234 FOSSIL MEN. 



being the rate in modern subsidences now observed, 

 we shall require periods in comparison with which 

 the received chronology of historians shrinks into in- 

 significance. This rate is, however, confessedly " purely / 

 conjectural/' and there are many considerations which 

 seem to show that it is based on insufficient data. 

 Such modern elevations as are on record, as for ex- 

 ample those in Italy, the Greek Islands, and South 

 America, have been rapid and paroxysmal ; and the 

 raised beaches of western Europe and of North 

 America show that this must have been its character 

 in former times. Slow and gradual movement, even if 

 interrupted, could not have produced these sharply 

 defined terraces. Modern depressions have, with few 

 exceptions, been gradual ; but their rate is so unequal 

 that we cannot reason with any certainty as to the past. 

 While, therefore, it must be admitted that the physical 

 changes of elevation and subsidence which have taken 

 place since man's arrival may have occupied long 

 periods, it cannot be said that they must have done so. 

 It is much the same with the arguments derived 

 from aqueous erosion. This must have gone on simul- 

 taneously with the elevations and depressions, and 

 must have been greatly modified by these. When we 

 stand by the grassy and tree-clad slopes of a river 

 valley, and consider that they have been just as they 

 are during all the centuries of history, it is difficult 

 to resist the prejudice that they must always have been 

 so, and that vast periods have been required for their 

 excavation at the slow rate now observed ; but if we 



