514 A. Blytt on the probable Cause of 



beds had time to be deposited. Instead of clay and marl, 

 limestone, Septaria-beds, ironstone, &c., were formed. These 

 beds were undoubtedly formed much more slowly than the 

 sand-, clay-, and marl-deposits which alternate with them. 

 They are analogous to the forest-beds in peat-mosses. Forest- 

 beds often separate peat-deposits with different species of 

 plants. This shows that the forest-beds indicate long dry 

 periods, during which the formation of peat ceased, and the 

 flora became changed* ; when the quantity of rain again in- 

 creased and the formation of peat commenced anew, the 

 forest-trees which grew around the mosses were changed, and 

 the forest-beds thus make divisions between different substages 

 or zones in the peat. 



Among the beds deposited in water (whether fresh- or 

 salt-water formations) it is chiefly the above-mentioned che- 

 mically-formed beds that are formed in dry periods. And just 

 as in the peat-mosses forest-beds often separate peat-deposits 

 with a different flora, so limestone- and Septaria-beds also fre- 

 quently intervene between clay-, marl-, and sand-deposits 

 with a more or less different fauna, so that these chemically- 

 produced deposits often form boundaries between geological 

 stages and substages. This is the case, for example, in the 

 Fluvio-marine series of the Isle of Wight, the main features 

 of which we shall now pass on to describe with the aid of 

 Forbes's detailed and classical statements. We shall then see 

 that we have fewer climatic changes than I previously sup- 

 posed, and that the series of beds in the Isie of Wight coin- 

 cides as admirably with the curve of eccentricities as the 

 Parisian deposits, although somewhat later on in time than 

 was hitherto supposed ; thus the agreement with the palseon- 

 lological results becomes complete. 



We begin from below, with the Upper Eocene Barton 

 Clay. Judging from the fossils this is synchronous with the 

 Gres de Beauchamp in the Paris-basin. It has 5 Septaria- 

 beds, and corresponds to arc 14 of the curve, which has the 

 same number of precessional periods. The Barton Clay is 

 covered by the Headon Sands (previously referred to the 

 Upper Bagshot), which have no alternations, and which were 

 probably formed in a comparatively short time. 



A great gap now follows in the series in the Isle of Wight. 

 In the Paris-basin the freshwater Calcaire de St. Ouen was 

 formed at this time. This is only 6-7 metres thick, but it has 

 10 alternations, which should represent 200,000 years according 



* See " Theori om Indvandringen af Norges Flora under vekslende 

 regnfulde og t0rre Tider," in Nyt Mag. for Naturv. xxi. 1876 (pp. 52, 53 

 of separate copies). 



