ON THE MOVEMENTS OF THE EARTu's CRUST. 353 



Thus for the first and second of the calculated cycles and tbeir subdi- 

 visions it is as follows : 



Cycle I. ^3,250,000-2,720,000 years, 0.0304. 



4-2,720,000-2,150,000 years, 0.0332. 



4-2,150,000-1,810,000 years, 0.0203. 

 Cycle II. 4-1,810,000-1,250,000 years, 0.0247. 



4-1,250,000- 700,000 years, 0.0340. 



4- 700,000- 350,000 years, 0.0280. 

 Cycle III. ^ 350,000 to the present time, 0.0291. 



Now as, according to our hypothesis, the sea-level under high lati- 

 tudes will rise and fall with the eccentricity, then it must not only rise 

 and fall once for each arc of the curve, but the " mean sea-level " for 

 longer periods must also rise and fall with the mean value of the eccen- 

 tricity, and such cycles as cycles I and ii must then correspond to two 

 cycles in the geological sequence of deposits. The limits between the 

 cycles of the curve must correspond to the periods of denudation which 

 divide the geological cycles, and the middle must correspond to the 

 periods of overflow. 



The correctness of the two hyi)otheses put forward in my memoir on 

 the Alternation of Strata may (as already 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 themselves, 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 alterations of strata, without giving definite numbers. 



A third difficulty is the distinguishing between the alternations of de- 

 posits which are due to precessions and those which have their cause in 

 other more transitory and local conditions. In the case of shore-forma- 

 tions this difficulty is especially perceptible ; but it has prov^ed 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 up- 

 lifted above the sea. And movements 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 lies 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 

 H. Mis. 224 23 



