ABSTRACTS OF PAPEES 21 



Dismissing in a few words the Teport set for presentation at this time, 

 Doctor Udden presented the following joaper, as being germane to the 

 topic under discussion : 



CHRONOLOGY IN GEOLOGY 

 BY J. A. UDDEN 



It seems to me that oue of the important aims in tlie study of sedimentation 

 should be to attain more precise information on the relation between quantity 

 of deposits and time represented. It has often been stated in discussions of 

 geologic time that this is merely relative, and that the geologist can not be 

 concerned with years, centuries, or millennia, as they are counted in history. 

 Nevertheless, many attempts have been made at estimates of time expressed 

 in years for all the sediments that have been measured, including even sedi- 

 ments of pre-Cambrian ages. Some such results have been regarded as rough 

 •estimates of the age of the earth since sedimentation began. They have 

 mostly been based on two as yet largely unknown factors, the annual quantity 

 of deposits brought down to the sea by the present rivers and the total thick- 

 ness of all sediments, the calculation consisting in dividing the larger factor 

 by the smaller. Lately it seems that some important observations have been 

 made by the use of a more exact method, bearing on the duration of the post- 

 glacial deposits in the Baltic regions, and De Geer has been able to count in 

 years the time elapsed since the glaciers disappeared in Sweden. Incidental 

 to this work, he has shown that tliere is a great variety in the thickness of 

 annual deposits of different kinds of material and at different points away 

 from the source of the sediments. The annual layers of the finest *ilt farthest 

 away are measured in millimeters, while the thickness of the annual layers of 

 coarser sediments closer to the glacial source is measured in decimeters. In 

 the proximal region of deposition the rate is somewhat like a hundred times 

 greater than in the distal region. 



Still more recently, in our own country, Robert W. Sayles has presented the 

 results of studies on the banding of glacial clays in New England, and has 

 secured evidence that seems conclusive of annual periodic variation in sedi- 

 mentation in glacial clays along the Connecticut River. He makes reference, 

 also, to other instances of banded clays that appear to be the result of sea- 

 sonal variations in deposition. 



Even if we considered that this work in correlating time with thickness of 

 sediments has as yet been possible only in the special case of the latest sedi- 

 ments, it seems to me that there is reason to hope that a method of measuring 

 time from the stratification of sediments may be found to have wide applica- 

 tion. 



We can, perhaps, never expect to be able to make out layers representing 

 seasonal deposition in the greater part of the sediments of the past; Several 

 circumstances make this impossible. In the first place, deposition has been 

 interrupted. Our unconformities may represent as much time as our sedi- 

 ments. In the second place, there must be many shifts of an irregular nature 

 in currents carrying sediments. These may entirely ol>literate the annual 

 variations conseiiuent on seasonal rhythm. Then thei'e is the important fact 



