AGE OF THE FARTH—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 forthgoming 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 scdiments 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- 
