516 A. Blytt on the prohahle Cause of 



calcareous," which forms the boundary between the two sub- 

 stages, the Lower and Upper Bembridge Marls. In these 

 Upper Marls, which likewise contain brackish and freshwater 

 shells and even lignites, I have assumed 4 climatic alternations: — 

 there are two pyritous bands and a marly bed, and at the top, 

 at the limit of the overlying Hamstead* stage, a bed with fer- 

 ruginous concretions capped with marl. But the two pyritous 

 bands and the first of the above-mentioned marls constitute 

 no palseontological boundary, and are far from being so pro- 

 minent as the Septaria-bed. I therefore regard it as the most 

 probable assumption that the whole of the Bembridge Marls 

 indicate only 3 alternations of climate, and thus for the whole 

 stage we have 6 climatic periods. 



Finally, we come to the Hamstead* Beds, These at the 

 lowest part consist of brackish and freshwater marls, in which, 

 besides a pyritiferous horizon of little importance, and which 

 forms no palseontological horizon, we find indications of two 

 dry periods. One of these, the so-called "White Band," a 

 more or less hardened ferruginous bed rich in fossils, forms 

 the boundary between the two substages, the Lower and 

 Middle Hamstead Marls ; and higher up there is a bed of 

 ironstone concretions, which nearly coincides with the limit 

 between the substages of the Middle and Upper Hamstead 

 Marls. The uppermost part of the Hamstead Stage is formed 

 by the marine Corbula-bed, in which there is a bed with 

 Septaria. The stage therefore represents one oscillation with 

 3 climatic alternations ; but it is not completely preserved, 

 the top having been removed by denudation. 



If we now sum up the above statements, we obtain the 

 following numbers of oscillations of shore-lines and climatic 

 alternations : — 



Barton, 1 oscillation, with 5 climatic alternations. 



Headon Hill Sand, without alternations. 



Break in the series. 



Lower Headon, 1 oscillation (or a little more), 7 alterna- 

 tions. 



Middle and Upper Headon, 1 oscillation, with 6 alterna- 

 tions. 



Osborne, with 8-10 alternations, corresponding to 2 oscil- 

 lations. 



Bembridge, 1 oscillation, with 6 alternations. 



Hamstead, 1 (incomplete) oscillation, with 3 alternations. 



Besides the Eocene Barton there are 3 well-marked marine 

 * The name is also often written '• Hempstead," but this is incorrect* 



