23G ANNUAL REPORT SMITHSONIAN INSTITUTION, 1937 



the Declaration of Independence of the United States. Not only 

 the birth of a nation, but the birth of an individual is also a unique 

 event, and it is natural in writing the biography of an individual to 

 start with his birth, which is a paroxysmal and unique event. How- 

 ever, some other event — his accession to the throne, Mahomet's 

 flight (the Hegira), or his death — may also be unique, precise, and 

 better known. The chronology according to the birth of Christ, 

 unique as that event was, was introduced long after his life, and his 

 actual birth was probably not the real starting point. 



The same difficulty comes up if one wishes to measure geologic 

 time from the "birth time of the world," which is the title of a book 

 by J. Joly. We know too little about it, and, in fact, it is hard to 

 define what we mean by it. It is therefore not the place to begin our 

 reckoning. 



The same thing applies to other points in the geologic story. The 

 farther back we go the more difficult it will bo to get a general agree- 

 ment as to what should be the starting point, how precise it is, and 

 what it means. To be sure, there are showers of volcanic ash (ben- 

 tonite) which must mark a rather precise epoch, but they are hard to 

 recognize and are more or less local. While they may well be used for 

 dating some small bit of geologic time, they can hardly be a generally 

 used starting pohit. 



It is a trite saying in geology that the present is the key to the 

 past. The present is unique. It is also best and most precisely 

 known. It should be, therefore, and is, the startmg point for any 

 general estimates of geologic time. From it and from what has hap- 

 pened within the history of man we work backward. The one diffi- 

 culty about the present is that it is changing, but for most of our 

 geologic time calculations it makes very little difference what historic 

 date we use as a datum, since few of the methods of measuring geo- 

 logic time can pretend to an accuracy of 1 percent, and most of them 

 deal in units of thousands or millions of years. 



It may well be, however, that A. D. 1920 will, to the geologist a 

 miUion years hence, be marked to within 10 years by the abundant 

 relics of discarded internal combustion engines, the relics of the great 

 war close below, a line of fossils indicating a cosmopolitan flora and 

 fauna, and the widespread burning of coal, of which, to be sure, there 

 would be signs in the previous 100 years. 



Our physical laws and scientific results tell us how two states are 

 connected, but in many cases it is a niatter of indifference with wliich 

 we start; one or the other must be arbitrarily given. The present, 

 then, is our starting point, and the baseline of our estimates is the 

 record of human observations, whether it be those recently described 

 so graphically by Ludwig in his book, The Nile, ^ such as the lower- 



« Ludwig, Emil, The Nile, pp. 26, 328-329. Viking Press, 1937. 



