fitton's section at atherfield. 315 



gault. But the fossils, both of this stratum and of the upper green- 

 sand of the Isle of Wight, are for the greater part unknown, or at 

 least unpublished. 



Postscript {July 1847). — Trigonometrical Section of the Coast. 

 — The Table and sectional drawing which accompany this paper were 

 laid before the Geological Section of the British Association at 

 Southampton, in September 1846 ; and on the same occasion Captain 

 Ibbetson produced a geometrical elevation of the coast near Atherfield, 

 exhibiting, on a very large scale, the succession of the strata there. 

 After the meeting at Southampton, Captain Ibbetson was so good as 

 to place his elaborate drawing at my disposal, and I then hoped to 

 avail myself of it to a much greater extent than is practicable in the 

 present volume. I am now obliged to confine myself to giving a geo- 

 metrical outline of the cliffs between the chalk and Atherfield, from 

 a drawing by Captain Ibbetson, on the same scale with my own ap- 

 proximate section, and likewise preserving the true relative propor- 

 tion between the heights and horizontal distances. This outline also 

 shows, by the parallelism of the lines bounding the great divisions 

 which Captain Ibbetson has adopted, the absence of disturbance and 

 interruption during the formation of the lower greensand ; and adds 

 the important fact, that while the strata below the gault are inclined 

 at an angle of 2° to the horizon, those above the gault are nearly 

 horizontal. I have added to my own sectional drawing an indication 

 of this fact, on the authority of Captain Ibbetson. 



In the reduction of my own section I have sacrificed all attempt at 

 effective representation of the coast, by an endeavour to confine the 

 heights to the same scale as the distances. Captain Ibbetson' s mea- 

 surement makes the total thickness of the section from the Wealden 

 to the Gault 833 feet*. My estimate falls within 805 — the differ- 

 ence being about 30 feet ; and I am convinced that my numbers 

 throughout err by defect, and not by excess. In round numbers the 

 thickness may be safely taken at full 800 feet. 



Section at Compton Bay. 



When we have well examined the section near Atherfield, the same 

 strata will be easily recognized in that of Compton Bay, which is six 

 miles distant in a direct line. On the N.W. of Brook Point the 

 upper beds of the Wealden are exposed, with the characteristic fossils ; 

 to which has recently been added the JJnio Valdensis, described by 

 Dr. Mantell. The first member of the Atherfield series which meets 

 the eye is a grey and bluish clay, with the Perna group at the bottom 

 of it, containing particles of oolitic iron ore ; and here I found, in a 

 detached mass about a foot in thickness, one of the largest specimens 

 I have ever seen of Perna Mulleti, with Panopcea plicata and Car- 

 dium sphceroideum. I was informed that the stratum containing 

 these is visible at low water, ranging towards the south of east. The 



* Erroneously printed 843 feet at p. 190 of this Journal, vol. i. 



