ADDRESS. 27 
moment since the birth of the moon tidal friction had always been at 
work in such a way as to produce the greatest possible effect, then we 
should find that sixty million years would be consumed in this portion of 
evolutionary history. The true period must be much greater, and it does 
not seem extravagant to suppose that 500 to 1,000 million years may have 
elapsed since the birth of the moon. 
Such an estimate would not seem extravagant to geologists who have, 
in various ways, made exceedingly rough determinations of geological 
periods. One such determination is derived from measures of the thick- 
ness of deposited strata, and the rate of the denudation of continents by 
rain and rivers. I will not attempt to make any precise statement on 
this head, but I imagine that the sort of unit with which the geologist 
deals is 100 million years, and that he would not consider any estimate 
involving from one to twenty of such units as unreasonable. 
Mellard Reade has attempted to determine geological time by certain 
arguments as to the rate of denudation of limestone rocks, and arrives at 
the conclusion that geological history is comprised in something less than 
600 million years.!| The uncertainty of this estimate is wide, and I imagine 
that geologists in general would not lay much stress on it. 
Joly has employed a somewhat similar, but probably less risky, method 
of determination.2, When the earth was still hot, all the water of the globe 
must have existed in the form of steam, and when the surface cooled that 
steam must have condensed as fresh water. Rain then washed the con- 
tinents and carried down detritus and soluble matter to the seas. Common 
salt is the most widely diffused of all such soluble matter, and its transit 
to the sea is an irreversible process, because the evaporation of the sea 
only carries back to the land fresh water in the form of rain. It seems 
certain, then, that the saltness of the sea is due to the washing of the 
land throughout geological time. 
Rough estimates may be formed of the amount of river water which 
reaches the sea in a year, and the measured saltness of rivers furnishes a 
knowledge of the amount of salt which is thus carried to the sea. A 
closer estimate may be formed of the total amount of saltin the sea. On 
dividing the total amount of salt by the annual transport Joly arrives at 
the quotient of about 100 millions, and thence concludes that geological 
history has occupied 100 million years. I will not pause to consider the 
several doubts and difficulties which arise in the working out of this 
theory. The uncertainties involved must clearly be considerable, yet it 
seems the best of all the purely geological arguments whence we derive 
numerical estimates of geological time. On the whole I should say that 
pure geology points to some period intermediate between 50 and 1,000 
millions of years, but the upper limit is more doubtful than the lower. 
1 Chemical Denudation in relation to Geological Time, Bogue, London, 1879; or 
Roy. Soc. January 23, 1879. 
2 «An Estimate of the Geological Age of the Earth, Trans. Roy. Dublin Soc., 
vol. vii. series iii., 1902, pp. 23-66. 
