LOWER AND UPPER GREENSAND OF THE SOUTH OF ENGLAND. 
447 
Summary. 
I have pointed out in this paper the occurrence in the lower and upper greensand 
strata of the Wealden area, the Isle of Wight, and the South-Western counties, of 
beds of rock, formed to a large extent of the detached spicular remains of siliceous 
sponges, and thus distinctly of organic origin. Their true characters have not been 
generally recognised, and they have usually been described as deposits of sandstone, 
chert, malm, hearthstone, firestone, &c. In the lower greensand these beds are 
mainly developed in the lower or Hythe division, and they are exposed at Haslemere, 
Midhurst, Petworth, Godaiming, Tilburstow Hill, near Godstone, Sevenoaks, Maid¬ 
stone, and at Hythe. The sponge-beds vary from three-quarters of an inch to three 
feet in thickness ; between them, as a rule, there are intervening beds of sand or 
sandstone. The greatest total thickness of the sponge-beds exposed in one section is 
11 feet. Sponge-beds are less qpmmon in the higher or Folkestone division of the 
lower greensand, but they are numerous at Folkestone itself, and reach a total 
thickness of more than eight feet, and there is also a thin bed in this division at 
Sevenoaks. The lower greensand strata at Faringdon, in Berkshire, is of an 
altogether different character to those of the same formation, in the area treated of in 
this paper, and the sponges which abound therein are likewise entirely different, 
being calcisponges and retaining their entire forms. 
The sponge-beds in the upper greensand are of two distinct types, one of which is 
shown on the northern and western margin of the Weald, and the other in the Isle of 
Wight, and further westwards in the counties of Wilts, Somerset, Dorset, and Devon. 
In the first-named district the sponge-beds are of a soft, grayish-white, siliceous or 
siliceo-calcareous rock, known under the names of malm, hearth, or firestone. In 
this the sponge-spicules principally occur in the negative form of minute empty casts, 
the presence of which renders the rock extremely light and porous. The beds can be 
traced nearly continuously along the northern and western margin of the Wealden 
area, and they are well shown at Godstone, Merstham, near Beigate, Betchworth, 
Farnham, and Selborne. Further northwards they are present at Wallingford, in 
Berkshire. The beds vary in thickness from 15 feet at Merstham to 60 feet at 
Farnham. 
In the more typical upper greensand of the Isle of Wight and the south-western 
counties, the sponge-beds consist of thick layers of chert and porous siliceous rock at 
the summit of the series, immediately beneath the so-called chlorite marl; whilst in 
the lower division the sponge remains principally occur in loose quartzitic sands with 
siliceous accretions. The chert or sponge-beds at the top of the upper greensand are 
best exposed at Shanklin, Yentnor, and the Undercliff, in the Isle of Wight, at 
Warminster in Wiltshire, and Penzlewood in Somersetshire. They vary from 10 
to 25 feet in thickness. The sponge-beds of the lower division are scarcely 
recognisable in the Isle of Wight, but they attain a thickness of 10 to 20 feet 
3 m 2 
