the Displacement of Shore-lines. 515 



to my calculation. It might seem that this was a lono- time for 

 the formation of a stage of" so little thickness ; but while this 

 stage was deposited the marine fauna was changed to such an 

 extent that a great geological boundary has been drawn 

 through this point, the boundary between the Eocene and 

 Oligocene. The first marine Oligocene bed in the Isle of 

 Wight (the Marine Headon) has a fauna of which only 30-50 

 per cent, of the species occur in the Barton Beds. The 6-7 

 metres of the freshwater limestone in the Paris-basin probably 

 represents more than half the time which elapsed between the 

 formation of the Marine Barton and Headon. 



Then the sea rose again, and the Oligocene period com- 

 menced. The oldest Oligocene stage in the Isle of Wight is 

 the Lower Headon ; it is a fresh- and brackish-water forma- 

 tion showing one oscillation of the shore-line. I have given it 

 7-8 alternations. The stage contains five limestones^ separated 

 by deposits of sand and clay, and besides these, two horizons 

 with ferruginous concretions. Reckoning these, it has 7 

 periods. Marine fossils (^Cytherea, Mytilus) sometimes occur 

 in the middle of the stage ; freshwater and brackish forms 

 above and below. The Lower Headon thus represents one 

 oscilhition of the shore-line (or a little more) with 7 climatic 

 changes. 



The next stage or oscillation is the Middle and Upper 

 Headon. These have together 6 alternations of strata, 4 

 limestones, and 2 beds with iron concretions separated by 

 clays and sands. The Middle Headon is brackish at the base, 

 but soon becomes a purely marine formation, with an abundant 

 fossil fauna. The Upper Headon contains fresh- and brackish- 

 water animals. 



Above the Headon come the Osborne Beds, a nearly pure 

 freshwater formation. It has 8-10 alternations: — 2 Septaria- 

 beds, 2 ironstone bands, and 6 horizons with concretions 

 of argillaceous limestone, separated by clay, and marl. 

 10 alternations represent 2 oscillations and 2 arcs of the 

 curve. 



Over the Osborne comes the Bembrndge Stage. The Bem- 

 bridge beds consist of : first, a freshwater limestone, which 

 has 3 well-marked alternations of compact limestone with 

 clay and marl ; these 3 alternations recur in profiles from 

 the most different localities ; over this the marine Bern- 

 bridge Oyster-bed, and immediately above thisa S(^ptaria-bed, 

 of which Forbes says that it is " very remarkable and con- 

 stant.'"' Above this come the Lower Bembridge Marls with 

 brackish and freshwater animals, but without alternations ; 

 upon this a Septaria-bed, " sometimes siliceous, sometimes 



