Phcvial Period due to Sun's Influence. 463 



to the other two sets, or marginal series of bends, that determines the denudations 

 of the Wealden. I showed in 1862 (Quarterly Journal Geological Society) that the 

 medial beds of Hastings sand thinned out N.E. and S.W. in a horizontal line 

 passing from several hundred feet in thickness at Castle-hill, Hastings to 3 or 4 ft. 

 7 miles to the N.E. and S.W., and having a series of flexures in that direction. 



It is only where a river carrying a large body of water attacked the point 

 weakened and prepared by springs and underground river channels, that such 

 an opening as the Gorge of Brading could be made. The ground at Sandown 

 was formerly the highest on the island ; now it is the lowest, owing to the 

 instability of the upper beds of Lower Greensand reposing on the weald of clay 

 at the level of the sea. This is the cause of the gorges of the Weald at Guildford. 

 The Weald clay at Pease Marsh comes up there about 100 ft. above the level 

 of the sea ; and this has been the determining cause of the perforation at Guild- 

 ford by the Wey, theWeald clay being 600 feet under Guildford. At Bury Hill the 

 Weald clay is 250 feet high, and thus directed the Mole against Box Hill, where 

 theWeald clay is 700 ft. under Box Hill. Then at Hildenboro, at the mouth of the 

 Sevenoaks tunnel, the Weald clay was a solid mass 400 feet high, and protected 

 Sevenoaks from the attack of the Medway. The gradient or slope of the Medway 

 was too small for the river to bore at Hildenborough, and the river, therefore, 

 changed its course at nearly a right angle towards Maidstone, and there resumed 

 its original direction. Otherwise the Medway would have joined the Darenth 

 if it could have succeeded in perforating a gorge at Sevenoaks. At Merstham 

 there was an attempt by tributaries of the Mole to get through the chalk and join 

 the Wandle, which was not successful. Thus one river had to pass to Maidstone, 

 20 miles distant, where the Weald clay was 400 feet less high in the direction of 

 the escarpment of the Surrey Hills. Other rivers fell from 250 to 1,000 feet per 

 mile, between Pease Marsh and Guildford, between Bury Hill and Box Hill, 

 between Maidstone and Burham, and between Sandown and Brading, when 

 these four gorges were formed. The high dome or elevated land at Crow- 

 borough determined one set of rivers, and another dome or elevated mass, 

 called Hind Head Hill, near Haslemere, was the source of another set of three 

 rivers going also in different directions, which have opened out gorges through 

 the escarpment of the chalk similar to that at Brading. 



It was the fact of these two points being elevated higher than any other that 

 started the Wealden rivers from them, just as the highly elevated ground in the 

 Alps started the Rhine and other great rivers ; and other and less elevated 

 ground, like that near Donauessingen, started the Danube, and sent off smaller 

 rivers to oppose the Rhine and keep it off Donauessingen. No doubt the Rhine 

 now follows a course worked out for it by an underground river passing through 

 the Devonian rocks between Bingen and Bonn. I find from Mr. F. Tuckett 

 that the real springs of the Rhine are twice as high as those of the Danube. 

 If a straight line was drawn to the Delta from the source of the Rhine, it 

 would nearly touch the level of the springs of the Danube. 



The sketch of the Gorge at Brading may realise what has happened there, and 

 serve as a type of the valley excavations of greater rivers (Figs. 25, 27, 28). 



PLUVIAL PERIOD DUE TO SUN'S INFLUENCE. 



I have shown you the effect that great weight of ice sliding or water flowing 

 has upon the velocity of ice and water currents, and that the enormous erosion 

 of the surface of the earth visible in valleys and lake hollows could only have 

 taken place when there were twenty times as much ice and snow as at present. 

 These different atmospheric conditions point to some as yet unknown causes. 

 Such snow and ice could only be formed by great heat in summer and great cold 

 in winter. Some disturbing effect in the atmosphere of the sun could, I think, 

 alone account for the phenomena we observe, and that must have been of a 



