132 THE ANTIQUITY OF MAN. 



how long a time would be occupied by agencies now in operation 

 in effecting the changes of elevation, subsidence, erosion and de- 

 posit, observed. Reasoning on this principle, it is plain that a 

 vast lapse of time will be required, and that we may place the 

 earliest men and the latest mammoths at an almost incredible dis- 

 tance before the oldest historical monuments of the human race. 



But can we assume any given rate for such changes ? Not cer- 

 tainly till all the causes which may have influenced them can be 

 ascertained and weighed. We have only recently learned that 

 Scotland has risen twenty-five feet in 1700 years ; but we do 

 not know that this elevation has been uniform and continuous. 

 There is another older sea-level at forty-four feet above the 

 present coast ; and there is a still higher sea-level 524 feet above 

 the sea, which certainly goes back to the time of the mammoth. 

 Now we may calculate that if an elevation of twenty-five feet re- 

 quires 1700 years, an elevation of 500 feet will require twenty 

 times that length of time ; but if we should find on further investi- 

 gation that ten of the twenty-five feet were raised in the first century 

 of the seventeen, and that the rate had gradually decreased, 

 our calculation must be quite different, and even then might 

 be altogether incorrect, since there may have been periods of 

 rest or of subsidence ; so that " such estimates must be consi- 

 dered 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 approxima- 

 tions to the truth : but Sir Charles Lyell has, in his present work, 

 introduced a new and disturbing element, in the strong probabi- 

 lity which he establishes that the cold of the glacial period ex- 

 tended 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 erading 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 newer pliocene period, 

 there were great glaciers in the basin of Lake Superior, all our cal- 

 culations 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 



