756 MISS M. I. GAEDIXEE OX THE GEEEXSAIST) BED 



transmitted light, and alTvays by reflected light, it is a bright yellow 

 green. That of some other sands, e.g. the Cambridge Greensand 

 and the basement Woolwich beds, is a very blue green. Between 

 crossed nicols it gives either no reaction or a speckled look, some- 

 what like that of flint. The glauconite-percentage is only 15. 



Flint. — On first looking at a slide of tliis sand one is struck by 

 the large number of very sharply angular chips. These may be 

 roughly divided into two sets, the one transparent, the other 

 almost or entirely opaque. The transparent ones have a rough 

 pitted surface, which gives them a slightly greyish tint, and are 

 often marked by small black dots, which, when present in any 

 number, give the grains a darker colour. Between crossed nicols 

 they have a minutely tesselated appearance, the lighter parts being 

 of a bluish neutral tint. Their close resemblance to chips obtained by 

 crushing a flint seemed to leave little doubt as to their nature ; but, 

 since the glauconite gives a somewhat similar reaction, it seemed 

 possible that at least some of the more rounded grains, or those 

 which gave a less distinct reaction, might be weathered glauconite. 

 As a test, glauconite grains were bleached by boiling in hydrochloric 

 acid, and it was found that these could be distinguished from the flint 

 by their diff'erent surface, clearer colour, and less distinct outline. 

 The Upper Greensand (Highclere) was then examined for compari- 

 son. It is a very similar sand of quartz and glauconit-e. Though the 

 glauconite-grains are in all respects like those of the Thanet Rand, 

 yet there are no grains which could be mistaken for flint. There 

 seems no reason svhy the grlauconite-grains in the one should be 

 supposed to have lost their colour by weathering when they have 

 not done so in the other. Pinally the sand was placed in a boro- 

 tungstate solution of sp. gr. just below that of flint ; although a 

 few green grains fell through, all those floated out, with the 

 exception of a very few grains of both quartz and flint which 

 had probably adhered to the side of the funnel, were green. 

 Therefore, unless the glauconite increased in sp. gr. by weathering, 

 these grey grains cannot be glauconite. A consideration of this 

 evidence seems to leave no doubt that these lighter grey grains are 

 flint, although they form the abnormally large proportion of 

 20 per cent. 



The more opaque gi'ains are in general form like the clear ones. 

 In both, forms which resemble microscopic spear- aud arrow-heads 

 are not uncommon. There is a more or less distinct transition 

 from the clear to the opaque, and some are opaque in parts and 

 clear in others. By reflected light many show the same greyish 

 colour as the clear grains, though many are almost black. In fact, 

 by reflected light one often cannot tell whether a particular grain is 

 transparent or opaque, though both are easily distinguished from 

 the quartz- or the green glauconite-grains. Crushed fragments of 

 the weathered white coating of a flint are very like some of the 

 more opaque and transitional forms by transmitted light, though 

 these diff'er by being white in reflected light. The slightly weathered 

 surface of a black flint is such a mere fllm that it has not been found 



