GREAT AGE OF ALLUVIUM. 127 



11. It is mainly at rapids and cataracts that rivers are now deepening their 

 beds. 



12. The details that have been given enable us to form some idea of the length 

 of time that has elapsed since the close of the drift period. 



The evidence on this point, rests on the assumption I have made in the preced- 

 ing details, that certain old river beds that existed on the last continent, became 

 so filled with modified drift during the sojourn of the surface beneath the ocean, 

 that when it rose, the old rivers were compelled to seek new channels, and in some 

 cases we have the amount of their erosion in the solid rock since that period. If 

 this explanation be admitted, it follows that probably such cuts as the Niagara has 

 made in the rocks below the cataract, in Genesee river, below Rochester, and 

 between Mount Morris and Portage ; in fact, all the ten cases referred to under the 

 third inference, have been formed during the alluvial period : or since the close of 

 the drift period. Nay, these old beds seem to have been filled with modified drift, 

 and, therefore, the gulfs eroded since the last emergence of our continent from the 

 waters, do by no means reach back to the drift period : that is, if we suppose the 

 coarser and legitimate drift to have been produced while the continent was sink- 

 ing. But since it is so difficult to fix the limits between drift and modified drift, 

 we will regard the drift period as not closing till the work of erosion had com- 

 menced upon the rising continent. And even with such limits, what an immense 

 period has elapsed since the period of the striation of rocks and the dispersion of 

 the erratics closed, and the alluvial commenced. 



But other facts in the history of alluvium correspond to the evidence which 

 erosions present of the great antiquity even of the drift period. I refer specially 

 to the vast deltas that have been pushed forward at the mouths of the large rivers 

 of the globe, and the enormous accumulation of debris on the face of steep moun- 

 tains. As mentioned in another part of this paper, the delta of the Mississippi, 

 at its present rate of increase, must have required over 14,000 years to accumulate. 



The growth and extent of coral reefs lead us to the same conclusion as to the 

 length of the alluvial period. But perhaps the erosions of the surface form an 

 argument for the earth's great antiquity more readily apprehended by men gene- 

 rally than any other. 



