318 
ELEMENTS OE GEOLOGY. 
Splienopte^Hs gracilis, Fitton. From 
the Hastiiigs Sands near Tun¬ 
bridge Wells. 
a. A portion of the same magnified. 
Fig. 296. are innumerable traces of a fossil 
vegetable, apparently Sphenop- 
' teris^ the stems and branches of 
which are disposed as if the 
M plants were standing erect on 
the spot where they originally 
grew, the sand having been gen¬ 
tly deposited upon and around 
them; and similar appearances 
have been remarked in other 
places in this formation.* In 
the same division also of the 
Wealden, at Cuckfield, is a bed 
of gravel or conglomerate, consisting of water-worn pebbles 
of quartz and jasper, with rolled bones of reptiles. These 
must have been drifted by a current, probably in water of 
no great depth. 
Eh’om such facts we may infer that, notwithstanding the 
great thickness of this division of the Wealden, the whole of 
it was a deposit in water of a moderate depth, and often ex¬ 
tremely shallow. This idea may seem startling at first, yet 
such would be the natural consequence of a gradual and con¬ 
tinuous sinking of the ground in an estuary or bay, into 
which a great river discharged its turbid waters. By each 
foot of subsidence, the fundamental rock would be depressed 
one foot farther from the surface ; but the bay would not 
be deepened, if newly-deposited mud and sand should raise 
the bottom one foot. On the contrary, such new strata 
of sand and mud might be frequently laid dry at low wa¬ 
ter, or overgrown for a season by a vegetation proper to 
marshes. 
Punfield Beds, Brackish and Marine. — The shells of the 
Wealden beds belong to the genera Melanopsis^ Melania^ 
Paludina^ Cyrena^ Cydas^ TInio (see Fig. 294), and others, 
which inhabit rivers or lakes ; but one band has been found 
at Punfield, in Dorsetshire, indicating a brackish state of the 
water, where the genera Corbula^ My tikis ^ and Ostrea occur ; 
and in some places this bed becomes purely marine, contain¬ 
ing some well-known Neocomian fossils, among which Am¬ 
monites Peshayesii (Fig. 284, p. 311) may be mentioned. 
Others are peculiar as British, but very characteristic of the 
Upper and Middle Neocomian of Spain, and among these the 
Vicarya Lujani (Fig. 297), a shell allied to ISTerinea, is con¬ 
spicuous. 
By reference to the table p. 308 it will be seen that the 
* Mantell, Geol. of 8.E. of England, p. 244. 
