Ch. XXI.] 
ALLUVIUM OF WEALD VALLEY. 
295 
the opposite coasts of France and England, composed of chalk, 
present ranges of white cliffs facing each other. A powerful 
current might then rush, like that which now ebbs and flows 
through the straits of Dover, and might scoop out a channel in 
thegault. We must bear in mind that the intermittent action of 
earthquakes would accompany this denuding process, Assuring 
rocks, throwing down cliffs, and bringing up, from time to time, 
new stratified masses, and thus greatly accelerating the rate of 
waste. If the lower bed of chalk on one side of the channel 
should be harder than on the other, it would cause an under 
terrace, as represented in the above diagram, resembling that 
presented by the upper green- sand in parts of Sussex and 
Hampshire. When at length the gault was entirely swept 
away from the central parts of the channel, the lower green-sand 
(3, diagram No. 70,) would be laid bare, and portions of it would 
No. 70. 
The doited tine represents the sea-level. 
become land during the continuance of the upheaving earth- 
quakes. Meanwhile the chalk cliffs would recede farther from 
one another, whereby four parallel strips of land, or perhaps 
rows of islands, would be caused. 
The edges of the argillaceous strata, No. 2, are still exposed 
to erosion by the waves, and a portion of the clay, No. 4, is 
already removed. This clay, as it gradually rises, will be swept 
off from part of the subjacent group, No. 5, which will then 
be laid bare, and may afterwards become land by subsequent 
elevation. 
Why no ruins of chalk on central district. — By this theory 
of the successive emergence and denudation of the groups, 1, 2, 
3, 4, 5, we may account for an alluvial phenomenon which 
seems inexplicable on any other hypothesis. The summits of 
the chalk downs are covered everywhere with flint gravel, which 
