128 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



numerous in many places between this spot and the downs above 

 Folkestone, as near Charing. On the chalk hills sloping from this 

 line of escarpment down to the Thames, such masses are likewise 

 common. 



It is possible that in Kent the upper and lower divisions of the 

 Lower Tertiaries may have partially contributed to the supply of 

 these dispersed sandstone blocks ; but on the hills of Hertfordshire, 

 Oxfordshire, and Wiltshire, and the extensive elevated chalk-platforms 

 forming Salisbury Downs, where these masses are occasionally so 

 numerous, I believe that they are all derived from the middle division 

 of the Lower Tertiaries, outliers of which still remain to prove the 

 former extension of these strata over those areas, for they have, with 

 these few exceptions, been wholly removed by subsequent denudation*. 



The absence of all organic remains, so peculiar a feature of the 

 sands of the Woolwich and Reading group westward of London, 

 strengthens the supposition of these equally unfossiliferous sandstones 

 being derived from this source -f. 



With the large softer masses of saccharoid sandstone of Salisbury 

 Plains and Marlborough Downs, there are however found a number 

 of small blocks and pieces of a very fine-grained, hard, compact, sili- 

 ceous sandstone generally of peculiar botryoidal forms, not conglome- 

 rate, and often with traces of long rootlet-like processes J ; but I have 

 not seen such rock-specimens in the Lower Tertiaries §, nor have I 

 been more successful in finding them i7i situ in the Bagshot series. 

 Their age must be considered uncertain ; at the same time the occur- 

 rence of specimens ^vith these rootlet-like processes weighs certainly 

 in favour of their origin from the Bagshot Sands, as we know that 

 similar impressions exist, although very rarely, in the blocks of sand- 

 stone found in the Upper Bagshot Sands. But in the sands or the 

 laminated clays associated with them such impressions are not met 

 with, although, as in the Lower Tertiaries, impressions of plants and 

 leaves have been found. These rootlet-like casts and impressions 

 are, however, organisms too indistinct to be of any definite value as a 

 proof of age. 



As before observed, much weight must not he attached to the rarity 



* It may be objected that no blocks of sandstone are now found scattered over 

 the surface of Salisbury Plains. They are certaiiily scarce, but I have found a few 

 in the valleys, although generally there hidden by the drift. Their very scarcity 

 has, however, probably hastened the destruction of the few that have existed, and 

 have been used for various economical purposes. 



t At least the only exception is the occasional bed of the Ostrea Bellovacina 

 and shark's teeth at the base of this series, but the concretionary masses and 

 the white sands occur more in the body of the strata. Still, traces of these fossils 

 may possibly be found in some of the blocks around Newburj', or in some of the 

 Hertfordshire pudding-stones ; but hitherto no such impressions have, I beUeve, 

 been Tuet with. 



X They are common also in the flint-gravel of the valleys of the district. 



§ That is to say, in the district westward of London. The fine-grained hard 

 concretionary stone, before mentioned (p. 103) as occurring in the Woolwich series 

 beneath or near London (Charlton), sometimes presents however a veiy similar 

 appearance and fracture. 



