THE GEOLOGIC DAY 



ALFRED C. LANE 

 Lansing, Mich. 



The fact that geologic formations, whether hthologic or faunal 

 units, given the same name — the Potsdam, for instance — are not in 

 all parts strictly contemporaneous, has been often noted. Terms 

 like "homotaxial" have been applied to them to avoid the assump- 

 tion of contemporaneity. 



A good part of the time of the geological congresses has been 

 devoted to a discussion of problems of a similar nature. Yet it is 

 rarely, if ever, noted that the same problem comes up in connection 

 with the civil day, and, in fact, with any possible measurement of 

 time. 



It has just been vigorously impressed (April 19) that 5:13 San 

 Francisco time is not the same as Washington time, but about three 

 hours later. Therefore the day ends and begins later at San Fran- 

 cisco than at Washington. The sidereal day is a little different from 

 the solar day, the astronomical year from the civil year. The rea- 

 son for this lies near to the heart of things. Time is measured by 

 change, and change, as we know it, must progress from point to 

 point. The progress may be as rapid as an explosion or as the trans- 

 mission of light, or as slow as the precession of the equinoxes. Never- 

 theless, there is always a rate of progress which is measurable, and 

 any interval of time must be marked by a certain stage in the change, 

 and so begin differently at different points. 



There is no reason to be surprised, therefore, if we find this to 

 be true in dividing up our geological time. We measure it generally 

 by some slow change : the evolution of life, or orogenic movements, 

 or the accumulation of salt in the sea, the changes in sedimentation 

 consequent on elevations of continents, or changes in climate, like 

 the passing away of the glacial epoch. 



Take postglacial time for an illustration. It is clear that the ice 

 must have retired from the valley of the Ohio before it retired from 



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