196 THE STORY OF THE EARTH AND MAN. 



equal duration of these two great eras of the earth's 

 history. But as we cannot yet prove this, we may 

 consider what light we can derive from the nature of 

 the rocks produced. These may be roughly classified 

 as of two kinds : First, the beds of sediment, sand, 

 clay, etc., accumulated by the slow chemical decay of 

 rocks and the mechanical agency of water. Secondly, 

 the beds formed by accumulation of the harder and 

 less perishable parts of living beings, of which the 

 limestones are the chief. With reference to the first 

 of these kinds of deposit, the action of the atmosphere 

 and rains on rocks in the earlier times might have 

 been somewhat more powerful if there was more car- 

 bonic acid in the atmosphere, that substance being the 

 most efficient agent in the chemical decay of rocks. 

 It might have been somewhat more powerful if there 

 was a greater rainfall. It must, on the other hand, 

 have been lessened by the apparently more equable 

 temperature which then prevailed. These differences 

 might perhaps nearly balance one another. Then the 

 rocks of the older time were quite as intractable as 

 those of the newer, and they were probably neither so 

 high nor so extensive. Further, the dips and emer- 

 gences of the great continental plateaus were equally 

 numerous in the two great periods, though they were 

 probably, with the exception of the latest one of each, 

 more complete in the older period. In so far, then, as 

 deposition of sediment is concerned, these considera- 

 tions would scarcely lead us to infer that it was more 

 rapid in the Palasozoic. But the Palaeozoic sediments 



