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. 
As it can be shown that somewhat less than a third of the sediments 
have been precipated as oceanic deposits,? 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 berestricted 
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. 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 
1 Trans. Roy. Dublin Soc., vol. 7, 1899, p. 48. 
2 Address to section C, British Association, 1908, p. 6. 
3 Data of Geochemistry, p. 29. 
