AN ESTIMATE OF THE GEOLOGICAL AGE OF THE EAETH. 283 



Approaching- finally the question as to whether a correction on the 

 geological age of the earth previously arrived at is fairh^ due, accord- 

 ing to our lights, on the score of the greater mass of detrital sediments 

 now reposing on the land areas compared with those of the earliest 

 times, we have, as we have seen in these ver}^ sediments, rocks of a 

 physical character which forbids us to pronounce, in man}- cases, on 

 the relative effectiveness of igneous and sedimentary rocks, as con- 

 tributing to solvent denudation. We have also factors of both earlier 

 and later times acting to accelerate solvent denudation. Of these the 

 least speculative is the influence of vegetation, which is a post-Early- 

 Paleozoic factor mainly. Again, the land uplifted from the primeval 

 ocean, after the free acids were for the most part neutralized, was, we 

 must infer, overlain with insoluble siliceous residues. To make any 

 deduction or addition is not warranted. There appears no good reason 

 to suspect that our broad uniformitarian principles are leading us into 

 considerable error where, more especially, such disturbing causes as 

 we are compelled to recognize are both of positive and negative signs. 

 But the whole consideration should undoubtedly lead us to widen the 

 margin we allow for error in our estimate of geological time. 



VIII. — THE ALKALIES OF SEDIMENTS AND THE GEOLOGICAL AGE OF 



THE LATTER. 



A very interesting but difficult line of inquiry is suggested in the 

 probable facts of geological denudation which we have reviewed. 



If the detrital sedimentaries of more recent geological age are 

 derived, or in part derived, from preexisting sediments, we would 

 anticipate that the detritals of successive periods should generally 

 show diminishing alkali percentages. The inquiry is complicated by 

 the necessitv of observing that rocks of similar origin are in each case 

 compared. The finer-grained sediments will be, as we have seen, the 

 richest in alkalies, for the reason that the more soluble constituents of 

 soils are just those which are reduced to the finest dimensions. Hence, 

 when in the course of time the mechanical sorting of the river exerts 

 such effects as the sieve of the investigator, the finer sediments laid 

 down in sea or lake come to differ in their chemical nature from the 

 coarser. Again, the percentage of soluble material in the soil ma}^, as 

 we have also seen, depend to some extent on the nature of the parent 

 rock, and hence one soil may differ from another in the percentages 

 of alkalies contained in the derived silts. However, by careful atten- 

 tion to the petrological and, above all, the phj^sical character of the 

 slates or clay slates we compare, some record of progressive change 

 might be expected to be revealed. 



Although our investigation labors under the difficulty that the exist- 

 ing records were not sought with a view to its prosecution, there are 

 some broad indications of the evidence we seek which we are justified 

 in referring to. 



