370 Prof. J. Joly on the 



pita ted as oceanic deposits, the average depth of the sedi- 

 mentary rocks on the land is less than one mile; about 

 4000 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 he taken as on an average covering the 

 continents to a depth of 2 kilometres. 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 

 thaw an average of 2411 feet deep over the continents. The 

 sediments in the sea would be additional to this. Q Data of 

 Geochemistry,' p. 29.) 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. 



III. 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 may be regarded as known with considerable 

 accuracy. It may be assumed that one gram of uranium in 

 equilibrium gives rise to, closely, 10*7 x 10~ 8 c.c. of helium 

 (measured under standard conditions) per year. Thorium and 

 its products of change are just as wide-spread in occurrence as 

 uranium. The contribution of helium derived from the thorium 

 group must, therefore, in most cases be also taken into account. 

 Failing direct measurements of the rate of generation of 

 helium by thorium, it is possible to estimate this in terms 

 of the out-put due to uranium by a comparison of the ioni- 

 zation effects of the two families of substances. This com- 

 parison has been made by Boltwood. Allowance has further 

 to be made for the different ionizing activity of the alpha 

 rays from the uranium and thorium series due to their 

 differing velocity and range. The final result is that 1 gram 

 of thoria (Th0 2 ) is equivalent, in its rate of production of 



