GROWTH OF GEOLOGY. 45 



Moreover, we must recall the fact that the sedi- 

 mentary rocks are in scores of cases interrupted in 

 a manner which forces us to infer periods of up- 

 heaval or subsidence or volcanic intrusion, still 

 further extending the demand for millions of years. 



In an exceedingly interesting paper, Goodchild * 

 has tried to estimate the time required for the vari- 

 ous sedimentary formations considered seriatim, 

 and the time represented by great unconformities, 

 and computes the total time since the commencement 

 of the Cambrian period at over 700,000,000 years. 

 But life was already ancient in the Cambrian times, 

 and this leads, as Goodchild indicates, to an enor- 

 mous increase of the seven hundred millions. 



'Argument from the Saltness of the Sea. Another 

 interesting line of argument is that which has led 

 Prof. Joly to conclude that eighty to ninety millions 

 of years represent the maximum period of time since 

 the oceans were formed. His argument is that since 

 the salt sea was once fresh, and since the saltness is 

 due to dissolved salts carried into the sea by rivers, 

 an estimate of the annual amount brought down by 

 the rivers will show how long it must have taken to 

 give the sea its present salinity. Taking sodium 

 alone, it is computed that the amount in the sea is at 

 least ninety millions of times greater than the quan- 

 tity which rivers pour in annually (about 160,000,- 

 000 tons). Joly's argument is clear and simple; 

 everything depends, however, on the reliability of 

 the data. 



(c) Biological Arguments. Apart from domesti- 

 cation and cultivation we know almost nothing in re- 

 gard to the present rate of variation of living crea- 

 tures, though researches like those of Prof. Weldon 



* Proc. Roy. Phys. Soc., Edinburgh, xiii., 1897, pp. 259-308. 



