272 PKOCEEDINGS OP THE GEOLOGICAL SOCIETY. [Apr. 16, 



get nearer to the Challc, and at last tvoulcl lie on that rock. Now it 

 is just at the part where one would expect this to happen that the grey- 

 wethers occur in hy far the greatest number, which naturally leads to 

 the inference that they have some connexion with that formation, 

 and indeed, have most likely heen derived from it. 



On the surface of the chalk- country westward of Marlborough 

 (Sheets 14 and 34 of the Geological Survey Map) there are literally 

 tens of thousands of greywethers. Speaking of their occurrence in 

 this district, Prof. Ramsay says, — " A few of the places where they 

 are most numerous are marked ' large stones ' on the Ordnance Map ; 

 but these yield no idea of their surprising number, or of the extent 

 of ground they cover, no indication being given of their occurrence 

 over many large areas where they strew the ground so thickly that 

 across miles of country a person might leap from stone to stone 

 without touching the ground on which they lie. Many of these flat 

 masses of grit are four or five yards across, and they are often four 

 feet in thickness *." I saw one block, in a valley on the northern 

 side of the Kennet, that measured 13 x 10 x 7 feet, that is to say, 

 contained, allowing for irregularity of surface, about 850 cubic feet. 

 In the distance it looked like a small hut. 



Greywethers are not only found on the surface of the Chalk and 

 older formations, but also on the London Clay (though not in such large 

 numbers), and that too at a distance of some miles from the outcrop 

 of the underlying Woolwich and Reading Beds, as is the case to the 

 north-west of London ; which fact favours the notion that they have 

 come from the overlying Bagshot Beds rather than from a formation 

 below the London Clay. 



As it is known that here and there sandstone occurs in various 

 parts of the Bagshot Series, there is nothing unlikely in the view that 

 greywethers may have been thence derived. Indeed Mr. Prestwich 

 has noticed that most of the stones have " a lithological structure 

 very similar to that of the blocks found irregularly dispersed some- 

 times in the lower, but more especially in the upper division of the 

 Bagshot Sands between Esher and Strathfieldsaye." 



I do not think, however, that aU greywethers came from the Bag- 

 shot Sands. Many, I have no doubt, have been derived from the 

 Woolwich and Reading Beds ; indeed I have seen a large mass of 

 sandstone in place in an outlier of that formation at Langley Park, 

 near Beedon, to the north of Newbury f. Again, on the south-east 

 of London there is a thick pebble-bed in that formation, which in the 

 neighbourhood of Bromley is often hardened into a pudding-stone, 

 large blocks of which may be seen in the railway- cutting at Becken- 

 ham. The blocks of pudding-stone so common on the surface of the 

 chalk- district of Hertfordshire, &c., I think (with Mr. Prestwich) 

 also belong to this Series. Other greywethers possibly, but not large 

 ones, came from the Basement-bed of the London Clay, which in 

 some places contains a bed of sandstone. But I hold that the occur- 

 rence, in vast numbers, of these sandstone-blocks westward of Marl- 



* Memoir illustrating Sheet 34 of the Geological Survey Map, p. 41. 



t See Memoir illustrating Sheet 13 of the Geological Survey Map, p. 35. 



