R. H. Rastall — Minerals of Lower Greensand. 271 



them lie on the cleavage parallel to 100, and therefore show a constant 

 extinction angle of about 30° and the emergence of a negative bisectrix. 

 Staurolite is unusually abundant, in crystals of all sizes, some being 

 as much as 0*3 mm. in diameter : the crystals are nearly always 

 angular and shapeless, very few showing any traces of crystal forms. 

 The colour is always orange-yellow and the pleochroism distinct (pale 

 yellow to orange). Rutile is extraordinarily abundant, both in good 

 crystals and in shapeless grains of all sizes. The colour also varies 

 remarkably, from quite pale yellow to the deepest crimson red with 

 submetallic lustre. The presence of brookite was suspected, but has 

 not been confirmed. Tourmaline is in crystals of various sizes and 

 shapes, but not very abundant. The majority are of a brown colour 

 and much rounded, more angular blue grains being rare. Zircon is 

 numerically the most abundant of all the heavy minerals, and some 



Fig. 7. — Kyanite crystals, near Woburn. x 100. ■ 



crystals are unusually large, up to 0*2 mm. in length and 0*1 mm. in 

 breadth ; the majority are much smaller than this. The number of 

 different varieties observed is great, and the variation very wide. 

 Black and brown iron-ores are plentiful, but in no way remarkable. 

 Muscovite is abundant in minute flakes. 



The slides made from this material present several features of 

 considerable mineralogical interest: apart from the great abundance 

 and variety of the zircons as before mentioned, there are also several 

 good examples of twin-crystals of rutile of various types, especially 

 "elbow twins". In a curious flat tabular twin of this kind, the 

 angle between the two portions is about 120°. As the crystal does- 

 not lie quite flat the angle cannot be determined with great accuracy,, 

 but the twin clearly belongs to the e (101) type, the commonest law 

 in this mineral. As shown by reflection in incident light the large 

 faces of both individuals are exactly co-planar. The flat tabular 

 form is not common in this mineral, and suggests that the crystal 

 developed between the foliation-planes of a schistose rock. Another 

 elbow twin is remarkable in that one individual shows in addition 

 lamellar twinning, apparently on the same law. This crystal is pale 

 yellow in colour. A small and very irregular fragment also shows- 



