'WUtdker — On Suhaerial Denudation. 447 



those were hard siliceous slates, the slopes of the river- valleys are 

 steep precipitous cliffs, from the summits of which we look over the 

 old plateau, somewhat wasted, doubtless, but still approximating to 

 its original form. Where those rocks are New Ked, or Oolite, or 

 Chalk, the slopes of the river-valleys are mostly gentle and far- 

 spread ; a few isolated hills and ridges may have summits that ap- 

 proximate to the level of the old plateau, but these are few and far 

 between, and none of them, perhaps, actually reach it by one or two 

 hundred feet. 



I believe that anyone, in any part of the world, who will apply 

 the key here given to the problem of the production of the present 

 " form of the ground " will find that if he adjust the wards properly 

 it will unlock it for him. 



TTT . — On Subaerial Denudation, and on Cliffs and Escarpments 



OF THE Chalk and Lower Tertiary Beds. 



By "William "Whitakek, B.A. (London), F.G.S., 



Of the Geological Survey of England. 



[PART I.] 



[A paper read before the G-eological Society of London, May 8, 1867.]^ 

 1. — Introduction. 



FOE some years geologists have more or less agreed in the view 

 that the present features of the earth, whether hill, valley, or 

 plain (with some small exceptions, as volcanic outbursts) have been 

 formed directly by denudation ; though indirectly disturbances, 

 whether faults upheavals or sinkings, have of course had their 

 effect in determining the flow, so to speak, of the denuding agent. 



So far all is harmony, the differences of opinion being only on the 

 comparative effect of the two forces, disturbance and denudation : 

 but beyond this all is discord, and of late there has been much 

 debate on the question by what means the surface of the earth has 

 been worn away, and its rocks carved into their present form. 



Many papers have been written on the origin of valleys escarp- 

 ments lake-basins etc., some of which are clear statements of care- 

 fully observed facts, with unprejudiced and logical reasonings there- 

 from ; whilst others, on the contrary, are little else than assertions 

 of belief, and some are made up largely of groundless suppositions 

 and false analogies. It seems hardly to be known that to fit one to 

 take part in such an enquiry a long and careful examination of nature 

 is needed, and that, to quote the words of a geologist of the last cen- 

 tury, " it is not to common observation that it belongs to see the effects 

 of time and the operation of physical causes in what is to be per- 

 ceived uj)on the surface of the earth."^ 



It may not be amiss therefore to analyse the evidence given by 

 some special classes of rocks ; and to avoid being charged with advo- 

 cating opinions on slight acquaintance with the formations chosen for 



1 The title given by the editor to the short abstract in the Society's Journal (Vol, 

 xxiii. p. 265) is not quite correct. 



2 Hutton's Theory of the Earth, vol. ii. p. 238. 



