540 ST11ATIGRAPHICAL GEOLOGY 



are scattered many rolled flint pebbles ; these pebbles are always 

 black, and sometimes have the remarkable peculiarity of falling 

 into fragments when tapped by a hammer, for though to the eye 

 they seem compact, they are really traversed by a multitude of 

 invisible cracks. This basement-bed is generally from 6 to 12 feet 

 thick, and often contains fossils, of which the chief are Aporrhais 

 Sowerbyi, Calyptrea aperta ( = trochiformis), Pitaria obliqua, 

 Panopcea intermedia,, Pectunculus brevirostris, and Ditrupa plana. 



The thickness of the London Clay near London is between 400 

 and 500 feet, but it thins gradually westward to about 270 at 

 Wokingham, 50 near Newbury and Hungerford, and only 15 in 

 the outlier of Great Bedwin in Wiltshire. 



Fossils are not common throughout the London Clay, but 

 sometimes occur in great profusion, especially where beds of sandy 

 clay occur. The cliffs of the Isle of Sheppey are noted for the 

 fossils they yield ; plant remains are there especially abundant, 

 and bones of birds, turtles, and snakes have also been found. 



In the Hampshire basin, with the exception of an outlier at 

 Newhaven, the most easterly exposure of this division in this area 

 is at Bognor in Sussex, where beds of clay and calcareous sandstone, 

 highly fossiliferous, are exposed on the shore, and are supposed to 

 belong to the lower part of the London Clay. 



Borings at Portsmouth Dockyard below the summit of the 

 formation proved a thickness of 290 feet, and exposures made in 

 extending that dockyard from 1868 to 1870 were described by 

 Mr. Meyer, 2 who found the clay divisible into three parts, each 

 commencing with a layer of pebbles and passing upward from 

 stiff clays into sandy clays. The lowest zone is about 100 feet 

 thick, the middle zone has a thickness of 1 34 feet, and the sandy 

 beds in the upper part of this yielded many fossils. The upper- 

 most zone consists mainly of clays containing Cyprina planata, 

 Pholadomya margaritacea, liostellaria lucida, and other species. 



In the Isle of Wight the London Clay is about 320 feet thick 

 at Whitecliff Bay, but only 230 at Alum Bay. 3 At neither place 

 are many fossils to be found, but at Whitecliff the higher 

 part contains Panopcea intermedia, Cyprina planata, Callista 

 tenuistriata, and Pinna affinis. At 262 feet from the base there 

 is a layer of black flint pebbles, and above this are sandy and 

 laminated clays. In Dorset the London Clay consists chiefly of 

 brown sandy loam with seams of sandy ironstone and a bed of flint 

 pebbles at the base. Its thickness in the Purbeck district is from 

 70 to 80 feet, but it thins westward, and is finally overlapped by 

 the Bagshot Beds near Dorchester. 



Bagshot Beds. In the London Basin these beds occupy a 



