AT THE BASE OF THE THANET SAND. 757 



possible to get pieces of it to compare ; but some of these grains 

 look by reflected light very like small, black flint pebbles, and the 

 double thickness of weathered coating even, though thin, might be 

 sufficient to make them opaque. In the boro-tungstate solution 

 these opaque grains fall with the clear ones. Altogether there 

 seems to be no reason for considering these grains to be glauconite, 

 as their comijarative opacity and the faint reaction which they 

 sometimes give in parts between crossed nieols at first inclines one 

 to do. The evidence there is seems to be in favour of the sup- 

 position, suggested by their form, that these grains are also flint. 

 Counting these in with the others, the flint-percentage rises to 40. 

 This and the other percentages have been obtained by counting 

 between 3000 and 4000 grains. The flint-grains are of about the 

 same size as the quartz -grains, ^. e. about 1 millim. in their longest 

 dimension. 



It is these opaque grains quite as much as the glauconite which 

 give the dark colour to the sand ; for when the clay is washed out, 

 what remains is dark grey, quite black when wet ; but when the 

 clay is washed out of the Upper Greensand, which is a sand very 

 like this without the flint, the residue is of a light green colour. 



Besides these flint-grains, larger ones which might almost be 

 called small pebbles, about jL. inch in diameter, are often found, and 

 at Pegwell Bay much larger flints, some slightly rounded like 

 those just above the chalk, and others which are regular pebbles. 

 Six were picked out of a piece of cliff about 2 feet square, but in 

 most parts they were not quite so numerous. 



Twinned Fdspar occurs in no great quantity. "What there is is 

 very generally twinned in two directions. 



Magnetite and Sjjinel. — Amongst the grains which come down in 

 a boro-tungstate solution of sp. gr. 2-9 black opaque grains are the 

 commonest. Amongst these some are very perfect octahedra. 

 Only some seem to be magnetic, so that probably both magnetite 

 and a dark spinel are present. 



Zircon also forms a large proportion of the heavier minerals. It 

 occurs in very perfect crystals and in grains. The crystals differ 

 considerably in size and form. Two from amongst the larger and 

 smaller respectively measured '116 millim. x '036 millim. and 

 •06 millim. x -02 millim. Often only the (100) and (lOi) planes 

 are developed, but sometimes also the (110) and (111) and other 

 pyramid planes. Very frequently one pair of the (101) planes is 

 developed at the expense of the other, so that the crystal has a 

 truncated appearance. There are often needle-like inclusions 

 parallel to some of the pyramid faces. 



Garnet (?). — In about the same quantity is present a mineral of 

 which only broken fragments of fairly large size have been seen. 

 It is very clear, colourless, highly refracting and isotropic. The 

 fragments are often very sharply broken, and sometimes the fracture 

 has a conchoidal look. Inclusions of black and green grains are not 

 uncommon. 



Rutile is not present in such quantity as the minerals already 



