THE AGE OF THE EARTH 69 



years, out it is almost certain that he has not illuminated 

 the earth for 500 millions of years." 



Thus the physicists have approached the problem from 

 several sides, and, drawing the mesh tighter and tighter, have 

 shown, not how long the earth has been capable of support- 

 ing life, but what is the limit beyond which it is certain that 

 it was not so constituted. Lord Kelvin is of opinion that 

 this limit does not exceed 20 million years. Physical methods 

 involve long calculations, and the indisputable accuracy of 

 mathematics gives to the results an appearance of rigid 

 exactitude which may be misleading. It is, however, obvious 

 that mathematics cannot produce an accurate result unless 

 the data be accurate, and if there be any uncertainty in the 

 conclusions just formulated, it must be due to errors in the 

 data upon which they are based. Each of the estimates 

 starts with certain assumptions. We are very far from 

 calling these in question, but if any person is ever found 

 competent to act as umpire between the physicists and the 

 geologists (who, as we shall show directly, prove a longer 

 period than 20 millions of years), he will inquire first whether 

 these assumptions are justified. Is the retardation assigned 

 in right proportions to the moon and the sun respectively ? 

 Is it true that the shape of the earth has not altered since it 

 cooled to the point of solidification ? Do the figures which 

 represent the increasing heat of the earth from without 

 inwards hold good for all latitudes, and are they independent 

 of local causes, such as the proximity of mountains, etc. ? 



Geologists approach the problem from the opposite side. 

 They ask the direct question, How long has it taken to 

 deposit all the fossil-bearing strata, and the earlier sedi- 

 mentary rocks which were capable of supporting life, 

 although no fossils are preserved in them ? Sir Archibald 

 Geikie answers that it must have taken more than 20 

 million years. These strata attain in the aggregate to the 

 thickness of 100,000 feet. The chalk alone reaches to a 

 thickness of 10,000 feet in certain of the western districts of 



