Page:Quarterly Journal of the Geological Society of London, vol. 27.djvu/348

 divisions over a considerable area a very doubtful and difficult task. In the present paper I propose to include in the Punfield formation those beds at the top of the series which present the same mineralogical characters as the typical beds of Punfield Cove, and which, like them} show signs of fluvio -marine origin in their intercalated bands of marine shells. The great mass of variegated strata containing only freshwater and terrestrial fossils I regard as the Wealden proper.

1. Brixton Bay. The Punfield strata are seen here between Atherfield Point and Barnes High. They are well illustrated in the detailed section of Professor E. Forbes and Mr. H. W. Bristow *. Their junction with the overlying " Perna-beds of the Upper Neocomian " is fully described by Dr. Fitton, who had a favourable opportunity for observing it in 1847† ; this junction is now concealed. The Punfield strata of the Isle of Wight, which attain a thickness of 230 feet, fall naturally into two groups, which, for the sake of convenience in description, I have distinguished by local names (see vertical section, fig. 3).

A. The Cowleaze Series, consisting of dark grey, blue, or green cypridiferous shales and clays, with indurated sandy ferruginous and argillaceous bands, and thin beds of limestone crowded with shells, Paludina, Cyrena, Cerithium, Melanopsis. Unio, &c. " Beef," or fibrous carbonate of lime, occurs abundantly in these limestones, which not unfrequently contain marine shells, and are sometimes quite made up of small oysters, when they exactly resemble the well-known " Cinder -bed " of the Purbeck series. It is evident that the Cowleaze series was deposited under conditions very similar to those which prevailed during the formation of the Purbecks. While, however, in the latter the clays are subordinate to the limestones, in the former the limestones are much less developed than the clays. The thickness of the Cowleaze series is about 180 feet ; and the best exposure of it is in Cowleaze Chine, where nearly 100 feet of its lower portion is exhibited in a clear section. The Cowleaze beds contain in places plant-remains ; and in some of the bands teeth and scales of fish are very abundant. Among the marine shells occur Exogyra sinuata and E. Boussingaultii, both much dwarfed, also Modiola (two species), Cardium, and Cerithium. I am not aware that any Ammonites have been found in these beds in Brixton Bay, though an undoubted specimen was obtained from them at Bed Cliff in Sandown Bay. The limestone bands of the Cowleaze series are used locally, like those of the Purbecks, for rough paving.

B. The Barnes Series. These beds rise from the shore at Cowleaze Chine, and crop out at the surface at Barnes High. Owing to the superior hardness of their upper beds they make a very marked feature and can easily be traced inland through the island. While

plate 2. fig. 2.
 * Memoirs of the Geological Survey, Geology of the Isle of Wight (1862),

† Proc. Geol. Soc. vol. iv. p. 198 ; Quart. Journ. Geol. Soc. vol. iii. p. 293.