Gh. XXII.] 
ORIGIN OF TERTIARY STRATA. 
309 
return to the valley of the Weald, the strata of the North 
Downs are inclined to the north, at an angle of from 10° to 15°, 
and in the narrow ridge of the Hog's back, west of Guildford, 
in Surrey, about 45°; those in the South Downs dip to the 
.south at a slight angle. It is superfluous to dwell on the 
analogy which in this respect the two escarpments bear to 
those which flank the valleys above alluded to ; and in regard 
to the greater distance which separates the hills of Surrey from 
those of Sussex, the difficulty may be reduced, simply to a 
question of time. If the rise of the land and its degradation 
by aqueous causes was accomplished by an indefinite number of 
minor convulsions, during an immense lapse of ages, we behold 
in the ocean a power fully adequate to perform the work of 
demolition. If, on the other hand, we embrace the hypothesis 
of paroxysmal elevation, or, in other words, suppose a sub- 
marine tract to have been converted instantaneously into 
high land, we may seek in vain for any known cause capable of 
sweeping away even those portions of chalk and other rocks 
which, all are agreed, must once have formed the prolongation 
of the existing escarpments. It is common in such cases to 
call in one arbitrary hypothesis to support another, and as the 
upheaving force operated with sudden violence, so a vast dilu- 
vial wave is introduced to carry away, with almost equal 
celerity, the mountain mass of strata assumed to have been 
stripped off. 
Materials of the tertiary strata whence derived. — If, then, we 
conclude that the wreck of the denuded district was removed 
gradually, it follows that it was deposited by degrees elsewhere. 
If any part of the sea immediately adjacent to the district 
which was then emerging, was of considerable depth, the drift 
matter would be consigned to that submarine region, since every 
current charged with sediment must purge itself in the first 
deep cavity which it traverses, as does a turbid river in a lake. 
Suppose that while the wave sand currents were excavating the 
longitudinal valleys, D and C (No. 81, p. 310), the deposits 
a were thrown down to the bottom of the contiguous deep 
