350 ON THE MOVEMENTS OF THE EAKTh's CRUST. 



climates eveu under high latitudes. But the great changes in the dis- 

 tribution of land and sea compel us to assume that, hand in hand with 

 them, occurred a periodical alternation of climate, which has been far 

 greater and more radical than the change produced by the precessional 

 l^eriods. 



Eamsay, Croll, J. Geikie, and others have thought that they found 

 more or less certain traces of Glacial periods in the older formations 

 (see, e. </., J. Geikie, "The Great Ice Age," ed. 2, 1887, pp. 566 et seq.). 

 Some of these traces seem to prove that, at any rate, there have been 

 more Glacial periods than the Post-tertiary one. Nevertheless von 

 Richthofen remarks {Filhrer fur Forschungsreisende^ p, 362) that these 

 supposed traces of Glacial periods are perhaps only a phenomenon of 

 abrasion, and that the action of the waves upon the shore could pro- 

 duce conglomerates with striated stones. As regards these supposed 

 old Glacial periods, the most certain traces (see J. Geikie, /. c.) appear 

 to be furnished by the Devonian Sandstone, "Old Red," in England and 

 Scotland, by the commencement of the Carboniferous period (Scotland), 

 by the Permian conglomerate (England), and by the Eocene (Switzer- 

 land). The most striking evidence (with striated stones) is from the 

 periods when the land had great extension. 



As regards these great overflows, it must be remembered that it is 

 only in folded chains and in strongly elevated regions {e.g. in the Alps, 

 Himalaya, Colorado, etc.) that sea-formed deposits of the later and lat- 

 est 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 fresh- water deposits in such forma- 

 tions 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 me- 

 ters. Afterward tlie 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 direc- 

 tion. When there are large flat countries with basin-shaped depres- 

 sions, a small elevation may sufijce to produce great geographical 

 changes. 



Possibly also these overflows ma}' be due to changes in the eccen- 

 tricity of the orbit. 



We will now test our hypothesis by a comparison between the astro 

 uomical periods and the geological series of deposits. 



The curve of the eccentricity of the earth's orbit has been calculated 

 from Leverrier's formulai by J. Croll (" Climate and Time," 1875, p. 312) 



