STRATA OP THE HAMPSHIRE BASIN* 143 



Almost every geologist who has studied the section in question has 

 assumed the existence of a great anticlinal fold, of which the sum- 

 mit is supposed to he seen in Totland Bay ; and as the effect of this 

 great flexure, the strata seen at the base of Headon Hill are supposed 

 to he repeated, with an opposite dip, in Colwell and Totland Bays. 

 The manner in which this supposed anticlinal is regarded as having 

 affected the strata is illustrated in Prof. Edward Forbes's memoir 

 (see PL YII. fig. 1), and also in Sheet 47 of the Horizontal Sections 

 published by the Geological Survey. 



The circumstance which seems to have given rise to this correla- 

 tion in the first instance was the occurrence, both at Colwell Bay 

 and at the base of Headon Hill, of a band containing numerous 

 oysters and other marine shells, and especially characterized by 

 abundant and well-preserved specimens of Cytlierea incrassata. Sow. 

 Now the " Yenus-bed,'^ as it was called by collectors, so well seen 

 in Colwell Bay, soon came to be identified with that at the base of 

 Headon Hill ; and the term " Middle Headon " was applied to both. 

 At the same time the freshwater beds, lying respectively above and 

 below these marine bands, were correlated with one another and 

 termed "Upper" and "Lower Headon." 



But Cytlierea incrassata, Sow., is not by any means to be regarded 

 as an eminently characteristic shell marking a particular geological 

 horizon, xiccording to the Geological Survey, it makes its first ap- 

 pearance in the Barton Clay, and ranges up certainly as high as the 

 Bembridge Marls. As I shall show hereafter, the " Yenus-bed " of 

 Colwell Bay contains a very different fauna from that at the base of 

 Headon Hill. A third "Yenus-bed" is found in the midst of the 

 Bembridge series ; and the failure of geologists, prior to the work of 

 Edward Forbes, to discriminate between this and the other " Yenus- 

 beds " led to the Headon strata being placed on the same horizon as 

 the Bembridge. 



This correlation of the " Yenus-beds " of Colwell Bay and Headon 

 Hill has been productive of a great amount of confusion, as will 

 soon be made apparent when we critically examine the conclusions 

 of different authors who have written upon the subject. As early 

 as the year 1821 Mr. G. B. Sowerby pointed out that the so-called 

 marine bed of Headon Hill had little claim to be so regarded ; for 

 the number of freshwater forms found in it is so great that it can 

 only be considered as having been deposited under estuarine con- 

 ditions, Sedgwick, who reexamined the sections in the following 

 year, seems to have clearly perceived the points of difference between 

 the beds in Colwell Bay and in Headon Hill. 



Both Edward Forbes and Mr. Bristow appear to have experienced 

 the greatest difiiculties in their attempt to correlate with one another 

 the beds above and below the "Yenus-beds" of Colwell Bay and 

 Headon Hill respectively.. Thus they found it necessary to assume 

 that the beds called " Upper Headon " at Colwell Bay have suddenly 

 expanded within a distance of less than two miles from 49 feet to 

 85 feet ; while the thick beds of limestone which are so conspicuous 

 in the latter locality are entirely wanting in the former. The diffi- 



