162 Mr Rastall, The Minerals of some Sands 
all they are generally assumed to be similar to the “ Gravels of . 
the Ancient River System” of Cambridge, and the field evidence, 
such as it is, goes to support this view. In the neighbourhood of. 
Exning and Snailwell the gravels and sands form a ridge, no 
doubt once continuous, but now cut through in places by existing 
rivers. This ridge is no doubt analogous to the Quy-Cambridge-_ 
Girton ridge, which is similarly cut through by the Cam. I have- 
been unable to find in the scanty literature any mention of the 
loam or brick-earth here described, although brick-earth is said to_ 
exist in some abundance at Bury St Edmunds. 
It is unnecessary to describe in detail any of the methods” 
adopted in collecting, preparing and separatmg the material. 
These processes were all carried out by the methods mentioned 
in the paper just cited. The only addition here made was the 
adoption of a new optical arrangement to facilitate the examina- 
tion of the separated minerals, which may be briefly described. 
The heavy residues from all the sands dealt with contain a 
considerable proportion of opaque grains, all of which look very 
similar when examined by transmitted light in the usual way. 
Hence it became highly desirable to devise some method by which 
grains of different colours could be readily distinguished. It was 
important also to make such an arrangement that it was possible 
to revert to the normal method of examination without disturb- 
ance of the microscope or of the slide, enabling any individual 
grain to be examined by each method alternatively. After some 
trials it was found that the best results were obtained by strong 
oblique illumination, and that various colours of background were ~ 
required for objects of different characters. Thus highly-coloured 
transparent or semi-transparent minerals are made most con- 
spicuous by a black background, while for completely opaque and 
metallic grains a partially illuminated background gives the best 
results. ‘The method finally adopted was as follows: a powerful 
electric lamp, of 16 or preferably of 32 candle-power, was placed to 
one side of and as close as possible to the revolving stage of an 
ordinary Swift petrological microscope, the ight being raised very 
slightly above the plane of the stage. The illumination of the 
field of view was thus very nearly horizontal. The plane mirror 
was arranged to reflect a beam of light up the tube in the usual — 
way for examination by transmitted light, with or without crossed — 
nicols. ‘To obtain a dark background all that is necessary is to — 
insert a card of the required shade, black, grey or white, into the — 
space between the lower nicol and the stage. It is convenient to _ 
have a single card, half black and half white, or with a grey strip — 
in addition ; this card can be moved about as desired for different _ 
grains, and it can also have a hole cut in it to be brought to the — 
centre when a solid background is not desired. This card in no — 
