0\ THE (iEOLOGICAL AGE OF THE EAKTII. 37o 



If now tlie sodiuiii-methocl affords a correct key to the age of the earth, 

 it remains to criticise methods yielding discordant results. And first, as 

 I'egards the method by rate of deposition, we find Sir A. Geikie claiming 

 a period, as we have seen, in perfect accord with that shown by solvent 

 denudation. Professor Sollas, however, considers the actual record indicates 

 a shorter period. I think, however, he has very fully and fairly shown 

 that much difficulty attends the actual measurements as well as the 

 application of the measurements. 



In the first place it may be observed that the data have been obtained 

 from an inadequate number of rivers. Again, the detrital matter discharged 

 by rivers is most difficult to estimate, owing to the rapid variation of 

 transporting power with current-velocity, and also owing to the fact that 

 a very large amount of sediment is transported by creeping along the river 

 bed. Both these facts render measurement so difficult that only the most 

 painstaking observations could be relied on for an approximate estimate. 

 And suppose we possessed the required estimate of detrital material, how 

 are we to dispose of it so as to represent what we may call the average 

 mode of maximum deposition ? Some rivers form deltas which creep out- 

 ward year by year. Here the rate of deposition is evidently not balanced 

 by subsidence. There is in fact no one law of deposition, nor can there 

 be. Once more, can we ever know the total maximum thickness of the 

 sediments 1 It must be that the sediments of one period supply in great 

 part those of the next. The very quantity we estimate in the rivers in 

 order to find our denominator has been robbed, perchance, from our 

 numerator. Have we, in fact, when all care is taken, measured the true 

 maximum thickness, or would sediments long ago removed afford vastly 

 greater maxima 1 Now observe the method is exposed, here at its weakest 

 point, mainly to errors of deficiency : and the method in its latest develop- 

 ment, in the able hands of Professor Sollas, aftbrds but some twenty-six 

 millions of years, and according to Mr. Wallace twenty-eight millions of 

 years. 



There is another method based on biological progress which seems 

 to go to the opposite extreme and claims immense periods of time. 

 Lyell claimed on biological grounds 240 millions of years since the 

 Cambrian ; Haughton, 200 millions ; Darwin speaks of a pre-Cambrian 

 period as long as the sum of the subsequent geological ages. Professor 

 Sollas has, with justifiable authority, dealt with this matter. And indeed 

 we may well ask if the argument does not assume an unwarranted 

 proportionality in the rates of evolution throughout successive ages. 

 Surely the organism of later date owes something of its stability to 

 heredity 1 



Ca]\ we assume tliat when trial was less likely to be attended with 

 error, owing to a less severe competition, species and genera were not 

 struck off more rapidly than later under more restricted conditions, and 

 when ages of increasing restraints had impressed upon the gei'm-plasm a 

 more stereotyped heredity ? It appears difficult to imagine that the 

 organism as we see it to-day so willing to take advantage of every loop- 

 hole and fill up every vacancy should have dropped its opportunist 

 character in early times and failed to profit by the more generous 

 environment. If this is so, can we accept with Huxley the period 

 required for the development of the horse as an indication of the length 

 of the history of ungulates 1 But this argument has been taken up 

 already l.iy Mr. Adam Sedgwick, who has urged that in the evolution 



