98 A. S. Lucas — The Headon Beds of the Isle of Wight 



tlie certified facts. They are based on notes and recollections of five 

 days spent in working the cliffs at the beginning of last year. I had 

 read Forbes's memoir and Prof. Judd's paper, but had not seen 

 Messrs. Keeping and Tawney's paper, but was independently brought 

 to the same conviction as they as to the accixracy of the Survey read- 

 ing. No attempt will be made to consider the relation of these beds 

 to the deposits exposed at Hordwell, Brockenhurst, or White Cliff 

 Bay, as it is especially desirable to concentrate attention first on the 

 determination of the identity or distinctness of these adjacent beds 

 in the west end of the Isle of Wight. 



One difficulty in reading the sections in Headon Hill arises from 

 the constant denudation of the yielding materials of the strata, which 

 pour down in taluses and obscure the lower beds. Only vertical cliff- 

 faces are at all reliable. And it is quite impossible, from this cause, 

 at any one time, for any one not perfectly familiar with the exact 

 locale of the smaller subdivisions to trace out the complete sequence. 

 The standard sections are the result of a synthesis of separate obser- 

 vations, now one series of smaller beds appearing in situ, and now 

 another, till by a combination of results the whole section is com- 

 pleted. The only way in which a passing observer can verify the 

 standard sections is to note whether the part of the cliff exposed at 

 the time is in accurate correspondence with the same part of the 

 section. During our stay, for instance (Jan. 4-9, 1881), the marine 

 series were well shown at the S.W. corner of the Hill, while at the 

 N.E. only the upper portion was seen, the white Cerithium band 

 standing out conspicuously, at the height of 110 feet above the sea, 

 in a clean vertical face, from which a foi'mer front had slipped down 

 in a talus right on to the Lower Headon Limestone, bringing the 

 beds quite away from their proper position. 



Another difficulty in making out the arrangement of these beds 

 is the normal variability in thickness and character of fresh-water 

 beds, and especially of Limestones, when pursued horizontally. 

 It behoves section-makers to state with the utmost exactitude the 

 spots at which their sections are taken. The Osborne Limestone, 

 18 ft. thick at Heatherwood Point, notoriously thins out to the E., 

 and is wanting altogether under the cottage. As noted by Forbes 

 in a wood-cut (Memoir, p. 132), one of the Limestones in his time 

 could be seen at one spot to increase quite suddenly owing to a basal 

 thickening. The thickness of the Great Limestone of Headon Hill 

 varies very greatly in the hill itself, as is well pointed out by Prof. 

 Blake " at the expense of the sand " beneath. Such lenticular 

 masses can hardly be strictly correlated with other beds in adjacent 

 areas after they have thinned out. But the more extensive beds in 

 which they are intercalated can very often be directly associated 

 with lateral extensions, and thus the jDlace of the Limestone be 

 fixed. Thus the Osborne Limestone is included in distinctive 

 clays and mottled marls, and these are found to thicken corre- 

 spondingly when the Limestone thins out. The age of the Lime- 

 stone is thus at once determined. When the Limestone bands 

 are continuous, they are, of course, of the very greatest service 



