4^4 PEOCEEDIIfGS OP THE GEOLOGICAL SOCIEIT. [June 3, 



necessity for the same principles of classification in the palaeozoic 

 and in the recent coral-fauna. There was a gradation from the 

 Hugosa to the Aporosa. 



Prof. Htjxlet remarked that the structures of the specimens of 

 the different genera proved that there were great difficulties in ac- 

 cepting Agassiz's opinion that these old forms were not Zoantharia, 



2. On the Pebble-beds of Mlddlesex, Essex, and Heets. 

 By Seakles Y. Wood, Jun., Esq., E.G.S. 



In the manuscript memoir which, in May 1867, I placed in the 

 Library of this Society, illustrating the Geological Survey I had made 

 of that part of the Thames-valley region which is comprised in sheets 

 1 and 2 of the Ordnance Survey-map, I entered in some detail upon the 

 subject of certain pebble-beds which I referred to the age of the Bag- 

 shot Sand ; but being anxious to compress as far as possible the paper 

 on the Postglacial structure of the east of England referring to that 

 memoir, which is printed in the 23rd volume of the Society's 

 Journal (p. 394), I made no allusion in it to these beds. A late 

 communication to the Society by Mr. T. M'K. Hughes, on the two 

 plains of Hertfordshire and their gravels*, induces me to lay a brief 

 notice of them before the Society. 



The beds so referred by me are composed almost exclusively of 

 rolled flint pebbles, with a few of jasper intermixed, not a pebble of 

 any other material being present except at one locality, " College 

 Wood," one mile and a half north-west of Erierning, where I found 

 many quartzites of similar rolled form to occur in them. The com- 

 position, form, and structure of these beds much resembles the well- 

 known pebble-bed beneath the London Clay, regarded by Mr. Prest- 

 wich as a local modification of the basement bed of that claj, the 

 principal difference being the absence of the smaller and spherical 

 pebbles so numerous in the latter, and in the absence of the beached 

 condition which that bed in one or two localities presents. The 

 peculiar feature attaching to the beds under consideration, in addi- 

 tion to their almost exclusively flint composition, is that they are 

 invariably present where any of the Lower Bagshot Sand remains, 

 and never occur except on the numerous outliers of that sand which 

 are scattered through South Essex f. They have often a consider- 

 able admixture of clay ; and where they occur on the Bagshot out- 

 lier at Stock, in Essex, the section shows the pebbles in the form of 

 bands imbedded in laminated clay. They often, but Dot invariably, 



* Quart. Journ. Geol. Soc. vol. xxiv. p. 283. 



t I should explain that in speaking of the Lower Bagshot of Essex, I have 

 treated the uppermost bed of brick-earth and the sand below it as included in it, 

 regarding the London Clay as terminating at the junction of that sand ; and my 

 mapping of this formation is on that principle. A thin bed of dark sand occurs 

 in the London Clay itself, about 50 feet from the top of that formation, which I 

 met with in several places, and is quite distinct from the sand which terminates 

 the London Clay. 



