AGE OF THE EARTH 27 
for all the phenomena which the crust of the earth 
exhibits. It is now more generally supposed that in 
very early times forces similar in kind to those in 
action at the present day may have exhibited con- 
siderably greater violence. 
To produce the present condition of the surface of 
the earth by the action, gradually accumulated, of 
such processes of denudation and upheaval as are now 
going on around us, vast ‘periods of time are clearly 
necessary. The early evolutionists, having once got 
rid of the idea that the date given by Bishop Usher 
as that of the creation of the world is a necessary and 
integral part of religion, immediately allowed their 
imaginations to run riot with regard to the amount 
of time at their disposal. Since this question of the 
extent of geological time has an important bearing on 
the problem of organic as well as upon that of inorganic 
evolution, it will be well to pay some attention to 
more recent views upon the subject. 
Some years ago the generous ideas of biologists as 
well as of geologists were to a great extent shattered 
by the calculations of Lord Kelvin. These were based 
upon three separate sets of data, which we may enume- 
rate without entering into a lengthy explanation of 
the calculations involved. The evidence made use of 
consisted of (1) the rate of the earth’s rotation, as 
affected by tidal retardation ; (2) the rate of secular 
cooling of the earth, as deduced from the rate at which 
the temperature of the earth’s crust rises on passing 
inward from the surface ; and (3) the rate of cooling of 
the sun by radiation. The three calculations were 
