132 Mr Rastall, The Mineral Composition of 
The Mineral Composition of some Cambridgeshire Sands and 
Gravels. By R. H. Rasrauu, M.A., Christ’s College. 3 
[Read 25 November 1912.] 
Ir has been recognized of late years that a detailed and 
careful study of the mineral composition of sands and gravels 
often yields valuable information as to the sources from which ) 
the material was derived and frequently throws much light on: 
the geographical and other conditions prevailing at the time of 
deposition *. j 
Some time ago I had occasion to apply this method of 
research to the Lower Greensand of Bedfordshire, Cambridge- 
shire and Norfolk. The results of this investigation have not 
yet been published, but they were of some interest, and it occurred | 
to me that it might be useful to compare with them the mineral | 
composition of some of the Pleistocene and Recent deposits of the 
same district, in order to see whether it was possible to trace the” 
derivation of material from the Lower Greensand as well as from ) 
far-travelled glacial deposits. 
A careful examination of numerous specimens of the so-called | 
Neocomian sands showed that all possessed certain peculiar 
features in common; of these features the most important for 
the present purpose are the abundance of kyanite, staurolite and 
tourmaline, and the complete or almost complete absence of! 
garnet, amphiboles and pyroxenes. This is not the place to) 
discuss the source from which these materials were derived: 
it must suffice here to say that they were certainly not of local 
origin, but must have come from some distant land-area not now 
exposed at the surface. The presence of tourmaline is not of: 
much significance, since this mineral is very stable and resistant, | 
and is common in nearly all sandy sediments, being often passed” 
on with little change from one formation to another. But the 
freshness and angularity of the chips and crystals of kyanite and 
staurolite in the Neocomian sands suggest that they were then 
recently separated from the parent rock, and not derived from— 
some older sediment. ; 
* Thomas, ‘The Petrography of the New Red Sandstone of the West of 
England,’ Quart. Journ. Geol. Soc. Vol. xv. 1909, pp. 229—245. j 
