MEASURING GEOLOGIC TIME— LANE 243 



geophysical research will be shown, although for the present the possibilities of 

 stretching it to fit various ideas are so great that it runs the danger of being treated 

 with mistrust on the physical side, as an india rubber theory. 



In any case, cycles are always spoken of as millions of years long — 

 so long that we have no historic knowledge of more than a small 

 fraction of a cycle. They are not adapted for units of measurement. 



The precessional cycle belongs to the solar system, as does the 

 sunspot cycle, and there are numerous other planetary cycles. Any 

 cycles of astronomic origin of greater sweep than the solar system are 

 much longer and would appear in the life of the earth possibly as pro- 

 gressive factors or cycles as long as any we have mentioned. Such 

 would be the great orbit which the whole solar system is describing 

 sidewise around the center of the Galaxy, some 3,000 light years 

 (5.84 X 10^^ miles) away at a velocity of about 300 kilometers (183 

 miles) a second, a cycle of some 20 million years. Such cycles are 

 obviously not suitable as time units. We must then fall back on the 

 year as the unit. 



PROGRESSIVE PROCESSES 



Leaving, then, the periodic processes and rhythmic cycles for the 

 moment, and accepting the year as a unit even though our accuracy 

 may be only expressed in millions of years, or "decamyriads" of 

 Vernodsky, we pass to those progressive processes, the type of which 

 is the hour glass, by which we measure time. 



These are "one-way streets." Among them are the processes of 

 degradation or lowering of the land, whether by wind or water or ice, 

 by the streams lowering the area of their basins or cutting back as in 

 waterfalls or cutting down, the erosion by glaciers, and along the shores 

 of the ocean. Of these activities, a host have been studied, such as the 

 retreat of Niagara Falls and of the Falls of St. Anthony at Minne- 

 apolis, the retreat of the cliffs of the Weald on which Darwin made esti- 

 mates, the recession of the English shores, and the shore from Chicago 

 to Milwaukee. Along Lake Huron, Gordon and I once found a road 

 and plow marks running off the bluff. The road came in again half 

 a mile farther on, and by comparing with old maps, Gordon was able 

 to compute the rate of retreat as 5.7 feet a year. 



But what is eroded in one place must be deposited elsewhere, and 

 to match degradation there must be deposition. In all such calcula- 

 tions there is a fundamental "rule of three." So much has been done 

 in a given, historically known time, and so much has been done in 

 all. At the same rate this would have taken a proportionately longer 

 time. 



In some cases we may be reasonably sure that the rate of change 

 is not the same. Many activities are asymptotic. They grow less 

 and less as time goes on, if no other factor interferes. For this we 



