LIFE ON THE EARTH. 127 



conception. If for the purpose of arriving at more 

 definite ideas we select from the pile of strata some 

 part whose origin is clear, and whose formation was 

 continuous, the time still comes out enormous. The 

 Coal-formation, in South Wales, is 12,000 feet thick, 

 and through a great part of its thickness there, gives 

 proof after proof that this whole series of strata was 

 deposited bed by bed at or nearly at the level of the 

 sea; the estuarine alveus continually sinking, and 

 continually receiving additions of fresh sediment. 

 Many beds of Coal, amounting to one hundred and 

 twenty feet in thickness, alternate with these sedi- 

 ments. Not a single bed of limestone occurs in the" 

 series; towards the bottom are marine shells, above 4 

 only freshwater shells and land-plants; the whole 

 mass agreeing with the facts of estuarine accumu- 

 lation at the mouth of a great river. If the growth 

 of the sediment were at the same rate as the waste 

 of the land, 12,000 x 12 x 111 =the years employed i 

 the production of the sediments = 15,984,000 years. 



But there are circumstances in the Coal-formation 

 which greatly modify this result. It is evidently a 

 deposit in quiet water quiet as compared with the 

 agitation of the open ocean. In such a case the ad- 

 dition of sediment would not follow the law of open 

 sea-deposits but rather that of lakes, and so the 

 strata would be for the most part accumulated in 



