492 A. Blytt on the probable Cause of 



As regards these great overflows, it must be remem- 

 bered that it is only in folded chains and in strongly elevated 

 regions [e. g. in the Alps, Himalaya, Colorado, &c.) that sea- 

 formed deposits of the later and latest geological periods occur 

 at very considerable elevations above the sea. These great 

 elevations, if we consider them in relation to the whole, can 

 only be regarded as quite local phenomena. At the time 

 when the deposits were formed they lay much lower, and 

 when we now find an alternation of marine and freshwater 

 deposits in such formations we must not suppose that the sea 

 rose and sank in relation to the land by thousands of feet at 

 each oscillation. During the period of formation the shore- 

 line need only have moved up and down a few metres. After- 

 wards the whole system of strata was lifted high above its 

 original level by locally acting " geotectonic " forces. 



Therefore I assume that even the great overflows do 

 not depend upon any very considerable displacement of coast- 

 lines in a vertical direction. When there are large flat coun- 

 tries with basin-shaped depressions, a small elevation may 

 suffice to produce great geographical changes. 



Possibly also these overflows may be due to changes in 

 the eccentricity of the orbit. 



We will now test our hypothesis by a comparison between 

 the astronomical periods and the geological series of deposits. 



The curve of the eccentiicity of the earth's orbit has been 

 calculated from Leverrier's formulaa by J. Croll (' Climate and 

 Time,' 1875, p. 312) for a period of four millions of years ; 

 three millions of years backward, and 1 million forward from 

 the present time. The curve is also calculated according 

 to the same formulae by McFarland (Amer. Journ. Sci. [3] 

 V vol. XX. 1880, p. 105) . His calculation extends from 3,250,000 

 years backward to 1,250,000 years forward in time. He has 

 calculated with shorter intervals of time than Croll (Croll 

 50,000, McFarland 10,000 years), which, however, has had 

 no particular influence in altering the form of the curves. 

 McFarland has in the same place calculated the curve for the 

 same period of time from new formulse of Stockwell's. The 

 two curves, taken in the gross, show a uniform course through- 

 out their length, but as regards the first half Leverrier's curve 

 is thrown somewhat backward, Stockwell's formulae are con- 

 sidered to be more accurate than Leverrier's. 



Both curves are given by McFarland. If we compare them 

 together it appears : — 



1 . The curves coincide with only a small essential difference 

 from the present day until 1 million years back. 



