406 A. Blytt on the probable Cau.^e of 



first place beg for induloence for the many faults and im- 

 perfections with which snch an attempt must be affected, and 

 express a hope that in any case the hypothesis may be found 

 worthy of being further tested. 



Havino- endeavoured in several memoirs on the distribution 

 of plants, on peat-mosses, shore-lines, terraces, and morainic 

 ridges, to show that climates undergo periodical changes, I 

 published in the Transactions of the Society of Sciences for 

 1883 (No. 9) a memoir on Alternation of Strata and its 

 possible significance for the chronology of geology and the 

 theory of the modification of species. The essential contents 

 of this paper, as regards the present question of geological 

 chronology, were as follows : — 



Alternation of strata, under which term is understood an 

 alternation of geological formations of different constitution, 

 can be produced by local conditions of rapidly passing 

 change, without the action of general and persistent causes. 

 But there are also causes of the latter kind which effect 

 an alternation of the strata. Two such periodically acting 7 

 causes are traceable in the geological series of deposits — a ' 

 shorter, somewhat regular one, and a longer, more irregular 

 one. The former effects a change of climate, the strength of 

 the marine currents alternately 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 alterna- 

 tion 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 connexion with the two cosmical periods 

 revealed by Astronomy — the precession of the equinoxial 

 points, and variations in the eccentricity of the earth^s orbit ; 

 although in the memoir referred to it is not attempted to 

 show in what manner such a connexion 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 4G changes of deposit may be recognized, bed by bed, 

 in the Tertiary formations of the Paris basin. 



This result may encourage us to test still further the cor- 

 rectness of the two suppositions. As regards the precession 

 this has been attempted in my paper " On the probable Cause 

 of the Periodical Change in the Strength of the Marine 

 Currents." * 



* T7f7. Selsli. Fork. Christicmia, 14 December, 1883 ; ArcMv f. Math, 

 og Natwv. ix. Cliristiania, 1884. 



