378 Prof. J. Joly on the 



point. The calculation which equates the amount of sodium 

 in the ocean with the estimated bulk of the detrital sediments, 

 knowing the loss by solution attending the derivation of these 

 from the average igneous rock, has, as we have seen, been 

 found to give results in fair agreement with the measurements 

 of all the quantities involved. The radioactive results must 

 now postulate an amount of pre-Cambrian denudation far 

 greater than the whole amount of sediment previously esti- 

 mated would have given rise to. This point is really quite 

 apart from the question of the age of the ocean. It is purely 

 one of gain and loss, of balance of" accounts. Nor can evasion 

 of the difficulty be found by ascribing exceptionally local 

 restrictions to pre-Cambrian denudation. It is probable 

 that none was more world-wide in its effects. 



If these conclusions appear untenable we may deal with 

 the results in another way. The ages found from the 

 higher lead ratios may be placed at a reasonable distance 

 from the base of the Cambrian and joined by a right line to 

 the origin. To bring them on to the line so determined the 

 results for the Carboniferous, Devonian, and Silurian- 

 Ordovician must be shifted to the left. This procedure is 

 equivalent to assuming that while the total thickness as- 

 cribed to the stratigraphical column is approximately correct, 

 the proportionate thicknesses assigned to pre-Carbonii'erous 

 and post-Carboniferous strata are erroneous : too much has 

 been assigned to the latter. The readjustment of the strata 

 involves diminishing the post-Carboniferous deposits about 

 50 per cent., and increasing the pre-Carboniferous by nearly 

 40 per cent. 



The assumptions involved in making these adjustments are 

 inherently 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 emen- 

 dation 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 over-estimated in thickness, we can, by discarding about 

 one half the recorded thicknesses since Carboniferous time, 

 produce an effect on the diagram equivalent to moving the 

 origin to the right. With this particular numerical as- 

 sumption 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. 



