494 A. Blytt on the probable Cause of 



The correctness of the two hypotheses put forward in my 

 memoir on the Alternation of Strata may, as ah-eady indicated, 

 be tested in one way by the comparison of geological profiles 

 with the curves of the eccentricity of the earth^s orbit. A 

 first attempt was made at the time with the Upper Eocene 

 and Oligocene beds of the Paris-basin. 



Many difficulties, however, stood in the way of this work. 

 First and foremost the calculation of the curve is less certain 

 for distant periods. This difficulty is to a certain extent got 

 rid of by the circumstance that, as the curves repeat them- 

 selves, it may be less essential. 



Another difficulty is in the finding of long and accurately 

 described profiles without gaps in the series of deposits. 

 Survey-profiles are not sufficient. Geologists often only state 

 that there are few, some, or many alternations of strata, 

 without giving definite numbers. 



A third difficulty is the distinguishing between the alterna- 

 tions of deposits which are due to precessions and those which 

 have their cause in other more transitory and local conditions. 

 In the case of shore-formations this difficulty is especially 

 perceptible ; but it has proved to be less than I supposed at 

 first. 



A fourth difficulty consists in the determination of the 

 number of oscillations of coast-lines. The higher a place was 

 situated, the more rarely was it overflowed ; the lower it lay, 

 the more rarely was it uplifted above the sea. And move- 

 ments of the solid body of the earth, as might be supposed, 

 have not been so uniform everywhere as those of the sea. 



A fifth difficulty Hes in the finding of perfectly typical 

 profiles of the stages produced by the oscillations. When the 

 sea rose and sank slowly the number of marine alternations of 

 strata will be less, and of land and freshwater formations 

 greater, the higher the place lay and the shorter the time 

 which it remained submerged in the sea during each oscilla- 

 tion. But this difficulty is of importance only when the 

 continuous profiles are so short that they do not embrace 

 several oscillations. 



In the absence of longer, connected and accurately traced 

 profiles I have first endeavoured to determine the number of 

 oscillations of coast-lines as regards the Tertiary and Qua- 

 ternary periods. Each of these oscillations, of which there 

 have been about 36 from the commencement of the Tertiary 

 period until now, has, in temporarily submerged localities, 

 produced an alternation of marine beds with freshwater or 

 terrestrial formations. To each more considerable oscillation 

 corresponds a geological " stage." In these '"stages" there 



