ESTIMATES OF TIME 835 



are uplifted and re-eroded, the total amount of erosion and deposition 

 is, however, much greater than the quantity of igneous rock which has 

 contributed to the salt of the sea. Clarke obtains a volume of igneous 

 rock equivalent to a shell enveloping the earth and ranging from 2,050 

 to 1,740 feet thick as the quantity necessary to yield the sodium. 



It is seen that chemical denudation makes possible valuable estimates ' 

 of the total erosion of geological time. As a basis for attempting to 

 measure the age of the earth, it is, however, defective only in lesser degree 

 than the methods resting on the measurement of stratigraphic sections. 

 ^Nevertheless, the method has been used as if it were very exact and con- 

 clusive. Consequently a review of the assumptions made by the several 

 investigators on this subject should be given. 



Joly assumes that the sodium derived from the sedimentary rocks, 

 mostly as ichloride from ancient sea-waters, is equivalent to that derived 

 from similar areas of igneous rocks, so that the rate of annual supply 

 to the sea is the same per unit from sedimentary and igneous terranes. 

 He further assumes a constancy in the supply through all geological 

 time. Both assumptions remove the subject from the realm of exact 

 science to that of vague probability. 



Holmes considers, on the contrary, that the evidence points to sodium 

 being supplied by erosion of sedimentary rocks, mostly as chloride, at a 

 higher rate than from equal areas of igneous rocks."^^ Furthermore, in 

 regard to the rate of denudation, reasons have been shown in this article 

 for holding that it may now be five, ten, or fifteen times higher than the 

 mean for all geologic time. Joly, in one of his later estimates, using Mur- 

 ray's data, obtains an age of less than 150,000,000 years. '^^ Clarke has 

 supplanted Murray's data by more accurate calculations which reduce 

 Joly's estimates to about 90,000,000 years."^^ "- 



Holmes shows how, by varying the mode of using the data, quite dif- 

 ferent ages may be obtained, still under the assumption of uniform rate. 

 If only river sodium uncombined with chlorine is used as a measure of 

 the annual contributions from igneous rocks to the sea, an age of 180,- 

 000,000 years is obtained. Taking the mean rate of denudation as one 

 foot in 8,600 years and the area of igneous rocks, (constituting approxi- 

 mately 20 per cent of the drainage areas, as a measure of the new sodium 

 contributed annually, would give an age to the oceans under the hypoth- 

 esis of uniform rate of 340,000,000 years. ^^ It is seen that the funda- 



'8 A. Holmes : The age of the earth, 1913, pp. 68-70. 

 " Radioactivity and geologj', 1909, p. 247. 



■^8 F. W. Clarke: A preliminary study of chemical denudation. Smith. Misc. Coll., vol. 

 56, no. 5, 1910, p. 16. 



■^a A. Holmes : The age of the earth, 1913, pp. 61-74. 



