MEASURING GEOLOGIC TIME— LANE 237 



ing of the Nile bed 25 feet in 3,000 years, and, near its mouth, the earth 

 end raised 52 inches in 1,000 years; or those poetically described by 

 Alfred Noyes in his Watchers of the Sky: 



* * * The records grow- 

 Unceasingly, and each new grp,in of truth 

 Is packed, like radium, with whole worlds of light. 

 The eclipses timed in Babylon help us now 

 To clock that gradual quickening of the moon 

 Ten seconds in a century. Who that wrote 

 On those clay tablets could foresee his gift 

 To future ages? * * * 



THE UNIT OF MEASUREMENT 



The unit must be identifiable in the geologic record, and must be of 

 uniform length. Of all the units of which one might think, the year is 

 preeminent. The periodic processes are obviously those which will 

 best give us the unit. The second and the day are obviously too short, 

 and, moreover, as yet no daily variations have been recognized in the 

 geologic record, whereas the changes of the seasons have left indelible 

 records. These have been recognized not merely in tree rings, present 

 and fossil, in the growth lines of fish scales and stalactites, the bands 

 of anhydrite in the salt beds which show evaporation under changing 

 conditions of temperature, but also in a wide range of deposits. 



To such bands the Swedish term "varves" is applied, since in Sweden 

 they have been worked by G. de Gcer into a systematic chronology 

 of post-glacial time. The greater melting of the ice in summer is 

 marked by coarser clays. Finer deposits settled in the lakes on the 

 margins of the glaciers during the winter. Baron de Geer's work has 

 been continued and extended by his pupil, E. A. Antevs,'* and by 

 Chester A. Reeds and others. But beside these distinctly glacial 

 varves or bands there are also bands of more and less organic matter 

 which are almost surely seasonal, especially when layers with traces of 

 ephemerids (mayflies) or pollen distinctly mark the seasons, as De 

 Geer found. Some of the more important papers are cited by Brad- 

 ley,^ from whom we take plate 1. 



One difficulty at once arises: How do we know that some of these 

 bands do not represent the two seasons a year that characterize 

 certain regions? To this there is a two-fold reply. First, such regions 

 are exceptional and are not likely to occur in regions of glaciers and of 

 temperate flora; second, studies such as those of C. G. Abbot ^ seem 

 to show the double sunspot cycle of years (fig. 1). Further study 



* Antevs, E. A., The last glaciation. Amer. Qeogr. Soc, Research Ser. No. 17, 1928. See also bibliography 

 in Rating the geologic clock, Rep. 16th Int. Geol. Congr., Washington, pp. 145-167, 1936. 

 » Loc. cit., and esp. U. S. Geol. Surv. Prof. Paper 158-E. 

 ' Loc. cit., footnote 1. 



