ON GEOLOGIC TIME-DIVISIONS 



355 



by adopting appropriate stage names. We shall here need two 

 sets of stage names, one for the marine Tertiary and the other 

 for the fresh-water Tertiary deposits. 



12. There is some advantage in retaining the terms Canadian 

 and Trenton as names of series in the Ordovician. The faunas 

 of the Trenton limestone, the Utica and Hudson River shales are 

 very intimately related, and that relation should be indicated by 

 grouping the three together as stages of a single series. The 

 Calciferous and Chazy should similarly be grouped into one 

 series. 



I believe if you have patience to read all this that you will 

 see how I would stand with reference to the several questions in 

 your circular letter. Any classification is arbitrary at best. A 

 dozen or more equally good schemes might be proposed, but we 

 should adopt and strengthen as far as possible that which is in 

 most general use, notwithstanding the fact that it might be 

 improved in many respects. Samuel Calvin. 



