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, fissuring 

 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 dotted line 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 



