R. H. Rastall — Minerals of Lower Greensand. 213 



Even after very prolonged digestion with strong hydrochloric acid 

 it is difficult to get rid of all the hrown iron oxide, which clings 

 very closely to the grains, and often forms oolitic structures 

 apparently possessing a skeleton of silica. 1 This sand also contains 

 a good deal more carbonate than those from most other localities, 

 though in this respect it does not approach the calcareous sandstone 

 of the Ely outlier. 



When the residue from the preliminary panning, so very necessary 

 in this instance, is digested for a long time with acid, it is found that 

 a large proportion of the separated material consists of hrown grains 

 of oxidized glauconite ; owing to their low density most of these can 

 be got rid of by shaking and washing in a watch-glass or shallow 

 dish before the bromoform separation. This is an advantage from 

 the point of view of economy of bromoform. Even while the 

 separation in the heavy liquid is in progress it is evident that the 

 average size of the grains that sink is greater than usual, and 

 especially greater than in those specimens from the western and 

 southern areas. This is abundantly confirmed in the microscopic 

 examination of the material, when the difference is very noticeable 

 at the first glance. 



Several distinct samples from different horizons in the cliff were 

 separated with the most careful treatment adapted to the special 

 circumstances ; all were, however, so closely similar that one general 

 description will suffice. The characteristic minerals are kyanite, 

 staurolite, rutile, zircon, tourmaline, and garnet. The inevitable 

 ilmenite was abundant, and both hornblende and diopside were 

 present in small quantities, as well as an immense number of minute 

 flakes of muscovite. 



The crystals of kyanite are abundant and large and as a rule not 

 much rounded, some being quite angular and blade-like; they show 

 clearly the normal extinction angle of 30° and the emergence of 

 a negative bisectrix normal to the principal cleavage, on which the 

 crystals always' lie. Staurolite is common in angular chips, much 

 larger than usual, and also in grains of smaller size and more 

 rounded form ; it is of the ordinary bright orange colour, with distinct 

 pleochroism. Entile is abundant in large grains, generally much 

 rounded, varying in colour from a bright orange to a deep blood-red ; 

 some show good twin-lainellge on the common law. Tourmaline is 

 moderately abundant; it is usually in small rounded grains of 

 a greenish or brownish colour, but there are also a few larger and 

 more angular individuals of a greenish-blue; these generally show 

 little crystal form, but possess very sharp angles: they have 

 evidently not travelled very far from their source. 



Zircon is very common and it is very difficult to give a compre- 

 hensive general description, since it shows so much variation ; on the 

 whole, however, it maybe said that the great majority of the crystals 

 belong to the granitic type of Krushtchov. 2 They vary much in size, 



1 Phillips, "On the Constitution and History of Grits and Sandstones" : 

 Q.J.G.S., vol. xxxvii, p. 17, 1881. 



2 Mem. Acad. Imp. Sci. St. Petersb., ser. vil, vol. xliii, No. 3, 1894 ; 

 Min. Petr. Mitth., N.S., vol. vii, p. 423, 1886. 



