372 ON THE MOVEMENTS OF THE EARTh's CRUST. 



occur in tlie Barton Beds. The 6 to 7 meters of the fresh water lime- 

 stone 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 againj and the Oligoceue period commenced. The 

 oldest Oligocene stage in the Isle of Wight is the Lower Headon ; it is 

 a fresh and brackish water formation, showing one oscillation of the 

 shore-line. I have given it seven to eight alternations. The stage con- 

 tains five limestones, separated by deposits of sand and clay, and be- 

 sides these, two horizons with ferruginous concretions. Eeckoning 

 these, it has seven periods. Marine fossils {Cytherea, Mytilus) some- 

 times occur in the middle of the stage; fresh-water and brackish forms 

 above and below. The Lower Headon thus represents one oscillation 

 of the shoreline (or a little more) with seven climatic changes. 



The next stage or oscillation is the Middle and Upper Headon. These 

 have together six alternations of strata, four limestones, and two beds 

 with iron concretions separated by clays and sands. The Middle 

 Headon is brackish at the base, but soon becomes a purely marine for 

 mation, with an abundant fossil fauna. The Upper Headon contains 

 fresh and brackish water animals. 



Above the Headon come the Oshorne Beds, a nearly pure fresh-water 

 formation. It has eight to ten alternations : Two Septaria beds, two 

 ironstone bands, and six 'horizons with concretions of argillaceous lime- 

 stone, separated by clay and marl. Ten alternations represent two 

 oscillations and two arcs of the curve. 



Over the Osborne comes the Bembridge Stage. The Bembridge beds 

 consist of :— first, a fresh-water limestone, which has three well-marked 

 alternations of compact limestone with clay and marl; these three 

 alternations recur in profiles from the most different localities; over 

 this the marine Bembridge Oyster-bed, and immediately above this a 

 Septaria bed, of which Forbes says that it is " very remarkable and 

 constant." Above this come the Lower Bembridge Marls with brackish 

 and fresh water animals, but without alternations ; upon this a Septaria 

 btd, "sometimes siliceous, sometimes calcareous," which forms the 

 boundary between the two substages, the Lower and Upper Bembridge 

 Marls. In these Upper Marls, which likewise contain brackish and 

 fresh-water shells and even lignites, I have assumed four climatic alter- 

 nations : There are two pyritous bands and a marly bed, and at the top, 

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

 concretions capped with marl. But the two pyritous bands and the 

 first of the above-mentioned marls constitute no palieontological bound- 

 ary, and are far from being so prominent as the Septaria bed. I there- 

 fore regard it as the most probable assumption tli.it the whole of the 

 Bembridge Marls indicate only three alternations of climate, and thus 

 for the whole stage we have six climatic i^eriods. 



Finally, we come to the Hamstead* Beds. These at the lowest part 



* The name is also often wrjtteii "Hempstead," but tbie is incorrect, 



