C EN O ZOIC HISTORY. 587 



Certain general relations in this diagram are founded on defi- 

 nite data. They are the relatively greater length of Chesapeake 

 submergence, the shortness of Lafayette definition, and the 

 relatively short submergence of Pleistocene times. The Post- 

 Lafayette emergence is shown to be greater in vertical amount 

 than the earlier emergences, by the sub-aerial trough-cutting 

 which ensued, but there may have been similar or deeper 

 gorges in earlier times, which were finally effaced by base- 

 levelling. That the Inter-Columbia uplift was of shorter dura- 

 tion than the Post-Lafayette is indicated by the relative narrow- 

 ness of the inner trough. 



N. H. Darton. 

 U. S. Geological Survey. 



