AGE OF THE EARTH JOLY. 291 



The assumptions involved in making these adjustments are inher- 

 ently improbable, and it might be thought easier to assume that the 

 time values 'of the post-Carboniferous strata were, as compared with 

 the earlier strata, less. This emendation requires us to assume that 

 the more recent materials were laid down about three times as fast as 

 the earlier. 



These are the alternative modes of adjustment of radioactive time 

 to the stratigraphical column, leaving the latter on the whole intact. 

 If we assume that the recent sediments have been overestimated in 

 thickness, we can, by discarding about one-half the recorded thick- 

 nesses since Carboniferous time, produce an effect on the diagram 

 equivalent to moving the origin to the right. With this particular 

 numerical assumption the lead line will become steeper than it appears 

 on the chart, and the early Algonkian point will remain at such a 

 distance to the right of the Cambrian as will ascribe to the pre- 

 Cambrian sediments a thickness equal to that of the whole post- 

 Algonkian accumulation. 



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 yield- 

 ing 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 geologic 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 any way to the sedimentary record. 



What will prima facie appear most difficult to credit in the fore- 

 going 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 1,400,000,000 years to 

 collect, the average rate has been no more than 1 foot in 4,000 years! 

 This seems incredible ; and if we double the depth of maximum sedi- 



