282 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1911. 



quantity. We are, I believe, at liberty to assume that the rate of 

 deposition and sinking was anything from, say, 1 foot to but a few 

 inches in a century. A rate of accumulation of 4 inches in a century 

 interprets the geological column as indicating 103 millions of years. 

 Three inches gives us 148 millions. The order of the time value is 

 probably indicated in these figures. 



It is important to note that the facts of solvent denudation place 

 a quite definite limit on the amount of sediments which have been 

 formed during geological time. The sodium which has reached the 

 ocean has originated in the conversion of igneous into sedimentary 

 rocks. It is easy to calculate from the composition of a generalized 

 igneous and a generalized sedimentary rock and from the quantity of 

 sodium in the ocean that the denudation of about 84 million cubic 

 miles of igneous rock, producing about 60 million cubic miles of 

 sediment, accounts for the sodium in the ocean. Such a quantity of 

 sedimentary rock would, if all was now on the land, cover the present 

 land area (55 million square miles) to a depth of a little over 1 mile. 1 

 As it can be shown that somewhat less than a third of the sediments 

 have been precipated as oceanic deposits, 2 the average depth of the 

 sedimentary rocks on the land is less than 1 mile; about 4,000 feet. 

 The total sedimentation throughout geological time must be restricted 

 within this limit. Possibly the limit is too high, for there may have 

 been some sodium in the primitive ocean. It is difficult to show 

 wherein it is too low. This limit must define not only sediments 

 which keep their recognizable characters as such, but those which 

 may possibly have been metamorphosed beyond certain recognition. 

 It is significant that the guesses (for they can only claim to be such) 

 of several writers as to the amount of recognizable sediment upon the 

 land areas, do not diverge very far from the suggested limits. Thus 

 Van Hise thinks these rocks may be taken as on an average covering 

 the continents to a depth of 2 kilometers. Clarke thinks that the 

 sediments certainly do not occupy a bulk equal to the whole land 

 extending above sea level. This would amount to less than an aver- 

 age of 2,411 feet deep over the continents. The sediments in the sea 

 would be additional to this. 3 These estimates may be guesses, but it 

 is improbable that they are several times in error. The observed 

 amounts of sediment are not then in discord with the limitations 

 imposed by solvent denudation. 



THE AGE OF THE EARTH BY RADIOACTIVITY. 



The radioactive investigation of the age of the earth is based upon 



the accumulation in minerals of the inert products, helium and lead. 



The rate of production of helium by a given amount of uranium 



i Trans. Roy. Dublin Soc, vol. 7, 1899, p. 48. 



a Address to section C, British Association, 1908, p. 6. 



3 Data of Geochemistry, p. 29. 



