CHAMBERLIN— THE AGE OF THE EARTH. 251 



He has succeeded in identifying the yearly deposits of glacial waters 

 and in correlating them with annual moraines. In addition to this, 

 he has been able to match sections at distant points by comparing the 

 succession of peculiarities in the annual " varve " layers. While this 

 is a quite special method and has only limited application, it is impor- 

 tant to general time estimates, because it gives a means of checking 

 up other criteria that indicate glacial time, and these help check up 

 certain non-glacial criteria. As is well known, the duration of the 

 recent Ice Age was for a time a sharply battled question, and the old 

 views pitted against the new views came to be well defined. Though 

 not yet ready for precise announcement, it is already foreshadowed 

 that the De Geer method of measurement, when it shall have fully 

 covered the retreatal stage of the last glacial epoch, will show that 

 stage to be about three times as long as it was made by the most 

 representative of the old estimates. The main differences of opinion 

 as to the duration of the glacial period, however, grew out of the 

 evidence that instead of one simple short epoch there were several 

 epochs of glaciation separated by rather long interglacial intervals. 

 Now, by using the De Geer method to correct the criteria on which 

 time estimates of these glacial and interglacial epochs have been 

 based, a glacial period at least twenty times as long as that assigned 

 by the old estimate seems to be foreshadowed. Very likely this 

 degree of extension of an old-time estimate by a new one is excep- 

 tional ; at any rate, the glacial formations are exceptional deposits and 

 make up only a small part of the geologic column. 



In considering the standard water-lain sediments of the column, 

 it is to be understood that only rapid surveys or mere reconnaissances 

 have as yet been made of the larger part of the earth, and that inevi- 

 tably inconspicuous breaks in the continuity of the deposits have been 

 overlooked. As a result recent critical studies have revealed in some 

 cases surprising numbers of gaps in the continuity of deposition. 

 For example, Dr. Stuart Weller, in a study of what was formerly 

 regarded as a continuous section of the Mississippian, has found no 

 less than 12 breaks in continuity. The time-value of these, in his 

 judgment, is two or three times as great as that of the visible beds 

 themselves.® The time-values of such intervals are best judged by 



s Personal communication. 



