and Gravels near Newmarket. 167 
(a) Brown loam. In this specimen all the grains are rather 
small and even in size; a few garnets only being somewhat larger 
than the average. Among the opaque constituents magnetite is 
moderately abundant and there are a few grains of red iron oxide. 
Among the transparent constituents zircon and rutile are very 
abundant, both in sharply angular and well rounded crystals. 
-Hornblende, epidote and staurolite were also noted, while flakes 
of muscovite are abundant. Tourmaline is scarce, while kyanite 
‘is very rare indeed. Garnet occurs in angular chips and also in 
rounded grains. 
The notable features here are the abundance of zircon and 
rutile and the rarity of kyanite and tourmaline, together with the 
presence of abundant muscovite. 
; (b) Grey marl. In this specimen the grains are all very 
small indeed. and of uniform size. Magnetite is not very abun- 
dant and there are a few pale yellow and white opaque grains. 
Minute flakes of muscovite occur in great profusion, the other 
notable minerals being zircon and rutile: the crystals of both are 
for the most part sharply angular, though a few are rounded. 
Other minerals are'in such minute quantity as scarcely to be 
worth mention. Allowing for the difference of size of grains this 
specimen agrees very closely with the last. 
GENERAL CONCLUSIONS. 
Although the specimens here described are few in number, 
and show a certain amount of variation among themselves, they 
all possess certain characters in common, with the exception of the 
Dane Hill marl, which is quite exceptional. In all the others the 
most notable heavy minerals are zircon and rutile, while kyanite, 
staurolite and tourmaline, So abundant in the gravels near Cam- 
bridge, are notably rare or even completely absent. In the loam 
and marl of the Snailwell road pit at Exning muscovite is also very 
abundant. 
From the general mineralogical composition of the sands and 
gravels here examined it may be inferred that the greater part of 
the material has been derived from distant sources by glacial 
transport, minerals obtained at second hand from the Neocomian 
series being quite subordinate in amount, and notably in less pro- 
portion than in the gravels near Cambridge. Not enough evidence 
is yet available to enable any explanation of the causes of this 
difference to be offered, and more work is required on this subject. 
As for the actual manner of formation of these deposits, the 
coarser types appear to be due to rapidly moving water, either 
the waters of ancient rivers, or possibly in some cases fluvio- 
glacial, while as before stated the fine marls have possibly been 
deposited in still or slowly moving water in which ice was floating. 
. 
