Age of the Earth. 379 



The important question is, of course, as to how far such 

 assumptions are permissible consistent with any degree of 

 probability. There is much that is uncertain about data 

 respecting rock thickness, not only as regards the actual 

 field observations, but as to the real significance of what is 

 observed. Again, the relative time equivalents of deposited 

 rocks are not really known to us. Whether it is a detrital 

 sediment forming in an estuary or a coral-reef building in 

 clear water, the rate of growth must depend to some extent 

 on the downward movement of the sea-bottom; either induced 

 by the load or taking place from other causes. Some 

 sediments, are, however, plainly of rapid and some of slow 

 growth. Amidst such considerations we find no very definite 

 grounds for numerical computation. So far as crustal 

 yielding affects the question, the probable inference is, as 

 I have stated above, that the earlier strata were in their 

 greatest development more localized, and hence their time 

 value should be less than the more recent. As regards the 

 vertical distribution of definitely fast or slow collecting 

 materials, a careful comparison of the materials throughout 

 the geological column is required in order to gather any 

 evidence that may be forthcoming from these indications. 

 At present, however, there seems nothing to support the 

 different time values or amended thicknesses which must be 

 assumed if we are to adjust the radioactive results in anyway 

 to the sedimentary record. 



What will prima fade appear most difficult to credit in the 

 foregoing assumptions is the extremely slow rate which must 

 be ascribed to the accumulation of the sediments even at their 

 maximum. If the recorded depths of sediment have taken 

 1400 million years to collect, the average rate has been no 

 more than one foot in 4000 years ! This seems incredible: 

 and if we double the depth of maximum sedimentation it still 

 remains incredible. But, if possible, still more incredible is 

 the conclusion respecting solvent denudation to which radio- 

 active time drives us. If the sodium in the ocean has taken 

 1400 million years to accumu'ate, the rivers are now bearing 

 to the sea about 14 times the average percentage of the past 

 — not less than 9 times. It seems quite impossible to find 

 any explanation of such an increase. 



With these difficulties in view it is excusable to direct 

 attention to the foundations of the radioactive method and 

 ask how far they are secure. The fundamental assumption 

 is that the parent radioactive substance, uranium, has always 

 in the past disintegrated at the present rate. Is this assured ? 

 I am not now suggesting that the rate of change has been 

 affected by heat or pressure, but I assume that there may 



