250 AN ESTIMATE OF THE GEOLOGICAL AGE OF THE EARTH. 



mate uniformity can be claimed for these factors over the past is 

 doubtless true; but this does not eliminate the necessity of accurate 

 knowledge where such is aA^ailable in the application of the method, 

 for the possibility of errors occurring of the same sign must always 

 be borne in mind. 



Although the uncertainty attending the estimates of the volume of 

 sedimentary 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 

 connection with the knowledge 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 anal3'ses 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 b}^ F. W. Clarke, of the United 

 States Geological Survey, and the composition of the older crust of the 

 earth in this way approximately 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 approximate 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 

 approximation. 



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

 ments the results of its gradual abstraction by the influences of denu- 

 dation. There 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 chem- 

 ically related substance potassium will not fit the detrital sediments. 

 It exhibits a deficiency. For obvious reasons the calcium and magne 

 sium salts will also be deficient.^ 



^The reasons referred to are principally the continual abstraction and precipita- 

 tion of these bodies from the sea, giving rise to limestones and dolomites, and the 

 presence of calcium and magnesiimi in the ocean sediments. 



