Natural History. 59 



He then considers the evidence indicating that at least two 

 cycles of sub-aerial denudation have been involved for the 

 development of the geographical features of the east of 

 England, the first cycle having reached old age, the second 

 being, at present, in its maturity. " Look," he says, " at the 

 remarkable evenness of the Oolite and Chalk uplands in 

 Yorkshire, Lincolnshire, and other counties." " Remember," 

 he adds, " that the most reasonable view concerning the 

 original extension of the strata of these uplands would carry 

 them high up in the air over the Midland Triassic lowland, 

 and then ask how denudation could reduce the original 

 constructional extension of these strata to the even uplands 

 they now present." " These uplands," he says, " seem to be 

 remnants of a 'peneplain' of sub-aerial denudation, for the 

 reason that their drainage is accomplished, in great part, by 

 subsequent streams (as should be the case if the present streams 

 are the revived successors of those of a former cycle of 

 atmospheric denudation), and not by streams imperfectly 

 adjusted to the structures (as should be the case if the region 

 had been denuded afresh by marine action, and then elevated 

 to its present height)." "Marine denudation," he reiterates, 

 (and this is a well-known physical axiom) " distinctly requires 

 the suppression of all previous drainage systems, and the 

 inception of a new system of streams entirely independent of 

 those beneath ; while the hypothesis of sub-aerial denudation, 

 as distinctly requires the retention of a previously adjusted 

 drainage system as a foundation to start with." Marine 

 denudation demanding a drainage without subsequent streams ; 

 sub-aerial denudation equally demanding a considerable 

 number of subsequent streams at the time of upheaval. 



Prof. Davis then takes individual rivers in Yorkshire, 

 Lincolnshire, and elsewhere, and points out some of the most 

 important captures that have been made by the subsequent 

 streams, as in the case of the Humber and Trent, and he 

 suggests that portions of the original consequent or transverse 

 rivers may be looked for in various localities \ and, for my part, 

 I do not know any more interesting occupation for a geologist 

 than to search for such portions, and to try to make out how, 

 and by what means, their courses have been changed.* 



After the Trent was captured, it would no doubt, for a long 

 time, have a tendency to resort to its original course in times 



* See note at end. 



