MEASURING GEOLOGIC TIME: ITS DIFFICULTIES 



By Alfred C. Lane 



[With 2 plates] 



In measuring any tiling, three tilings are necessary: First, a definite 

 starting point; second a unit of measurement; and third, a method of 

 counting those units. All processes may also be divided into three 

 lands, which we have in the geologic story and which are variously 

 useful in measuring: The periodic, the progressive, and the 

 paroxysmal. 



The periodic processes are not merely those that depend upon the 

 days and the yearly seasons, but also longer cycles, such as the 23-year 

 double sunspot cycle, and the 25,000-year precessional cycle. Others 

 mentioned by Clough have been diligently sought. The only one of 

 these of which there appears to be as yet indisputable geologic evidence 

 is the double sunspot cycle, which Abbot * and Bradley ^ have studied. 

 To this matter we will return later. 



The progressive processes are the one-way activities, such as cooling 

 and the lowering of continents by rivers, which "draw down eonian 

 hills and sow the dust of continents to be." 



Finally, we have the paroxysmal processes, illustrated by the earth- 

 quake and the volcano. Whenever we have a progressive accumula- 

 tion of strain which reUeves itself when a particular amount is reached, 

 we may have a periodic recurrence of paroxysms. The classic illus- 

 tration of tliis is the Old Faithful Geyser with its 65-minute period. 



THE STARTING POINT 



The starting point of measurement, corresponding to an engineering 

 datum plane, should be unique and not easily mistaken or confused 

 with some other point. It should also be at least as precise as the 

 unit, preferably more so. A cyclical process does not in itself give a 

 good starting point, since conditions are repeated over and over again. 

 A year must be labeled by reference to some unique event, such as 



1 Abbot, C. G., Solar radiation and weather studies. Smithsonian Misc. Coll., vol. 94, No. 10, pp. 71-75, 

 1935. 



» Bradley, W. H., Nonglacial varves with selected bibliography. Exhibit (2) in Report of the National 

 Research Council Committee on the measurement of geologic time. May 1. 1937. 



235 



