494 
SILURIA. 
[Chap. XX. 
of the Chalk which once covered the Wealden, and these chiefly upon the 
Greensands and Gault at small distances from the hills, and tumultuouslj 
aggregated — the residue, doubtless, of some of the enormous mounds of 
debris which must have resulted from the breaking-up and destruction of 
the Chalk which once covered over the whole area between the opposite 
escarpments of the Forth and South Downs *. 
Further, if we look to the interior of this vast valley, we find the dome- 
shaped centre, consisting of the Weald Clay and Hastings Sand, just as 
entirely swept of any superficial fragment as in the smaller valley of 
Woolhope, except in those limited tracts of narrow dimensions where 
rivers of date long posterior to the great denudations have acted, and 
there left traces of their old beds, as in the case of the Medway, so well 
described by Messrs. Foster and Topley of the Geological Survey f . 
By what ordinary currents of the sea, I ask, could such a sweeping- 
out be made over an area of nearly 200 square miles, throughout which, 
except where an ancient river-bed like that of the Medway, but flowing 
long after the great denudations, have left some shingle and gravel, 
every portion of the soil is but the decomposition of the denuded rock 
beneath ? 
I therefore believe that, during the several upheavals (for I am no 
believer in one such movement) which took place, by which the central 
dome was raised, the present valley was subjected to powerful and per- 
haps long- continued currents of water, purging the surface of all those vast 
mounds of debris which must have encumbered it after each upheaving 
dislocation of the central "Wealden axis £, 
Agreeing, as I do, with my associate Lyell in a belief in the constancy 
of laws of Nature, I am rejoiced to be his follower in reasoning from the 
present to the past : at the same time I maintain, from the evidences pre- 
sented to me in the crust of the earth, that during former periods there 
were, at intervals, causes in action of much greater intensity than those of 
which the human race has ever had an example. 
In speaking of the ' relative amount of work done by mechanical force 
in given quantities of time past and present,' Lyell adds§, "It is not 
the magnitude of the effects, however gigantic their proportions, which 
can inform us in the slightest degree whether the operation was gradual, 
insensible, or paroxysmal. It must be shown (he says) that a slow pro- 
cess could never in any series of ages give rise to the same results." 
Now, to take one of the examples before cited, I affirm that by no pos- 
* Doubtless the present softly rounded outlines England &c." For proofs of the great force which 
of these beautiful Chalk hills are due to diurnal operated in the upheaval of this grandest of our 
erosion by atmospheric agency during long ages, British valleys of elevation, accompanied as the 
and some amount of unworn and frost-broken flints movements necessarily were by rents and openings 
is due to this cause. transverse to the main line of upheaval, see the 
t Quart. Journ. G-eol. Soc. vol. xxi. pp. 443 &c. admirable memoir of the late eminent mathetna- 
T I refer the reader, for further details on this tician, W. Hopkins, Trans. Geol. Soc. ser. 2. 
point, to a memoir I wrote many years ago (see vol. vii. p. 1. 
Quart. Journ. Geol. Soc. vol. vii. pp. 349 &c), § Principles of Geology, 10th edit. vol. i. p. 136. 
" On the Distribution of the Flint Drift of S. E. 
