Dr J. W. Dawson on the A7itiquity of Man. 61 



sea-level 524 feet above the sea, v^^liicli certainly goes back 

 to the time of the mammoth. Now we may calculate that 

 if an elevation of 25 feet requires 1700 years, an eleva- 

 tion of 500 feet will require twenty times that length of 

 time ; but if we should find, on further investigation, that 10 

 of the 25 feet were raised in the first century of the seventeen, 

 and that the rate had gradually decreased, our calculation 

 must be quite difi'erent, and even then might be altogether 

 incorrect, since there may have been periods of rest or of 

 subsidence ; so that " such estimates must be considered in 

 the present state of science as tentative and conjectural." 



Again, at the rate in which the Somme, the St. Lawrence, 

 and the Mississippi now cut their channels and deposit 

 alluvium, we can calculate that several tens of thousands 

 of years must have elapsed since the mammoth roamed on 

 their banks ; and we have been accustomed to rest on these 

 calculations as close approximations to the truth : but Sir 

 Charles Lyell has, in his present work, introduced a new 

 and disturbing element, in the strong probability which he 

 establishes that the cold of the glacial period extended to a 

 later time than we have hitherto supposed. If, when the 

 gravels of the Somme were deposited, the climate was of a 

 sub-arctic character, we have to add to modern eroding 

 causes the influence of frost, greater volume of water, spring 

 freshets, and ice-jams, and the whole calculation of time 

 must be revised. So if it can be proved that when the St. 

 Lawrence began to cut the ravine of Niagara, in the Post- 

 pliocene or New Pliocene period, there were great glaciers in 

 the basin of Lake Superior, all our calculations of time would 

 be completely set at nought. 



Such are the difficulties which beset the attempt to turn 

 the monumental chronology of geology into years of solar 

 time. The monuments are of undeniable authenticity, and 

 their teachings are most valuable, but they are inscribed 

 wdth no record of human years ; and we think geologists 

 may wisely leave this matter where the Duke of Argyll, in 

 his address to the Eoyal Society, lately placed it, as a doubt- 

 ful point, in so far as geological evidence is concerned, 

 whether the mammoth lived later than we have hitherto 

 supposed, or man lived earlier. Still, as we have already 



