Rev. A. Irving — The Bagshot Beds and London Clay. 403 



say, irrespectively of the particular horizon in the Bagshot Series 

 to which certain marginal portions of the Bagshot Beds may be 

 ultimately assigned, there seemed to be pretty clear evidence 

 furnished, from the London Clay itself, of the fact, that along their 

 northern and southern flanks the Bagshot Beds rest on beds belonging 

 to lower horizons in the London Clay formation than those on which 

 the Bagshot Beds rest in the more central portions of the area. The 

 present brief paper is an attempt to show that the above conclusion 

 is borne out by a much wider induction from data gathered from 

 the area of the London Basin in general. I have carefully gone 

 through the 448 well-sections appended to Mr. Whitaker's Memoir 

 on the London Basin, 1 and have selected from among them such 

 instances as seemed to throw any light upon this question. A large 

 majority of those sections include the London Clay ; but since, in a 

 great number of them, the thickness of that formation in feet is 

 given only, unaccompanied by any lithological notes on the strata, 

 they have been omitted, as affording no help to us in the present 

 investigation. There are, however, as many as 29 sections, in which 

 such notes appear. In every one of these the Beading Beds or the 

 Thanet Sands, and in most cases both these formations, were proved 

 below the London Clay, so that we are quite sure of a correct basal 

 horizon. In many instances also the Basement Bed of the London 

 Clay is noted. In 22 of these 29 sections, the thickness of the 

 London Clay is less than 200 feet, and in all these there is mention 

 of such inclusions in the strata as ' green sand,' septaria ('clay- 

 stones'), pyrites, flint pebbles, and 'shells.' These therefore agree 

 in this respect (without a single exception) with the lower half or 

 rather more of the London Clay of the Brookwood section, as shown 

 in the deep well at the Asylum on Knap Hill, which also (as shown 

 in my last paper) agrees with the London Clay strata pierced at 

 Wokingham on the northern, and at Aldershot on the southern, 

 margin of the Bagshot area. From this I conclude that in all these 

 22 sections we have (within their several limits) homotaxial equiva- 

 lents of strata of the lower half or rather more of the London Clay, 

 as that formation is proved in the Brookwood section. 



In the remaining seven sections there is no mention of septaria, 

 etc., above about 200 feet from the base of the London Clay. Of 

 course this does not absolutely prove the absence of them, in these 

 seven sections, but the number of instances is sufficiently large to 

 establish a high degree of probability that they were not found 

 at those higher horizons ; and in every case there is mention of them 

 at horizons below. The list from which this inference is drawn 

 includes sections at Aldershot Place, on the south side of the area, 

 at Hampstead, Harrow, Braintree and Chelmsford, on the north, and 

 at Canterbury in the extreme east. The great majority however are 

 in the metropolitan portion of the area. One of these (at Wimbledon) 

 corresponds so well with the Brookwood section, 2 that it deserves to 

 be given here more in detail, as follows : — 



1 Memoirs of the Geol. Survey of Great Britain, vol. iv. 



2 Geol. Mag. August, 1886. 



