12 
gravel are found on the Southdowns similar in character to those 
examined by Professor Prestwich, viz., consisting of a tenacious 
brown clay with seams of sand and coarse gravels, and at about 
the same levels of height above the Ordnance datum line. I 
should earnestly advise any members of this Society who have the 
time and strength for pedestrian excursions to explore these 
gravels. It is very likely that they would be rewarded by finding 
palzolithic implements, and, in any event, good service would be 
rendered to science by collecting a mass of the gravel and ascer- 
taining from what sources it was derived. If it turned out to be 
partly derived from rocks of the forest ridges of the Wealden, it 
would give an important confirmation to Prestwich’s theory of an 
anticlinal mountain range 2,800 feet in height, which makes more 
for enormous denudation and extreme antiquity of the human 
race than anything that has yet been recorded in Britain. 
THE ARGUMENT FROM DENUDATION. 
The average rate of denudation of continents, as shown by the 
amount of solid matter carried down by great rivers, is about one 
foot in 3,000 years. It is of course much beyond this average on 
the slopes of hills and when the strata are of soft materials ; but 
allowing it to have been even thirty times greater than the 
average in the case of the Wealden ridge, it would have taken 
200,000 years to denude 2,000 feet. The difficulty is not somuch 
to see how the denudation proved by Prestwich to have taken 
place can have required the interval of 200,000 years assigned by 
Croll and Wallace for the antiquity of the first great glacial 
period,as how it could have been compressed within that period,and 
whether it may not date back tothe preceding period of a still 
higher maximum eccentricity, which occurred about 700,000 
years ago. In Western North America we have proofs of a still 
greater denudation. The auriferous gravels of California, 
Oregon, and British Columbia, consists of an enormous mass of 
débris washed down by early glacial or pre-glacial rivers from the 
western slopes of the great coast ranges. During their depositions 
they became interstratified with lavas and tuffs from eruptions of 
voleanoes long since extinct,and finally covered by an immense flow 
of basalts, which formed a gently-inclined plane from the Sierra 
Nevada to the Pacific. This plane was attacked by the denuda- 
tions of the existing river courses and cut down into a series of 
flat-topped hills divided by steep canons, and the valleys of the 
present great rivers. In one case, that of the Columbia river, 
this recent denudation has been carried down to a depth of 2,000 
feet, and the river flows between precipitous cliffs of this height. 
Nearly the whole of the present gold mining is carried on by shafts 
and tunnels driven through superficial gravels and sheets of 
basalts and tufts, to the gravels of those pre-glacial rivers, which 
are brought down in great masses by hydraulic jets. In a great 
