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



Althouji-h the uncertainty tittending the esthiiates of the volume of sedi- 

 mentary rocks involved in the recognized geological method of prosecuting 

 inquiry into the Age of the Earth must be admitted, it will be seen further 

 on that the sodium contents of the sea considered in connexion with the know- 

 ledge we possess of the chemical composition of the sedimentary and igneous 

 rocks lends support to estimates that have been made of the total bulk of the 

 sedimentaries. Here the two methods of inquiry into the Geological Age of the 

 Earth appear to mutually support one another. 



This is involved in the following consideration which for the sake of clearness 

 may be outlined here. The mean composition of the siliceous sedimentary rocks 

 can be arrived at approximately by taking the average of the many reliable 

 analyses available of various classes of such rocks as are most abundant. For 

 present purposes, it is only necessary to consider the alkali content of these rocks. 

 Furthermore, the mean alkali content of the principal igneous rocks, can, in a 

 similar manner, be investigated. The mean composition of these rocks has been 

 estimated with more especial care by F. W. Clarke of the United States Geological 

 Survey, and the composition of the older crust of the Earth in this way approxi- 

 mately determined. It is in the first place found that the alkali content of the 

 latter is considerably in excess of that of the former. Accepting now an approxi- 

 mate estimate of the bulk of siliceous sedimentary rocks, and restoring to this the 

 sodium now contained in the ocean, the sodium content of the original crust, 

 or of the average of the eruptives, is obtained with a fair degree of approxi- 

 mation. 



Here we observe in the sodium-deficit of the detrital siliceous sediments the 

 results of its gradual abstraction by the influences of denudation. Tiiere can surely 

 be but this one legitimate explanation of the fact that the great bulk of the detrital 

 sedimentaries is deficient in sodium by just that amount of this body as is 

 contained in the ocean, plus a relatively small allowance for the deposits of 

 Rock Salt. It is to be observed that we can effect such a restoration in the case 

 of no other elemental body dissolved in the sea. The amount present of the 

 chemically related substance potassium will not fit the detrital sediments. It 

 exhibits a deficiency. For obvious reasons the calcium and magnesium salts will 

 also be deficient.* 



An interesting fact, however, is in the case of the potassium revealed as the 

 result of very simple calculation. The jDresent potassium discharge of the rivers, 

 if prolonged into the past, as the duration of this is determined by the sodium 

 constituent, would have fed into the ocean just about the missing quantity of 



* The reasons referred to are principally the continual abstraction and precipitation of these bodies 

 from the sea, giving rise to Limestones and Dolomites, and the presence of calcium and magnesium in the 

 ocean sediments. 



