400 PKOCEEDINGS OF THE GEOLOGICAL SOCIETY. [JunO 19, 



in the trough, runs to the sea in the opposite direction to that re- 

 presented by the denudation of the trough in which it takes its rise. 

 In this respect the Ouse and Cherwell resemble streams that, like 

 the latter, are tributaries of the Thames, such as the Evenlode and 

 the Darent ; they also resemble the Tove, a tributary of the Ouse. 

 All these rivers run through valleys whose denudation the Post- 

 glacial structure shows to have descended from the Glacial clay in 

 the reverse direction to that followed by the courses of the streams 

 themselves. The present levels of the different parts of these troughs 

 have greatly changed, even inter se, since the time when the denu- 

 dation formed them. 



We have thus identical features exhibited by the earlier part of the 

 Postglacial denudation along the southerly edge of the Glacial beds 

 in Essex and along their south-westerly edge in Buckinghamshire. 



Thus arises the presumption that the extensively denuded south 

 and south-west portions of the area referred to in the head of this 

 paper have acquired their present condition from a deuudation which 

 commenced with the elevation of the tracts now occupied by the 

 Glacial beds, and which at the outset wholly destroyed them to the 

 southward of their present limits, as well as extensively in a north- 

 erly direction. 



As a general proposition, the denudation increases as we recede 

 from the Glacial beds — whether it be in the southerly direction from 

 the counties of Essex, Middlesex, Hertford, Bedford, Buckingham, 

 and ^Northampton or in the northerly direction from the counties 

 of Leicester, Northampton, Huntingdon, Cambridge and West Suf- 

 folk, and Norfolk, in which, with the exception of one or two small 

 outliers, such as that near Grantham, and the central Lincolnshire 

 tract, they are destroyed for a great distance towards York. 



In the Index Map accompanying the memoir I have shaded in with 

 great care and minuteness aU the principal hill-contours from the 

 sheets of the Ordnance Trigonometrical Survey, and marked upon it, 

 in strong red lines, such of the arcs of denudation (to which I called 

 attention in 1864) as affect the area now under consideration. These 

 arcs may be seen to present a striking feature ; and as they do not 

 depend upon any authority less than that of the National Trigono- 

 metrical Survey from which they are reduced, they cannot be regarded 

 as any fanciful emanation from me. AU that I have done is to call 

 attention to them, to place them in a form readily accessible to the 

 eye, and to show the position occupied by the Glacial beds in re- 

 ference to them. These arcs of denudation, by becoming more 

 deeply scored out to the south and south-west, concur with the 

 structure displayed by the Glacial beds to which I have been ad- 

 verting, in indicating the augmentation of the Postglacial denuda- 

 tion in those directions. The arcs, which in aU their repetitions 

 preserve a remarkable concentricity, belong to two series — one having 

 the centre a little east of Canterbury, and the other about Brook- 

 point in the Isle of Wight. Those of the first series in every out- 

 ward repetition, except close to the centre in Kent, cut through the 

 Glacial beds in some part of their sweep ; and this the large-scale 



