AND OF THE NEIGHBOTIRHOOD OF WATFOED. 103 



Hence it will be seen that a careful study and comparison of 

 present forms with those of past times, yields us evidence that the 

 London Clay was deposited under tropical conditions, or in a 

 climate very different from that at present prevailing in the same 

 area. 



The London Clay, as I pointed out to you, is but little spread 

 over the area of Hertfordshire.* "What remains of the Tertiary 

 series is that which belonged to the Woolwich beds and the 

 basement bed of the London Clay, with only a small portion of 

 the Clay itself. 



One interesting point which some of the members of the Society 

 may have noticed is that in some parts of Hertfordsliire, as near 

 Radlett, there occur masses of rounded pebbles frequently 

 cemented together into hard stone. These plum-pudding stones, 

 as they are called, are merely the consolidated pebble-beds of the 

 Woolwich series. These pebbles were not only rolled upon some 

 shore where they were deposited, but were subsequently cemented 

 by infiltration of siliceous matter, so as to form a solid rock. 

 These old conglomerate masses were formerly made into querns 

 or hand-mills for grinding com, having been so used because, 

 probably, the softer parts between the pebbles more readily give 

 way than the harder, hence forming a continued roughened 

 surface. 



Besides the characteristics which I alluded to in the London 

 Clay, there is another interesting point bearing upon the life of the 

 period. Many of you may probably have observed in your visits 

 to some of our sea-shores, pieces of wood, or sometimes pieces at 

 the bottom of old piles, bored by a kind of shipworm called the 

 Teredo. Other specimens, which are not at all uncommon in the 

 London Clay, tell a somewhat similar story. The wood which, 

 from its structure, is known to be coniferous, grew upon land, and 

 was subsequently di'ifted into the sea; during the drifting it 

 became the home of the Teredina, and ultimately becoming water- 

 logged, sank to the bottom. This and other facts noticed, there- 

 fore show that the London Clay was formed not only in a deep 

 sea and gradually accumulated, but that there was contempo- 

 raneous land on Avhich vegetation grew and animals lived whose 

 remains were washed down by the streams and imbedded in the 

 accumulating mud. 



This Clay period at last terminated, and the other Eocene 

 deposits newer than the London Clay are not represented in 

 Hertfordshire. 



The next formed beds were sands (the Lower Bagshot), and 

 are still found capping as outliers the London Clay of Harrow 

 and Hampstead Hills, and are remnants of similar strata which 



* " Along the northern boundary the escarpment (of the London Clay) is 

 again conspicuous for the most part, as on the south of "Windsor, and along the 

 valley of the Coins : Stanmore Heath, indeed, with its capping of pebble- 

 gravel, being the highest ground in Middlesex." — Whitaker, 'Geology of 

 London,' p. 42. 



