Marcu 25, 1915 | 
NATURE 
107 

an end of the existing geological régime; and Thom- 
son endeavoured to set a limit to its past duration 
from a discussion of the rate of cooling of the globe. 
A parallel line of argument was based on the cooling 
of the sun, 
Now as regards the validity of the general criticism 
there can be, of course, no doubt. Huxley’s halting 
defence of what was then the orthodox position was 
easily broken down, and a wholesome check was given 
to the extravagance of the geologists. When we turn, 
however, from the destructive to the constructive part 
of Kelvin’s argument, the case is different. The time 
to be allowed for the geological record was stated at 
first with considerable latitude, but was afterwards 
narrowed down, unti!, in 1899, Lord Kelvin concurred 
in Clarence King’s conclusion that the globe was a 
molten mass about twenty-four million years ago. It 
is rather remarkable that so many geologists were 
found willing to submit to this narrow limitation. 
Doubtless they were impressed by the prestige of Lord 
Kelvin’s authority, and perhaps some of them were 
influenced by a vague feeling that a result arrived at 
by strict mathematical reasoning is thereby entitled 
to credence. But, as has been so often pointed out, 
and so often forgotten, what you get out of the mathe- 
matical mill depends upon what you put into it. The 
reasoning may be unimpeachable, but it merely proves 
that, if certain assumptions be granted, certain conse- 
quences will follow. It may be that Lord Kelvin him- 
self, in the enthusiasm of entorcing his conclusions, 
did not always recall the foundations on which they 
rested, and it is to be suspected that many geologists 
read no more than the conclusions. 
Kelvin’s argument was based necessarily upon a 
number of assumptions. At the present time, in the 
light of fuller knowledge, it is sufficient to note one, 
which in 1862 seemed little open to question. Kelvin 
recognised that, while the earth is certainly losing 
heat, ‘‘it is possible that no cooling may result from 
this loss of heat, but only an exhaustion of potential 
energy, which in this case could scarcely be other than 
chemical affinity between substances forming part of 
the earth’s mass.’’ This, however, he dismissed as 
“extremely improbable,” and proceeded on the assump- 
tion that heat is the only form of energy to be 
reckoned with. Since the discovery of radium we 
have learnt that the earth possesses a vast store of 
potential energy in a highly concentrated form then 
unsuspected. Strutt has calculated, from data of a 
very simple kind, that the observed temperature- 
gradient can be wholly accounted for by radio-activity, 
if the rocks to a depth of forty-five miles contain as 
much radium as those at the surface. In other words, 
the heat generated by radio-active changes within this 
relatively thin crust will, on that supposition, be 
sufficient to compensate that lost at the surface. 
Clearly, therefore, the actual rate of cooling of the 
globe—if indeed it is cooling—must be far less than 
that adopted in Kelvin’s calculation, and his estimate 
of the age of the earth must be enormously increased. 
This is not all. A study of the various radio-active 
elements contained in minerals and rocks has shown 
that it is possible, in certain favourable cases, to cal- 
culate directly their age in years. Some estimates of 
this kind have been made, and the results are liberal 
enough to satisfy the most exacting claims of what 
may be called the reformed Uniformitarian creed. 
With this turning of the tables one might suppose 
that the old controversy would come to an end. But 
the reversal of the situation is, in fact, more complete; 
for meanwhile there has arisen a formidable minority 
of geologists who contend, on geological grounds, for 
estimates of time no more elastic than Lord Kelvin’s. 
The question is still, in great part, one between geo- 
NO. 2369, VOL. 95 | 

logists and physicists, but it is now the geologists 
who offer us the stinted measure and the physicists the 
more liberal one. 
It is not my purpose to discuss in detail the various 
geological arguments which have been advanced for 
limiting the age of the earth to a span of 80 or 100 
millions of years. The method of procedure is broadly 
the same in all. A computation is made of the rate 
at which some fundamental geological process is 
going on; it may be the lowering of the land-surface 
by erosion, or its destruction by solution, or the de- 
position of sediment, or the addition of salt to the sea. 
Some estimate is then made of the total result of the 
process throughout geological time. Having the 
annual rate of increment and the total amount, simple 
division gives the measure of the time in years. The 
observational data employed in these calculations are 
of a very precarious kind, and it would not be difficult 
to point out instances of that levity in the handling of 
figures to which I have adverted. But the funda- 
mental weakness of all such reasoning lies in the 
assumption that the present rate of any of these geo- 
logical processes can be adopted as equivalent to its 
average rate throughout the whole time. 
The existing configuration of the globe, and all the 
physical conditions that go with it, have been attained 
in consequence of a prolonged evolution. If we be- 
lieve that, as the net result of all its vicissitudes, the 
land-area has on the whole been growing in extent, 
in complexity of distribution, in boldness of relief, we 
must believe also that differences of temperature, of 
humidity, of climate generally, between different parts 
of the globe have become progressively more accen- 
tuated, and that all geological activities have been 
quickened as the world has grown older. While there 
is difference of opinion concerning these secular 
changes, there can be no doubt as regards the great 
cyclical changes which have been repeated several 
times in the history of the earth: the cycle beginning 
in each case with an epoch of important crust-move- 
ments and including the train of consequences which 
follow upon this new step in the evolution of the 
earth. Such a cycle was initiated at an epoch not 
long remote by geological reckoning, and we are living 
in consequence in a time of more than ordinary geo- 
logical activity, with the continental masses rising 
higher than their average level, and with large tracts 
of newly deposited strata exposed to the attack of 
destructive agents. 
For these reasons I am of opinion that the present 
rate of erosion, and of its correlative sedimentation, 
is much higher than the average rate, and that any 
calculation based upon it must greatly under-estimate 
the duration of geological time. I do not ask you 
necessarily to concur in this conclusion, but at least 
to suspend judgment in the matter: for it will 
assuredly be a misfortune if geology, so lately freed 
from one bondage, should fall straightway into 
another no less galling. This at least is certain, that 
every one of the various geological processes which 
have been discussed in this connection, is controlled 
by conditions which cause its rate to be very variable. 
It is a clock which now hurries and now creeps, or 
stands still, and it can never be trusted as a time- 
keeper. Even for the most recent chapter of geo- 
logical history we can make no approach to certainty 
on these lines. Attempts have been made, for example, 
to estimate the time since the final retreat of the ice 
in North America from the rate of recession of the 
falls of Niagara; but the evidence shows that this 
rate has varied widely even during the last half- 
century, and Gilbert. after a careful study of all the 
data, refrains from offering any opinion on this point. 
Must we then abandon all hope of any practicable 

