370 ON THE MOVEMENTS OF THE EARTH'S CRUST. 



cycles has sixteeu arcs, wbicli corresj)ond to sixteeu smaller oscillations 

 of the shore-lilies, or sixteen geological stages. In each of these stages 

 there are as many alternations of strata as there are precessions in the 

 corresponding arc. And the " mean sea-level " rises with the mean ec- 

 centricity in the middle of the cycles, and falls at the boundary between 

 them, and hand in hand with the mean sea-level the temperature in the 

 higher latitudes also rises and falls. 



The theory here discussed agrees with Lyell's great 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 efi'ects 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 {ante, 

 p. 356), 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 Gres 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 alternations of cli- 

 mate 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 lentic- 

 ular intercalations were often formed in the beds, and as precipitous 

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

 chauge in appearance. But with all this irregularity there are certain 

 beds which appear far more constantly, and which we can recognize in 

 the diflereut 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 maintained unal- 

 tered ; 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 150 meters in thickness, and more than three times as thick as the 

 contemporaneous beds in the Paris Basin, which have a thickness of 

 only 48 meters. 



la the dry periods the depositiou of clay aud qaud was much less, the 



