314 SCIENCE PROGRESS 



suoaerial agencies. That some pre-Eocene denudation did occur is certain ; and 

 if, as is probable, it exposed the Lower Chalk, and perhaps even Lower Cretaceous 

 strata on the crown of the arch, 1 then there existed, from the moment of final 

 emergence from the sea in post-Eocene times, some of that differential resistance 

 to which the longitudinal hills and valleys owe their origin. And when once 

 this process was started it would be only a question of time before a structure 

 would be evolved practically indistinguishable from that arising after marine 

 planation. 



It has been thought necessary to give this brief outline of the older hypotheses 

 because Major Marriott, who rejects them both, seems hardly to have understood 

 their import. He lays the utmost stress on the resistance of the Chalk to erosion, 

 which he describes as the "cue which has been missed" (p. 592); and he also 

 insists strongly on the "differential erosion," which he tells us has not been 

 recognised (p. 603), arising from the lesser resistance from the sands and clays. 

 But these aspersions on geologists are quite unmerited. This differential erosion 

 is in truth, and has been for years, one of the most important factors in all 

 attempts to explain the sculpture of the earth's surface, and the special case of 

 the Chalk Downs was fully recognised by all the pioneers who first taught us 

 the meaning of escarpments. 2 And, after all, is the removal of the Chalk by 

 denudation 3 so extraordinarily difficult as Major Marriott would have us believe? 

 Even on his hypothesis a wedge of Chalk several miles in width has been removed 

 to form the escarpments (fig. 2); and his inference that, because the Upper 

 Chalk appears on the surface here and elsewhere, it has suffered hardly any 

 change since its deposition (p. 600) is unwarranted. This formation is of great 

 thickness (300-500 ft.) and can be separated into zones, each with its characteristic 

 fossils ; and, as it is seldom that anything higher than the lowest zone is found 

 on the crest of the escarpment, it is clear that a great deal is missing ; all the 

 evidence, however, goes to show that the absence of the higher zones is not 

 due to irregular deposition, but to removal by denudation— how far marine and 

 how far subaerial we cannot say. But the most obvious proof that great masses 

 of Chalk can be eroded away is to be found in the cliffs of England and France, 

 once clearly joined together, but now widely separated. , What is the floor of the 

 Straits of Dover but a "plain" of marine denudation? 4 And yet this plain is of 

 very recent origin. 



The hypothesis which Major Marriott attempts to substitute for the older ones 

 is that "the chalk never covered the Weald, but that the sandstones, sands, 

 clays, etc., of the latter once formed an island or bank laved by the sea in the 

 depths of which the chalk was deposited" (p. 595. The italics are mine), and he 

 gives a diagram (p. 596) showing the position of this fringe of Chalk after 



1 More information is required about this pre-Eocene denudation ; but the 

 argument would not be materially affected if there had been none of it at all. 



2 See quotations in Topley's Geology of the Weald, pp. 273-4. 



3 Major Marriott says (p. 523), " denudation, being a technical term in geology, 

 does not express the process illustrated" in his paper. But Prof. Marr {Scientific 

 Study of Scenery, 2nd edition, p. 24) defines it as " the stripping of portions of rock 

 from one place and their removal to another"; while Prof. W. M. Davis {Physical 

 Geography, p. 105) says, " the general process of wasting and washing, by which 

 surface structures of the earth's crust are attacked, is called denudation or erosion? 

 I therefore fail to grasp Major Marriott's meaning. 



4 Major Marriott (p. 594) attributes the formation of the Wealden portion of the 

 Channel to depression in a syncline, but I am not aware of any evidence pointing 

 in this direction ; and in any case there must have been much erosion. 



