the Displacement of Shore -lines. 513 



principle. Slow changes in the length of winter and summer 

 and in the force of the tidal wave produce periodical changes 

 of climate and displacements of shore-lines. The changes 

 take place so slowly that the effects begin to appear distinctly 

 only after the lapse of many thousands of years. There are 

 two astronomical periods which are the cause of the great 

 and fundamental changes of which geology bears testimony 

 to us from long past days, and which will still continue, for 

 millions of years, to effect similar changes in the geography of 

 our globe, in its climate, and its animal and vegetable life. 



Postscript. 



With reference to the profile of the Isle of Wight above 

 cited (p. 497), I must make a few remarks. Although with 

 some doubt, I have referred the Headon beds to the Upper 

 Eocene. But the difference between the faunas of the Gr^s 

 de Beauchamp and the Middle Headon is far too great for 

 these beds to be synchronous. 



The cause of the error is that I reckoned too many alterna- 

 tions of climate in the Isle of Wight beds. In these fluvio- 

 marine deposits there is by no means the same regularity as 

 in beds which are formed in basins with less sedimentation. 

 The river eroded its borders and shifted its bed, banks were 

 formed and carried away, according as the direction of the 

 stream varied and the channel changed. Hence lenticular 

 intercalations were often formed in the beds, and as precipi- 

 tous cliffs of the Isle of Wight break down, the minor details 

 of the profiles change in appearance. But with all this 

 irregularity there are certain beds which appear far more con- 

 stantly, and which we can recognize in the different profiles 

 even although their condition is somewhat altered. By the 

 aid of these constant beds we find order in the variations, 

 and it appears that the great features of the profiles are main- 

 tained unaltered ; and it is these great features that we must 

 follow when we wish to determine the number of climatic 

 alternations. In the Paris-basin, where sedimentation was 

 much less, chemically deposited beds play a much more 

 prominent part. In the Isle of Wight the stages are of 

 much greater thickness. The Oligocene deposits of the Isle 

 of Wight are 156 metres in thickness, and more than three 

 times as thick as the contemporaneous beds in the Paris-basin, 

 which have a thickness of only 48 metres. 



In the dry periods the deposition of clay and mud was much 

 less, the water of the river was purer, and chemically formed 



