THE AGE OF THE EARTH 67 



shape which it assumed when it first became cool enough 

 to solidify. It is, as we have said, an oblate spheroid, but 

 the difference in length between the axis joining the poles 

 and the axis passing from one side of the equator to the 

 other is much smaller than the contemplation of most models 

 of the globe would lead one to suppose. The equatorial 

 axis is only seventeen miles longer than the polar axis. 

 Therefore it cannot have been spinning much faster when 

 it first became solid than it does now. Lord Kelvin esti- 

 mates that the centrifugal force at the time of solidification 

 cannot have been more than 3 per cent greater than it is 

 at .present, and therefore, having regard to the known rate 

 of retardation of the earth's rotation, this event occurred 

 not more than 100 million years ago. 



Another line of argument which leads to much the same 

 result depends upon the evidence that the earth is losing 

 heat. The craters of extinct volcanoes, scattered over all 

 parts of the globe, testify to the existence of much greater 

 plutonic activity in former times than is anywhere exhibited 

 now. The sixty or more cones which may be seen from the 

 top of Mount Eden, in the neighbourhood of Auckland, New 

 Zealand, give to the landscape the appearance which the 

 surface of the moon would present were it clothed in green, 

 and the almost perfect preservation of the cups of Mount 

 Eden itself and of some of the surrounding volcanoes shows 

 that it cannot be very long, in geological time, since they 

 were in action. The subsidence of volcanic activity proves 

 that there is less heat than formerly beneath the surface of 

 the earth. 



Again, it can be shown that the nature of the record, pre- 

 served in this case by the rocks, might have been antici- 

 pated by a process of reasoning. It has long been known 

 that the heat of the earth is greater at the bottom of r a 

 mine than it is near the surface. Observations made in many 

 regions show that, after a level down to which the temper 1 

 ature is affected by the heat of summer and the cold of 



