60 JoLY — An Estimate of the Geological Age of the Earth. 



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 pre-existing sediments, we would anticipate that the detritals 

 of successive periods should generally show diminishing alkali percentages. The 

 inquiry is complicated by the necessity 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 may, 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 attention to the petrological 

 and above all the physical character of tlie slates or clay slates we compare, some 

 record of progressive change might be expected to be revealed. 



Although our investigation labours under the difficulty that the existing 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. 



Let us look at the analyses of the "Roofing-slates." In these a certain 

 fineness of grain and attendant similarity of history are probably in most cases 

 involved. These are types mainly of the finest sediments. It does not appear 

 that we have any reason to suppose that their deposition, consolidation, and 

 prolonged existence in the rocks added to or subtracted from their original 

 chemical constituents. With these we may probably compare Clay Slates of more 

 modern periods, and the finest muds now being laid down in estuaries and lakes. 



Referring to Clarke and Hillebrand's collection of rock analyses,* we find 

 sixteen analyses of Roofing-slates of Cambrian age from Vermont and New York. 

 The mean percentage of added potash and soda alkalies is 5*05. In Rosenbusch 

 {loc. cit., p. 425) the alkalies in a Welsh Roofing-slate are recorded as 5-38 per cent. ; 

 a Cambrian Clay Slate of the Fichtelgebirge, 5-53 per cent. ; a Lower Silurian 

 Clay Slate of the same region, 4"10 per cent. ; and a Silurian Clay Slate from 

 Christiania, 5-60 per cent. These are otherwise mutually fairly concordant in 

 chemical composition, and also concordant with those from the United States. 



* BuUetin U.S. Geological Survey, No. 148, 1897. 



