HEADON HILL AND COLWELL BAT IN THE ISLE OE "WIGHT. 



87 



dip to the "W. in Headon Hill ; and it is implied that the Survey 

 section is false, owing to the neglect of this consideration. "We 

 must point out, however, that this Section, Sheet 47, fig. 2, in which 

 alone the anticlinal is shown, does not go through the summit of 

 Headon Hill at all, and passes about a half mile inland at the latitude 

 of Widdick Chine : in it the Upper Bagshot beds are made to appear a 

 little above the sea-level at that spot ; and we have great confidence 

 that the calculations on which this is grounded are correct * ; they 

 would be brought up by the anticlinal of which the existence has 

 just been acknowledged. So far then, we may observe, no reason 

 has been shown why the Survey Section should not be received as 

 correct. 



Prof. JudcVs Section {op. cit. pi. vii.). — "We do not find any 

 indications of the direction in which this section is drawn ; but, from 

 the names of chines which occur in it, we presume that it is intended 

 to start from Alum Bay in a JST.E. direction through Headon Hill, 

 and after that to follow the coast-line ; in this case it is not precisely 

 comparable with the Survey Section, whose direction is indicated on 

 the map as passing inland and crossing from sea to Solent. It will be 

 seen at once why it does not correspond to nature, and agree with 

 the views of other observers. It will be noticed that the Marine 

 bed of the Headon Group ({3 of fig. 3, pi. vii.) is made to exist at the 

 sea-level near "Widdick Chine; and in the letterpress we read 

 (op. cit. p. 147) " it is admitted on all hands that at the north-east 

 angle of Headon Hill the marine band [Middle Headon beds] makes 

 its appearence just above the sea-level." On the contrary, ive cannot 

 imagine any one putting the bed in this position. At the spot indi- 

 cated the top of the Middle Headon is about 105 feet above the sea- 

 level ; so that the section, in our opinion, is erroneous : the dip 

 thereby is exaggerated ; and 105 feet of beds are intercalated which 

 do not exist. We shall prove this presently by a detail-section at 

 this point of the hill ; at the present moment we wish to point out 

 that, with the correction of this error of 105 feet, the argument 

 against the accuracy of the Survey section entirely breaks down. 



Thus, we are told that the height of the Bembridge Limestone 

 above the sea-level at this point is 250 feet (op. cit. p. 147) ; then, 

 the marine band being put at the sea-level, it follows that 250 feet 

 of strata must intervene between that and the Bembridge Limestone ; 

 " but the Geological Survey [vertical] section shows less than one 

 half of that thickness of beds, and in Colwell Bay the distance 

 between the Bembridge Limestone and the marine band is 120 



feet f But 250 feet of strata is precisely the thickness required 



by my interpretation." Since we have to subtract 105 feet from 



* The greater thickness of Upper Bagshot beds above the sea-level in the same 

 line of section in the old edition of Sheet 47, and in the Plates of the Memoirs, 

 seems due to the outline of the ground at the S. end being raised too high above 

 Ordnance datum ; probably the accurate height of the Beacon Hill was not 

 obtainable till 1870, when the revised edition of Sheet 47 was published ; other- 

 wise the sections are practically identical. 



t It would be 133 feet, according to Mr. Bristow's estimate (Eorbes's Menu 

 p. 142), to the top of the Bembridge Limestone in Colwell Bay. 



H 2 



