212 Geology of the Straits of Dover. (April, 
Surrey, Hants, and Sussex, containing there some hard beds 
of “‘ firestone’”’ and ‘‘ Malm rock,” which form a small es- 
carpment. Compared with the overlying Grey Chalk the 
Upper Greensand is a permeable bed, or one through which 
water passes with tolerable freedom. 
At the base of the chalk slope is a broad tract of clay, 
known as the Gault. This may be distinguished all round 
the country, at the foot of the Chalk escarpment, by most of 
it being in pasture. In Kent, Surrey, and Sussex, it ave- 
rages about 100 feet in thickness. This bed is very constant 
in character; it is a stiff bluish and blackish clay, wholly 
impervious to water, and makes a strong dividing line 
between the partially porous beds above and the alternations 
of highly porous and partially impervious beds below. 
The series of beds underlying the Gault is known as the 
Lower Greensand: the characters which they present are 
important, because it has been proposed that a submarine 
tunnel shall start from the English coast near Folkestone, 
in which case the tunnel would first be driven through these 
beds. 
The Lower Greensand in Kent presents four well-marked 
divisions. Underlying the Gault is a bed of sand, known 
as the Folkestone Beds. It makes a very light soil, and is 
thus strikingly distinguished from the neighbouring Gault. 
It is highly porous, water passing through it with great 
rapidity. ‘The division next below (Sandgate Beds) consists 
of clay and sandy clay; compared with the Folkestone Beds 
above, it is impervious to water, but it is much less so than 
the Gault. Its outcrop is frequently marked by a line of 
springs, and the water of wells sunk through the Folkestone 
Beds is held up by this stratum. 
The third division of the Lower Greensand is known as the 
Hythe Beds: it consists of alternate beds of limestone and 
calcareous sand, the former being locally known as ‘‘ Kentish 
Rag,” and the latter as ‘‘ Hassock.” ‘This division, like the 
Folkestone Beds, is highly porous. Below this is a band of 
clay, known as the Atherfield Clay, which geologists class 
with the Lower Greensand, because the fossils which it con- 
tains are, like those of that formation, of marine origin: 
for our present purpose it might be classed with the great 
mass of Weald Clay which underlies it, the fossils of which 
are of fresh-water origin. 
The Wealden Beds extend along the inland border of 
Romney Marsh, and form the high cliffs of eastern Sussex. 
They consist of alternations of clay and sand, the former 
largely predominating in the upper part, and the latter in 
