1868.] WOOD PEBBLE-BEDS. 471 



paper, that if the bed No. 6 be of an age intermediate between 

 Nos. 5 & 7, it should occur between the two latter where they are 

 in superposition. The bed No. 6, however, is of such limited and 

 exceptional occurrence, and is due, as I regard it, so entirely to the 

 contiguity, in the particular parts of its occurrence, either of the 

 pebble-beds beneath the London Clay, or of bed No. 4, that its 

 existence in any form sufficiently marked to be detected over bed 

 No. 5 is scarcely to be expected. But, in truth, the bed No. 6 is 

 sometimes, as at Bencroft AVood, only one mile from the termination 

 of Mr. Hughes's section, so completely un distinguishable in general 

 appearance from that usually presented by bed No. 5, that I felt 

 constrained, notwithstanding its much higher elevation, to delineate 

 it in the survey-map made by me, which I had the honour to place 

 in the library of the Society, under the same colour and symbol as 

 the bed No. 5, and confine myself to calling attention in the Memoir 

 which accompanied it to the difference of level which existed be- 

 tween them. 



The relative levels of the beds in this part of Essex are shown (as 

 nearly as the absence of any figures denoting the precise elevations 

 on the Ordnance Maps enables me to show them), in the accompany- 

 ing section, which extends from the termination of that given by Mr. 

 Hughes for twenty- two miles south-eastwards to the brow of the 

 Thames valley. The elevation of what is clearly the same bed as No. 6 

 at Finchley, as also of the patches at Totteridge, Barnet, and other 

 summits in Middlesex, that seem to belong to it, relatively to the 

 Bagshot of Hampstead are, I think, very similar, although the 

 absolute elevation of Hampstead Heath, and of these pebble-capped 

 summits of Middlesex, is probably some 50 feet or more higher than 

 those of Essex shown in the section. 



The patches of this High-Plain Gravel that stretch northwards 

 towards the centre of Herts have not, I think, any considerable 

 extension, and not any such as would, I submit, point to a sub- 

 mergence of this part beneath the sea prior to the Middle and 

 Upper Glacial deposits*. It appears to me that they have been 

 accumulated under the same physical conditions, and at the same 

 time as those that stretch southwards towards the Bagshot outliers, 

 except that, in lieu of the flint-pebbles having in the former area 

 come from the bed No. 4, they may have been derived from outliers 

 of the Lower London Tertiaries, which, owing to the rise of the 

 Lower Tertiaries towards Herts, were scattered over the higher 

 plain in that part at the time when it began to subside beneath the 

 Glacial sea. 



Discussion. 

 Mr. Prestwich was inclined to regard some of the beds referred 



* There are some quartzose pebble-graTcls at high elevations in Oxfordshire 

 noticed in the Memoirs of the Geological Survej (l\fenioir for sheet 13, p. 55 ; 

 see also ' Geology of the Country round Woodstocli,' p. 27); but any connexion 

 between them and the bed now under discussion would be difficult to trace. 

 Their relation to the Glacial Clay does not appear. 



