64 H. A. Baker — Quartzite Pebbles of tJie Oldhaven Beds. 



but not of such extreme rarity as has been supposed. They 

 are apjDarently always to be found where the Blackheath Pebble 

 Beds are exposed, if the seeker has succeeded in cultivating the 

 necessary " eye " for their detection, and pursues his search with 

 patient assiduity. The writer has obtained them from the Black- 

 heath Pebble Beds in the neighbourhood of Plumstead Common, 

 and at East Wickham, Bostall Heath, and Eltham. The pits at 

 East Wickham and Bostall Heath have yielded them in con- 

 siderable number to various workers. Mr. A. L. Leach and Mr. R. H. 

 Chandler have obtained a good many from these two localities. 

 They have also been found at Charlton and Keston. The outliers 

 of the North Downs have yielded them. They have been found 

 at Worms Heath and in the neighbourhood of Caterham. They 

 occur on beaches in the neighbourhood of the outcroj) of the Lower 

 London Tertiary beds. The writer has collected them in numbers 

 on the beach near Reculver, not far from Bishopstone Ravine, 

 and also at Shoreham (near Brighton) and at Dovercourt. They 

 do not occur at Pegwell Bay, where the Tertiary clifi-section is in 

 Thanet Beds only. 



In external ajDpearance these pebbles bear a fairly close resem- 

 blance to j)ebbles of partially decomposed flint, but they possess 

 certain characteristics which, when well visualized, lead to their 

 ready detection in the field. They vary in size, but are usually not 

 small, and are generally, though not invariably, characterized by 

 a flattened ovoid shape suggestive of their derivation from hardened, 

 tabular masses. They are usually very hard, capable of taking and 

 frequently possessing, a more than ordinary degree of polish. In 

 colour they vary from brown to grey or nearly white, being frequently 

 buff or drab. A common feature is a mottled marking or patchy 

 colour -distribution. Another interesting type sometimes met 

 with exhibits a kind of wavy banding of darker and lighter coloured 

 material, due, apparently, to a tendency towards aggregation of 

 the iron-oxides present in the rock. The texture of the rock, as 

 examined on fractured surfaces with a hand-lens, is usually fine- 

 grained, sometimes markedly so, but there is frequently con- 

 siderable variation to be seen, often in one and the same specimen. 

 There is often little apjjroach to uniformity, the conversion of the 

 rock from sandstone or grit into quartzite having apparently 

 proceeded irregularly, so that the crystalline "appearance of a 

 quartzite is seen only in patches, the remainder of the rock appearing 

 as a fine-grained saccharoidal sandstone. It is this variation in 

 texture owing to the sporadic character of the silicification which 

 frequently imj)arts a mottled appearance to the external surface of 

 many of the pebbles. 



When the rocks are sliced and examined microscopically it is 

 seen that they are not metamorj)hic rocks at all, in the strict sense 

 of the term. They have not been jjroduced by either contact or 

 regional metamorphism, but fall within the class of rocks resulting 



