MARKER : PRESIDENTIAL ADDRESS 
13 
were strongly marked, and the accumulation of sediment was rapid 
enough to yield an appreciable thickness in each year. From such 
data De Geer has estimated that the recession of the last ice-sheet 
occupied a duration of about 5,000 years ; and he further gives 7,000 
years as the lapse of time since the recession of the ice. 
As regards the longer astronomical cycles, it is clear that the 
argument involves a large element of hypothesis, and its application, 
as Blytt allows, is beset with practical difficulties. It possesses a 
special interest as lending a new significance to the details of strati- 
graphy, but as a means to the establishment of a geological chronology 
its value is at present only potential. The radium method of evaluating 
geological time seems to offer more immediate promise. 
In conclusion, it is pleasant to note how these applications of 
chemistry, astronomy and meteorology, not merely to general principles 
of geology but to a definite geological problem, emphasize the funda- 
mental unity of the sciences, and illustrate the powerful aid that may 
be rendered by one to another. 
