THE GREENSAND BED AT THE B ^SE OF THE IHANET SAND. / O.) 



42. The Gkeensand Eed at the Base of the Thanet Sand. By 

 Miss Margaret I. Gardiner, Bathiirst Student, Newnham 

 College, Cambridge, (liead June 20, 1888.) 



(Oomiuuuicated by J. J. H. Teall, Esq., M.A., F.G.S.) 



This bed may be seen at various points from Pegwell Bay in the 

 east to Chislehurst in the west of Xent, and there is a bed at 

 feudbury, in the N.W. corner of SSuttolk, which Mr. Whitaker 

 considers to be the same*. At Lewisham and Croydon, to the west 

 of Chislehurst, it is missing, and the light buff micaceous sand which 

 usually succeeds it in West Kent rests directly on the hint bed 

 above the chalk ; so that, unless either the 9 inches of greensand and 

 liint or the 2 feet of grey sand last seen in 1830 at Epsom by Prof. 

 Prestwich are the same, the succeeding beds of the Ihanet Sand 

 overlap it westwards. 



Specimens have been obtained from Pegwell Bay, Chislet near 

 Heme Bay, Upnor, Chislehurst, and Sudbury. Leaving for the 

 present che Sudbury sand out of consideration, this basement bed is 

 a very fine sand formed of about equal quantities of dark and light 

 grains mixed with more or less clayey matter. Its appearance in a 

 section varies considerably with the weather, for it is the dark 

 greenish grey of the darker grains which gives the colour when it is 

 wet : but when it is dry the clayey matter becomes a white powder, 

 and is a much more conspicuous constituent. A microscopic in- 

 spection shows the sand to consist of quartz, flint, glauconite, and 

 small quantities of felspar and various rarer minerals, with a few 

 casts of microscopic organisms. 



Quartz. — The quartz is in not much rounded grains of average 

 largest dimension about '1 miilim. One of the striking points 

 about the sand is the small proportion of quartz-grains, namely, 

 only about 45 per cent. 



Glauconite. — The glauconite-grains are small as compared with 

 those of most greensands. The majority are of rounded outline, 

 and consist of an aggregation of smaller grains, often wedge-shaped 

 in form and fitted together in a convolute manner. The cracks 

 between the parts of the grain are marked by a yeUow line, 

 probably of iron-oxide. This kind of aggregate seems to be the 

 commonest form of glauconite-grain, and occurs in those of the 

 Cambridge Greensand, Lower Greensand (Folkestone), Upper 

 Greensand (Highclere), and the basement bed of the Woolwich 

 Sands. Other green grains are subangular. Some of these are 

 only ]3ieces of the round grains, but others are probably coated 

 grains of tlint or quartz, since some may be seen to give a distinct 

 quartz-reaction. When mounted in balsam the glauconite is 

 opaque except just at the edge, but in water or glycerine by 

 * Geol. Surv. Mem. to Sheet 47. 



