xvn. THE SARSEN STONES. 447 



and yet stratified, have been carried away, and the solid parts have 

 suffered some displacement. 



I have never found shells in any of these stones lying in their 

 native beds, and have some scruple in mentioning that they do 

 occur in a layer in one of the blocks at Stonehenge. But as I did 

 not choose by chiselling that monumental stone to attract attention 

 to it, probably it may for many years to come escape all injury 

 except that which it must suffer from the strokes of time. 



The result of a careful inquiry by Mr. Prestwich was to convince 

 him that the ' Sarsen' stones belong to the lower eocene series 

 of Woolwich and Reading ; but Mr. Whitaker, who has found 

 a thin deposit of true London clay in the Vale of Kennet, prefers 

 to class them as part of the Bagshot sands. 



It is worth remarking in connection with the ' Sarsen' stones, 

 that drifted masses which seem to correspond with them (having 

 occasionally flint pebbles and siliceous fragments imbedded) lie 

 on the north side of the Wiltshire downs, near Swindon, and that 

 a fragment of the sandstone of considerable size was found in 

 post-glacial drift as far north as the Thames at Long Wittenham, 

 near Abingdon. 



To complete our view of the eocene strata in the basin of the 

 Thames, by including a portion of the higher beds, we must go 

 down stream. London and the immediate vicinity offers to in- 

 spection, in pits, wells, road and railway-cuttings, the whole series 

 up to the Bagshot sands, and down to the Thanet sands, which 

 are supposed not to extend westward into the district round 

 Beading. On Mr. Mylne's excellent Map d , and in his authentic 

 sections, the course of all these strata can be clearly traced, and 

 their general relations to the chalk and its large supply of water 

 satisfactorily studied. Mr. Prestwich, in several remarkable 

 memoirs presented to the Geological Society, has furnished full 

 descriptions of the strata, their thicknesses and distribution, and 

 catalogues of the fossils they contain e. 



The following is a very condensed view or general index to 

 the whole of the early cainozoic series in the lower part of the Vale 

 of the Thames. 



d Geological Map of London and its Environs, by E. W. Mylne, F.R.S., F.G.S. 

 (1871). 



e Consult Journal of the Geological Society, vols. iii., vi., viii., and x. 



