﻿656 MR. H. BURT ON THE DENUDATION OF [Nov. I9IO, 



Passing to the west of the Mole, we find that the gradient from 

 the summit of Hackhurst Down (733 feet O.D.) to the 600-foot 

 contour farther north is about 106 feet per mile; but I think that 

 there is a reasonable doubt as to whether the dip-slope here repre- 

 sents a pre-Eocene or a post-Eocene surface. In the face of such a 

 question we naturally turn to the only Eocene outlier in this region, 

 and we find near Hook Wood a patch of Woolwich and Keading 

 Beds, surmounted by Southern Drift, reaching 678 feet O.D. within 

 a mile of the escarpment. The theoretical summit here (see fig. 1, 

 p. 642) is about 730 feet, which gives a gradient of 50 feet per mile. 

 Not only is a measurement involving an Eocene surface more satis- 

 factory than one derived from the Chalk alone (since the latter 

 may be pre-Eocene), but we obtain from the Lower Greensaud 

 corroborative evidence that the smaller gradient more nearly repre- 

 sents the angle of the plain than the large one. Leith Hill reaches 

 965 feet O.D., and the distance between it and Hackhurst Down 

 (733 feet) is 4 miles ; consequently we have a gradient of about 

 57 feet per mile, according very closely with the one arrived at above. 



Along the line of the Hog's Back no data are available, and 

 indeed none can be expected, since that hill is so much below the 

 level of the plain that it must have been entirely covered at the 

 time of uplift by a thick layer of Tertiary strata. To the west of 

 the Blackwater we find the Farnham Plateau (612 feet O.D), 

 apparently at the level of the plain, and, measuring thence to 

 Easthampstead Plain (423 feet O.D), we obtain a gradient of 21 feet 

 per mile ; but the line of measurement is not strictly at right angles 

 to the strike. From Hindhead to the Earnham Plateau the average 

 gradient is 30 feet per mile, but this also is taken along a somewhat 

 oblique line. Lastly, measuring directly from Hindhead to East- 

 hampstead Plain, and so leaving the Farnham Plateau to the west, 

 the gradient is 27 feet per mile. Now chert, probably derived from 

 Hindhead or somewhere near it, is found in the Drift on the 

 Farnham Plateau, and more abundantly at Easthampstead; pre- 

 sumably, therefore, at the time when these drifts were formed 

 (which I take to have been but little later than the upheaval of the 

 plain) the Hythe Beds were already exposed at Hindhead. In that 

 case this hill cannot have been much higher than at present, since 

 only the upper layers of-the Hythe Beds (less than 50 feet?) have 

 even now been removed from its summit ; and it follows that the 

 gradients just quoted are approximately those of the original 

 inclination of the plain, though it is not certain that the latter was 

 uniform throughout. 



Holybourne Down owes its elevation in large measure to its 

 being higher up the slope of the plain (that is, nearer to the 

 central axis) ; but there are no Eocene outliers on the north 

 by which the gradients in that direction can be calculated. 

 Taking Wheatham Hill (813 feet O.D.), however, as indicating 

 the height of the plain along the central axis, and measuring 

 northwards to Holybourne Down, we find a gradient of only 

 7 J feet per mile. Of course, I cannot assert that Wheatham Hill 



