R. H. Rastall — Minerals of Barrington Bone-hed. 255 



to pieces readily when placed in water. The preparation of the 

 specimen was difficult, since very prolonged washing is needed to get 

 rid of the fine white chalky mud. When this had been removed 

 there was left a mass of fine gravel and sand with flints and Chalk 

 pebbles and many shells. The latter were mostly in a very 

 fragmentary condition, though some were unbroken. A large 

 proportion of the residue consisted of very fine sand, and in this 

 minute grains of brilliant green glauconite were visible. After 

 concentration by panning, the heavy minerals were found to be present 

 only in small amount, and the grains were also less in size than in the 

 other specimens. The only abundant mineral is white mica, in thin 

 flakes of varying size. Besides this there were a few crystals and 

 broken fragments of garnet, together with rounded grains of 

 hornblende and a very few zircons. One grain of rutile was seen 

 and one flaike of kyanite ; magnetite is not very abundant. The 

 general assemblage of minerals in this sample is much the same as the 

 others, the chief difference being the much greater abundance of 

 muscovite. This latter fact is probably to be correlated with the 

 preponderance of finely divided material. 



In connexion with the foregoing descriptions several points present 

 themselves for discussion. In the first place the chalky material 

 which is so abundant clearly belongs to two different types : the fine 

 chalky mud is undoubtedly derived from the underlying Chalk Marl 

 which forms the bed-rock of the deposit. But the pebbles and grains 

 of Chalk, of rounded form, which are such a characteristic feature, are 

 for the most part much harder than the Chalk of the Cambridge 

 district, and must have been transported by ice from more northern 

 localities. The very brilliant green grains of glauconite must have 

 been derived from the Cambridge Greensand, which underlies the 

 bone-bearing deposit over part of its extent. Grains of glauconite 

 derived from the !Neocomian show a duller shade of green of a quite 

 different tint, easy to recognize, though difficult to define in words. 



The general assemblage of heavy minerals here identified is, on the 

 whole, similar to that found in the other Pleistocene and Recent 

 gravels of the Cambridge district. There is, however, one notable 

 exception, namely, the occurrence at Barrington of abundant white 

 mica, a mineral which is notably absent from other localities, although 

 it is fairly common in some fine sands and marls near Newmarket, 

 and is abundant in a sand on the summit of the Gogmagog Hills. 

 The sporadic distribution of mica in these deposits is a curious 

 circumstance for which no plausible explanation has yet been found. ^ 

 Of the other minerals the most interesting are tourmaline, kyanite 

 and staurolite, which, as is shown in the paper previously cited, have 

 probably been derived from the Neocomian rocks. The garnet, 

 hornblende, zircon, rutile and others, are believed to have been for 

 the most part obtained from the far-travelled constituents of the 

 Glacial Drift, which has undoubtedly provided a great proportion also 

 of the larger elements composing the' superficial deposits of this 

 district. 



1 See Proc. Camb. Phil. See, vol. xvii, p. 142, 1913. 



