. 

 40 THE AGE OF THE EARTH 



But while measurements like these may furnish us with 

 some notion of the sort of speed of these changes, they 

 are not sufficient even to suggest an average ; for this w 7 e 

 must be content to wait till sufficient tidal observations 

 have accumulated, and the disturbing effect of the 

 inconstancy of the sep -level eliminated. 



It may be objected that in framing our estimate w r e 

 have taken into account mechanical sediments only, and 

 ignored others of equal importance, such as limestone and 

 coal. With regard to limestone, its thickness in regions l 

 where systems attain their maximum may be taken as 

 negligible ; nor is the formation of limestone necessarily 

 a slow process. The successful experiments of Dr. Allan 

 cited by Darwin, prove that some reef-building corals 

 may grow at the astonishing rate of 6 feet in height 

 per annum. 



In respect of coal there is much to suggest that its 

 growth was rapid. The carboniferous period well de- 

 serves its name, for never before, never since, have car- 

 bonaceous deposits accumulated to such a remarkable 

 thickness or over such wide areas of the earth's surface. 

 The explanation is doubtless partly to be found in favour- 

 able climatal conditions, but also, I think, in the youthful 

 energy of a new and overmastering type of vegetation, 

 which then for the first time acquired the dominion of 

 the land. If we turn to our modern peat-bogs, the only 

 carbonaceous growths available for comparison, we find 

 from data given by Sir A. Geikie that a fairly average 

 rate of increase is 6 feet in a century, which might per- 

 haps correspond to one foot of coal in the same period. 



The rate of deposition has been taken as uniform 

 through the whole period of time recorded by stratified 

 rocks ; but lest it should be supposed that this involves a 

 tacit admission of uniformity, I hasten to explain that in 

 this matter we have no choice; we may feel convinced 



