Page:Quarterly Journal of the Geological Society of London, vol. 29.djvu/115

1872.] MEYER PUNFIELD SECTION. 71 Purbeck was superior in position to the "Lobster-clay" of Atherfield— or (to state the matter more clearly) that the Punfield beds, from the "marine band" upwards, were Lower Greensand instead of Wealden, representing, however, only so much of the Lower Greensand of the Isle of Wight as would be included between the "Lobster-clay" and the Gault.

The assertion was a bold one, and should, I am well aware, have been supported at the time by fuller evidence. It was not, however, put forward thoughtlessly; and I have now to offer such further evidence as may tend towards a settlement of the points in question.

To test the truth or error of my impressions with respect to the relation of the Isle-of-Wight Wealden and Neocomian strata to those of Punfield, I revisited the coast-sections at Redcliff and Atherfield, and spent several days in studying the corresponding strata in Compton Bay. I sought carefully in either formation for such change in its passage westward as might indicate the relation of either to the marine, or semi-marine, Punfield beds of the Isle of Purbeck.

The Wealden beds showed no such change. But for slight variations in thickness and disposition, these Upper Wealden shales of the Isle of Wight are almost perfectly alike at Compton Bay, at Sandown, and at Atherfield. The section of the Wealden strata of Compton Bay has been already given with great accuracy in the Memoirs of the Geological Survey*; and on this point it is therefore needless to enter into detail.

Unlike the Wealden, the condition of the Greensand strata of Compton Bay is much altered in appearance from what may be seen at Atherfield. Their upper and middle portions are extensively and distinctly laminated†, and put on already very much the appearance of the more laminated portions of the Punfield strata. They contain more lignite and apparently fewer fossils in comparison than the corresponding strata at Atherfield. The "Crackers rock" is scarcely recognizable at Compton, its position being merely indicated by semi- indurated nodules of shell-rock. The "Lobster-clay," however, is, fairly represented; and although the Perna-bed and underlying grit, or passage-bed, are nowhere visible at their outcrop, on account of the fallen masses of the low undercliff, their presence at this point has been long since satisfactorily determined. Masses of the grit-bed, with its characteristic Neocomian fossils, may be picked up abundantly along the shore.

So far, then, the evidence obtainable from the Wealden and Neocomian strata in their most westerly exposure in the Isle of Wight tended mainly to strengthen my previous belief as to the real position of the Punfield beds of Punfield. The next step to be taken was to reexamine the Punfield Section, and to obtain fuller evidence, if possible, as to the condition of the strata beneath the so-called "marine band."

With this intent I returned to Punfield towards the end of October,


 * Mem. Geol. Surv. (Geol. I. of Wight), chap. iii. p. 9, fig. 7.

† Ibid, p. 9.