The Eocene Formation of Headon and Alum Bay. 3 



The boundary of the basin on the North side is formed by the 

 range of hills extending from Beechey Head to Dorsetshire. The 

 strata of these hills dip generally at an angle varying from 15° 

 to 5°. The slope, therefore, on the North side was a gentle one, 

 that on the South, extremely steep and abrupt. 



In discussing the formations deposited in the basin, we shall 

 follow the natural order in which they exist. 



I. The lower marine formation over the chalk. 



II. The lower freshwater formation. 



III. The upper marine formation. 



IV^. The upper freshwater formation. 



I. The lower marine formation, of which the clay and sand 

 clifFs of Alum Bay form a most interesting natural section. 



A diagram was exhibited shewing the succession of vertical beds in 

 the cliffs of Alum Bay, and these were described in detail in the order in 

 which they present themselves as we proceed northwards from the wall 

 of chalk, which forms the southern boundary of the bay. The paper 

 then proceeds : — 



The lowest of these strata, the plastic clay, is utterly destitute 

 of any fossil remains, and extends across the whole island from 

 Alum Bay to Whitecliff Bay. The same formation appears again 

 at Reading, and at Corfe Castle, and the Druidical Monument 

 called the Agglestone, near Studland, is a huge isolated block of 

 this bed. A stratum of sand containing green particles frequently 

 occurs near the chalk. It is seen in Alum Bay without fossils, 

 though at Reading it is found containing oyster-shells. 



After the plastic clay comes the London clay, which extends 

 in a narrow bed, from Alum Bay to Whitecliff. It is separated 

 from the plastic by a band of flint pebbles. At Whitecliff this is 

 succeeded by a bed of sandstone full of Ditrupa plana The 

 remainder of the section at Whitecliff consists of lignite em- 

 bedded in dark grey clay and sand, and sandy laminae contain- 

 ing large Septaria often including organic remains. 



Besides the clay and sand of which the main body of this 

 stratum consists several other substances are found : of these the 

 chief is sulphuret of iron, which is the principal mineralizing 

 matter of the vegetable and animal remains embedded in the blue 

 clay. The London clay also abounds in sulphate of magnesia, 

 otherwise known as Epsom Salts ; and it is said that an efflores- 

 cence of this salt may be frequently seen on old bricks in London 

 during fine weather. 



Next come the Lower Bagshot beds, consisting of variously 



