442 



Maw — On Subaerial and Marine Denudation, 



valleys and diverting the waterflow, and the alteration of the lie 

 of the ground through local dislocations, upheavals, and depressions, 

 the whole of the causes may have been going on in every variety of 

 degree, working in unison or conflict, ever ,siace the land surface 

 received its first impress of hill and valley contour, and may have 

 produced a variety of exceptions to that regularly graduated contour 

 resulting from the accumulative waterflow over the land surface. 

 May not also some connecting gorges and transverse valleys possibly 

 be the fragmentary remnants of sets of watersheds and valleys that 

 were first initiated on other surfaces than that on which the pre- 

 vailing hill and valley system had its origin ? 



It is impossible to limit the persistency of a set of valleys and 

 watersheds that have never been interrupted by the diverting causes 

 before referred to, or that have not been covered up by superincum- 

 bent formations ; let us, however, consider the case of an old land 

 surface diversified with watershed ranges and river valleys buried 

 beneath an overlying deposit. The deeper depressions of the buried 

 surface would, as shown in Figure 1, to a certain extent reproduce 



Fig. 1. — Exhibiting partial conformity of New Deposit to the sujbacent 

 Ancient Contour. 



themselves on the contour of the new deposit a a, and the consoli- 

 dation and subsidence^ (represented by the space between the line 

 A A and the dotted line a a) of the thicker parts of the new deposit 

 being greater than that in the thinner parts over the higher ground, 

 would still further cause the contour of the ancient surface to be 



^ From observations I have made on the contraction of clays and other materials 

 in drying, it appears probable that strata newly deposited passing from the degree of 

 wetness whilst submerged to a comparative state of dryness on emergence, would 

 contract about as follows : — 



Clayey strata from 8 to 10 per cent, of original bulk ; 

 Sandy „ 3 to 6 „ „ 



Even supposing that the new superimposed deposit was completed with a perfectly 

 level surface, and did not follow in any degree the irregularities of the subjacent 

 contour, the thicker parts over the deep valleys would have a greater amount of 

 contraction than the shallow parts over the higher ground, and thus tend to the 

 reproduction of the general form of the old ground (though with less strongly marked 

 irregularities of outline), and the principal lines of old waterflow and watershed on 

 the new surface. 



