834 J. BARRELL MEASUREMENTS OF GEOLOGIC TIME 



described by J. M. Clarke^ which shows starfishes buried while in the 

 act of feeding on bivalve mollusks."^* 



ESTIMATES OF TOTAL TIME BASED ON OCEANIC SALTS 



TJie use of chemical denudation as a measure of geological time began 

 in 1S7G with T. Mellard Reade. Joly^^ has carried foi'ward this method, 

 using the sodium chloride of the ocean, the quantity of which is known 

 with a considerable degree of precision. The chief sourice of the oceanic 

 sodium is without doubt the lime-soda feldspar of igneous rocks, from 

 which it has been derived by the aggregate weathering and erosion of 

 Ideologic time. 



If the amount of sodium derived annually from igneous rocks and car- 

 ried to the sea can be determined, and if it be assumed that this rate is 

 a true mean for all geologic time, then the age of the ocean is obtained 

 by simply dividing the total sodium by the annual supply. Various cor- 

 I'ections have to be applied, however. To obtain the total sodium derived 

 by weathering, the oceanic sodium should have added to it the amount in- 

 cluded in the sedimentary rocks, and have subtracted from it the amount 

 derived from juvenile waters, including the original salt, and the amount 

 due to marine denudation. All of these corrections, however, are regarded 

 with considerable assurance as small and would not lead to gross errors 

 if neglected. 



The amount contributed by rivers yearly to the sea has been ascer- 

 tained with reasonable accuraicy for representative areas by analysis of 

 their waters. Corrections to this must be made for the amount of sodium 

 chloride which is not derived from igneous rocks, but is borne by the 

 wind or by man from the sea, and also that redissolved from the sedi- 

 mentary rocks as sodium chloride and originating from the sea. The 

 last correction for chloridized sodium is large and uncertain in amount. 



The composition of the average igneous rock is known with fair pre- 

 cision. It contains between 3.4 and 3.9 per cent of soda. In suffering 

 weathering and erosion it retains on the average between 1.3 and 1.4 per 

 cent of its soda, this remaining in the detrital rocks. It loses between 

 2.10 and 2.5 per cent to the sea. 



From the above it is seen that chemical denudation serves to measure 

 tlie total amount of igneous rock which must have been eroded through 

 geological time in order to supply the salt of the sea. Because sediments 



■^* J. M. Clarke : Early adaptation in the feeding habits of starfishes. Jour. Acad. Nat. 

 Sci.. Philadelphia. 2d ser., vol. 15, 1912. pp. 113-118. 



■^5 Trans. Roy. Soc. Dublin (2), vol. 7, 1899, p. 23; Rept. British Asso. Adv. Sci., 1900, 

 p. 369 ; Radioactivity and geology, 1909, pp. 233-251. 



