated in the aggregate at about 50,000 

 feet in thickness, while those of the Mesozoic scarcely 

 reach 8,000. We might, therefore, infer that the 

 Palaeozoic period was perhaps five or six times as long 

 as the Mesozoic. 



If we take the second class of rocks, the limestones, 

 and suppose these to have been accumulated by the 

 slow growth of corals, shells, etc., in the sea, we 

 might, at first sight, suppose that Palaeozoic animals 

 would not grow or accumulate limestone faster than 

 their Mesozoic successors. We must, however, con- 

 sider here the probability that the older oceans con- 

 tained more lime in solution than those which now 

 exist, and that the equable temperature and exten- 

 sive submerged plateaus gave very favourable con- 

 ditions for the lower animals of the sea, so that it 

 would perhaps be fair to allow a somewhat more 

 rapid rate of growth of limestone for the Palaeozoic. 

 Now the actual proportions of limestone may be 

 roughly stated at 13,000 feet in the Palaeozoic, and 

 3,000 feet in the Mesozoic, which would give a pro- 

 portion of about four and a quarter to one ; and as a 

 foot of limestone may be supposed on the average to 

 require five times as long for its formation as a foot 

 of sediment, this would give an even greater abso- 

 lute excess in favour of the Palaeozoic on the evidence 

 of the limestones an excess probably far too great 

 to be accounted for by any more favourable condi- 

 tions for the secretion of carbonate of lime by marine 

 animals. 



