132 
exception is the occurrence of a thin band of feruginous sandstone. 
This can still be seen near the top of the Zig-Zag, whence I have 
traced it eastward for some distance. (Plate XV., Fig. i). 
The Boseombe Sands, although of Bracklesham age, are 
entirely unfossiliferous, except for the usual chalk fossils— 
sponges, echini, and a few mollusca—found in the flints. There 
can, however, be no doubt that the beds represent an old sea 
beach of sand and shingle that indicates marine invasion of the 
estuary of the ancient river that laid down the Bournemouth 
Beds, and covered them with its litoral deposits. How far west 
the sea invaded the river mouth we do not know, as these beds 
have, doubtless, been denuded off a large area in that direction. 
Its gradual approach from the east, where the beds are thicker, 
indicates a subsidence of the previous river bed, and a comparison 
of fossils from other Bracklesham strata shows that this coincided 
with a change to a warmer climate from the sub-tropical con¬ 
ditions that prevailed before its advent. The absence of fossils 
in our cliffs is largely due to the fact that, owing to their porous 
nature and nearness to the surface, the beds have been entirely 
decalcified, and all the usual organic remains that they contained 
have been dissolved out by percolating waters. 
THE BUILDING OF THE BOURNEMOUTH CLIFFS. 
Reference has been made in the preceding pages to the 
ancient river by which the Bournemouth Beds of Bagshot age 
were laid down. Following the excellent method adopted by Mr. 
Jukes-Browne in his “ Building of the British Isles,” it may be 
of interest to briefly trace the geological sequence of events and 
the work of this river in the district. 
At the close of the Cretaceous period England slowly emerged 
from the deep sea in which the chalk had been deposited. As 
the sea receded, rain, rivers and sea-erosion commenced the work 
of destroying the newly-formed land. The chalk being washed 
away, the flints enclosed in it were rapidly rolled into black 
pebbles on the sea-beaches which fringed the shores. This was 
done precisely as it occurs to-day and can be seen, for example, 
at Arish Mell and in other chalk-enclosed coves. These flint 
pebbles were rapidly spread over the bed of the shallow sea 
which then covered South Hampshire and Dorset, and they 
formed the first deposit in the recently elevated chalk floor. A 
few rivers bringing down sand and mud interspersed it with the 
pebbles. So were formed the Woolwich and Reading Beds, 
which lie upon an eroded surface of chalk, as can be seen at 
Alum and Studland Bays. I his period was closed by a subsidence 
throughout the area, which greatly deepened the shallow seas 
of early Eocene times. 1 he land was now far off, and the deposits 
then formed were of clay brought by distant rivers, which 
probably opened into the sea far to the east and south-east, there 
being no shore line in this district. This was the period of the 
London Clay which lies upon the Woolwich and Reading Beds. 
