142 Mr Rastall, The Mineral Composition of 
glacial period. They are much more angular than the first group, 
especially in the older sands, and show little evidence of derivation 
from pre-existing sandy deposits: in particular the angularity 
= 2 toll fil 
of the chips of garnet is often very marked, and to use a simple — 
word they look very new, having undergone little rolling or 
attrition: well-rounded garnets are very rare, except in the 
superficial and wind-borne deposits of the Breckland. 
The most striking feature of the sands here examined is the 
almost complete absence of muscovite, this mineral being one of : 
the commonest constituents of sands of all ages, with a noteworthy 
exception in the case of deposits of one class. It is stated by — 
Retgers* and Thoulet+ that mica and other minerals with very ~ 
perfect cleavage are almost completely absent from wind-blown 
sands, and this observation may have some significance in this 
ease. The only deposit here described containing muscovite is 
the high-level or plateau gravel on the summit of the Gog-Magog 
Hills. This is undoubtedly much older than the gravels and 
sands of the Cam system, and it is generally regarded as being 
of glacial age. The great abundance and variety of far-travelled 
erratics in it certainly lend support to this conclusion, and the 
sand itself is of a very different character to the river and surface 
deposits seen at lower levels. 
On the retreat of the ice the whole country must have been 
covered by vast spreads of sand and gravel, the relics of which 
are still to be seen in the Breckland of western Suffolk and 
south-west Norfolk. It is generally believed that the Glacial 
period on the continent was succeeded by a time of warm and 
dry climate, the Steppe period, and somewhat similar conditions 
must have prevailed in England. It is possible that during this 
time the sands were to a certain extent worked over by the wind, 
removing the muscovite and other light and flaky minerals. The 
chief difficulty in the way of this view is the extreme angularity 
of the garnets and some other minerals in the earlier river- 
oravels. 
The general conclusions arrived at from a study of the mineral 
composition of the sands here examined may be stated as follows: 
the materials have been derived from two sources, partly from the 
Neocomian sands of Cambridgeshire and the neighbourhood of the 
Wash, and partly from far distant sources by ice-transit; that is, 
from the solid matter transported on and in the ice from Norway, 
Scotland and the north of England. The mineral grains obtained 
from the former source are almost exactly like those characteristic 
* Retgers, ‘Uber die chemische und mineralogische Zusammensetzung der 
Diinensande Hollands,’ Neues Jahrb. fiir Min. 1895, p. 22. 
+ Thoulet, ‘Ktude minéralogique d’un sable du Sahara,’ Bull. Soc. Min. France, 
Vol. iv. 1881, p. 262. 
