326 ON THE MOVEMENTS OF THE EARTH'S CRUST. 



somewhat regular oue, and a longer, more irregular one. The former 

 effects a change of climate, the strength of the marine currents alter- 

 nately diminishing and increasing during thousands of years ; the latter 

 longer period effects a rise or fall of the sea in relation to the land, and 

 an alternation of deep-sea formations with shore-formations or fresh- 

 water deposits. The opinion has been expressed that these periods, 

 which are traced in the series of deposits, might possibly stand in con- 

 nection with the two cosmical periods revealed by astronomy — the pre- 

 cession of the equinoctial points, and variations in the eccentricity of 

 the earth's orbit; although in tlie memoir referred to it is not attempted 

 to show in what manner such a connection could be established. But 

 if, with the aid of these two hypotheses, we construct an "artificial" 

 series of strata, we find that one with no less than forty-six changes of 

 deposit may be recognized, bed by bed, in the Tertiary formations of 

 the Paris basin. 



The result may encourage us to test still further the correctness of 

 the two suppositions. As regards the precession this has been at- 

 tempted in my paper "On the probable cause of the periodical change 

 in the strength of the marine currents."* 



The contents of this memoir are essentially as follows: The preces- 

 sion of the equinoctial lines causes the summers in about 10,500 years 

 to be longer, and in the following 10,500 years shorter, than the win- 

 ters. The conditions are opposite in the northern and southern hemis- 

 pheres. The difference between the number of winter and summer 

 dfiys increases with the eccentricity of the earth's orbit. 



The cooling of continents under high latitudes, in the winter, produces 

 a diminished pressure of air over the sea. This low pressure draws air 

 from lower latitudes. For this reason, in the Atlantic, southwest winds 

 prevail. Thus, in the winter, the southwest winds of the North At- 

 lantic are on an average three times as strong as in the summer, in con- 

 sequence of the great refrigeration of the mainland. In the semi-period 

 when the winter falls in aphelion the average annual wind-force is con- 

 sequently greater. Now it is the prevalent wind that produces the 

 powerful marine currents, such as the warm current in the Atlantic 

 Ocean. The strength of the marine currents is dependent upon the 

 average wind-force for the last great time period. Now as this aver- 

 age wind-force is periodically variable in consequence of the precessions, 

 the strength of marine currents and the temperature of the sea must 

 also be subject to a periodical variability. For about 10,500 years the 

 warm sea-current will increase, to diminish in the next similar period, 

 and so on constantly through all time. When the winter falls in 

 aphelion, the difference between the littoral and inland climates will 

 increase. The propelling force of currents in the sea will increase and 

 diminish by 1 to 5 per cent, upon their total annual value according as 



* Fid. Selsk. Forh. Christiania, December 14, 1883; Archiv f. Math, og Naturv., ix. 

 Cbristiania, 1884. 



