138 Mr Rastall, The Mineral Composition of 
important of the latter the sample was collected. In the heavy 
residue the dominant mineral is pink garnet, and opaque grains 
of magnetite and other iron ores are also abundant. The grains 
are very variable in size, many of them, especially the garnet, 
being rather large. 
The following is a list of the minerals identified: garnet, 
tourmaline, staurolite, kyanite, hornblende, augite, hypersthene, 
zircon, rutile, epidote and magnetite. 
Garnet, both pink and colourless, is very abundant; some 
grains are rounded, most are subangular, while a few are very 
sharply angular. The grains of brown and pink tourmaline are 
generally very well rounded and appear to be derived, while the 
staurolite and kyanite grains exactly resemble those of the Lower 
Greensand, being almost certainly derived from that formation. 
(b) Swan's Gravel Pit, Milton Road, 45 ft. above O.D. 
This well-known pit, which is situated near the first milestone 
on the Cambridge and Ely road, is excavated chiefly in rather fine 
flinty gravel which contains a good many far-travelled rocks. The 
specimen was collected from a thick seam of rather ferruginous 
sand which forms the north-western corner of the pit. After 
cleaning in the usual way it was found to contain abundant dark 
grains together with glauconite. The proportion of grains sinking 
in bromoform was unusually large and most of them were so thickly 
coated with iron oxide as to require prolonged treatment with acid. 
It is noticeable in this and other cases that the iron oxide is often 
regularly deposited in concentric coats round some mineral nucleus, 
and when these shells are only partially destroyed a regular oolitic 
structure giving a black cross in polarised light can often be seen. 
The final residue consisted of abundant rather large grains, 
most of which were fairly well rounded, sharply angular crystals, 
other than garnet, being uncommon. The list of minerals re- 
cognised is as follows: garnet, tourmaline, kyanite, staurolite, 
epidote, hornblende, augite, hypersthene, rutile and zircon: 
possibly also brookite and anatase. The garnets vary much in 
colour, being most commonly pink or brownish pink, sometimes 
colourless. The tourmaline grains are very round, and include 
brown, pink and blue varieties. There are many rounded grains 
of yellow-green epidote, which in everything but colour have 
a strong resemblance to monazite. 
This sample is specially notable for the large size and rounded 
form of the kyanite grains. Generally speaking, the grains of 
heavy minerals are conspicuously more rounded than those of the 
sands previously described. 
J 
