HARKER : PRESIDENTIAL ADDRESS 
9 
geologists wlio offer us the stinted measure and the physicists the 
more hberal one. 
It is not my purpose to discuss in detail the various geological 
arguments which have been advanced for limiting the age of the eaith 
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 m^ay be the lowering 
of the land-surface by erosion, or its destruction by solution, or the 
deposition 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 pre- 
,carious 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 fundamental weakness of all such reasoning lies in the assumption 
that the present rate of any of these geological 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 believe 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 
accentuated, 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 greart 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-movements 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 reckon- 
ing, and we are living in consequence in a time of more than ordinary 
geological 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- 
