1889 ] 



THE DISPLACl 



5NT OF BEACH-LINES. 



25 



ocean's tidal-wave. To this force Darwin's „internal tide", his 

 „bodily tides", should now be added, but which I have no means 

 of calculating. 



Many millions of years ago, when the Moon was nearer, and 

 the tidal-force much greater, the length of the sidereal day also 

 increased more rapidly. But now its iucrease is indubitably 

 very slow; and for that reason we cannot expect any great 

 general changes of level in a short time. 



To an increase of 10 seconds in the length of the day there 

 corresponds (according to Todd 1. c.) a shortening of the equa- 

 torial radius by 5.6 m., and the double prolongation of the polar 

 one, 11.2 m. 



What value the lengthening of the day had, in reality, 

 during the tertiary period we do not know. Much greater than 

 in present times it can scarcely have been ; and it appears, there- 

 fore, in any case, to result from what has been said above, that 

 the vertical displacement of the beach-line during each oscilla- 

 tion can, in general, not have exceeded a few metres, if our at- 

 tempt at explanation of the phenomenon is correct. 



We must, therefore, investigate whether the displacement of 

 the beach-lines was so very considerable. 



We must, then, first investigate, how much there has been 

 deposited in each period of precession, and the thickness of the 

 strata of every stage. The thickness of the beds depends, 

 first and foremost, upon the locality of the place, whether it lay 

 close to or distant from land or the estuaries of rivers, and upon 

 the nature of the layers; chemical precipitations are usually less 

 thick than mechanical ones. As a mean for each synodical 

 period of precession (20—21000 years) I have found the follow- 

 ing values for the various alternations of strata: 



Marl and silicate of lime from 0.6—2.2 m. 



Clay and silicate of lime *- 3 » 



Marl, gypsum, silicate of lime, marine . . . 1.3—1.4 „ 



The same, fresh-water 2.8—2.9 „ 



Lime and marl 1.8—2.5 „ 



Marl, argillaceous lime-stone, ironstone, sandy marl 2.0 „ 



