290 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1911. 



We assume that the stratigraphical column for post-Carboniferous 

 time is in actuality much as we have plotted it, and draw a right line 

 from the origin through the three results for the age of Carboniferous, 

 Devonian, and Silurian-Ordovician periods as derived from the lead 

 ratio. This is equivalent to assuming that the error in the strati- 

 graphical record is to be sought mainly in the pre-Cambrian records. 

 We therefore plot the early Algonkian and post-Jatulian results on 

 this line. We have now to make the following amendment on the 

 recorded thicknesses of the pre-Cambrian sediments. Algonkian 

 sediments rise from 82,000 feet to 381,000 and 441,000 feet (A t or A 2 

 on the diagram), according to which of the two early Algonkian 

 results we select. Keweenawan sediments rise from 50,000 feet to 

 245,000 or 359,000 feet (K x or K 2 ), according as we select among the 

 two Jatulian results. 



While it is true that important unconformities exist in the pre- 

 Cambrian succession, the sedimentation equivalents of which are not 

 found as yet, it seems incredible that amounts of sediments consider- 

 ably greater than the entire thickness of post-Algonkian rocks and 

 from 60 to 70 miles in cumulative depth can have escaped investiga- 

 tion. There is another 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 much greater than would have 

 attended the formation of the whole amount of sediment previously 

 estimated. This point is really quite apart from the question of the 

 age of the ocean. It is purely one of loss and gain, 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 ascribed to the strati- 

 graphical column is approximately correct, the proportionate thick- 

 nesses assigned to pre-Carboniferous and post-Carboniferous strata 

 are erroneous; too much has been assigned to the latter. The read- 

 justment of the strata involves diminishing the post-Carboniferous 

 deposits about 50 per cent and increasing the pre-Carboniferous by 

 nearly 40 per cent. 



