Ch. 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 



