MEASURING GEOLOGIC TIME— LANE 239 



should be made of all these banded deposits to see if this or other 

 identifiable cycles can be determined. 



Another question is whether we can assume that the year has been 

 of the same length. In an expanding universe, unless the gravitation 

 constant also changes to match, the year would have been shorter 

 when the earth was nearer the sun. 



While one might also think of the addition to the weight of the 

 earth by meteorites and their effect upon its orbit, there is such a 

 lack of meteoritic material in the rocks accessible to the geologist 

 that any such effect during the time they were laid down would be 

 an infinitesimal of the second order. The same description would 

 apply to the effect of the loss of mass by radiation from the sun. 



But the year is a rather short unit for the longer geologic times. 

 Is there any practical longer unit? A number of longer periods have 

 been suggested by E. Huntington, Clough, and others, ^ but only 

 three have been seriously applied. One is the processional period of 

 21,000 to 25,000 years. In half the period the North Pole changes 

 from being inclined toward the sun when the earth is farthest from 

 the sun in her elliptical orbit around it, to being inclined toward the 

 sun when the earth is nearest it, thus making the two reasons for 

 summer heat reinforce each other in the Northern Hemisphere. 

 Moreover, since the earth moves faster the nearer it is to the sun, 

 there will be extra hot and short summers in the Northern Hemisphere 

 from the spring equinox to the faU. The difference is at present only 

 3 days, but when the orbit of the earth is more elliptic it may be as 

 much as 30 days. 



As the two hemispheres are unequally balanced in proportions of 

 land and water, the northern having much more land, it seems quite 

 possible that this cycle should register itself in climatic changes. 

 Bradley's bibliography (see footnote 2) cites papers by himself, 

 Gilbert, Korn, Stamp, and Wolansky, which discuss supposed signs 

 of such cycles in rhythmic alternations of strata. Oil geologists are 

 making further studies. Such cycles should be alternately more and 

 less conspicuous, depending upon a longer rhythm — that in which 

 the ellipticity of the earth's orbit varies. However, the effect upon 

 climate is indirect and depends upon geographic arrangements of 

 land and water which may vary quite independently. The whole of 

 human history recorded in accurate measurements has only covered 

 a quarter of a cycle. The geologist who a hundred thousand years 

 hence has four or five such cycles to look back upon will be better 

 able to judge their worth as time units. 



The longer cycle of varying ellipticity of the earth's orbit has even 

 greater disadvantages. We have passed through so little of it that we 



' Lane, A. C, bibliography in Rating the geologic clock, Rep. 16th Int. Oeol. Congr., Washington, pp. 

 158-167, 1936. 



