136 Mr Rastall, The Mineral Composition of 
presence of much muscovite in this sand, and its absence in other — 
localities, will be discussed in the concluding section. It is at any 
rate clear that this high-level deposit differs greatly from the 
low-level sands next to be described. 
II. Sands and Gravels of the Ancient River System. 
(a) Pit on Newmarket Road, half a mile beyond Barnwell 
Junction, 46 ft. above O.D. 
In this pit, which is now being worked for gravel, are seen 
seams of very white sand, interbedded with fine flint gravel. 
On examination with a pocket-lens, the sand is seen to be very 
rich in minute fragments of Chalk, flint chips and grains of 
glauconite. It also contains many small white prismatic objects, 
which, as Dr Bonney suggests, are probably prisms from the 
disintegrated shells of Inoceramus, together with minute spines 
of Echinoids. 
After washing, the sand effervesced very strongly with dilute 
acid, and a second washing removed a large amount of muddy 
material; hence the grains are evidently partly cemented by 
calcareous matter. 
In the first separation in bromoform, many brown grains 
came down: the majority of these proved to be grains of light 
minerals with a coating of iron oxide (limonite). After prolonged 
treatment with acid this coating was removed, and many opaque 
grains were left, of various colours, white, pink, green and brown. 
These grains were easily separated from the true heavy residue 
by shaking with water in a dish. They are very well rounded 
and uniform in size, and certainly come from the Lower Greensand. 
In the heavy residue the most abundant mineral is garnet, 
either colourless, pink or brownish red, for the most part in very 
angular chips of varying size. Green hornblende is also common, 
while blue-green arfvedsonite and pale green augite also occur. 
Other minerals noted are tourmaline, kyanite, staurolite, epidote, 
zircon and rutile, besides opaque iron ores. 
(b) Pit behind the Travellers’ Rest Inn, Huntingdon Road, 
1 mile from Cambridge, 83 ft. above O.D. 
This large pit is opened in gravel of very variable coarseness, 
with abundant seams of brownish sand. This gravel is notable 
in that it yields an unusual number of large boulders, up to 
one foot in diameter: the great majority of these are of sandstone, 
probably Carboniferous, but other rocks are fairly common. From 
this locality a large number of far-travelled erratics have been 
