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



applies to tourmaline. Muscovite is common in small flakes. Among 

 the rarities may be mentioned a very few crystals of orange sphene, 

 a few bits of colourless pyroxene, and a single very angular colourless 

 isotropic fragment, apparently garnet. A large pale-orange flake 

 with slight pleochroism, distinct striations, and straight extinction, is 

 doubtfully identified as brookite, but it may be an unusual form of 

 rutile. A black metallic-looking mineral in prismatic crystals is 

 ilmenite. 



Zircon is remarkably abundant and shows a great variety of forms. 

 Nearly all the types described by Krushtchov may be recognized, 

 together with a good many intermediate forms. The commonest 

 habit appears to be the square prism with pyramidal terminations, 

 100, 111, while similar prisms with their edges bevelled by narrow 

 110 faces are also abundant. In many cases the angle made by the 

 edges of the pinakoid or prism and pyramid can be measured with 

 sufficient accuracy to determine the form. The horizontal edge 



Fig. 3. — Zoned Crystals of Zircon, Great Gransden. x 200. 



made by the intersection of 110 and 111 is very rare here, though 

 common elsewhere. Another common type is the square prism 

 terminated by the bipyramid 311, the gneissic type of Krushtchov. 

 One very remarkable crystal showed a very long prism with 100 and 

 110, the terminations being different at the two ends. At one end 

 311 is dominant, while the other appears to show a simple pyramid 

 (Fig. 2). However, in these very small crystals with high refractive 

 index, it is difficult to be certain of the forms, owing to the strong 

 internal reflection and the rounding of the edges and corners. Pale- 

 pink and pale-brown crystals, often much rounded and with strongly 

 developed zonary structure, are also very common. According to the 

 latest researches these may be xenotime, which appears to be 

 isomorphous with zircon and even to form parallel and alternating 

 intergrowths with it. These zoned crystals are on the whole more 

 rounded than those without zonary structure. Fig. 2 shows a 

 number of sketches of interesting forms, some with conspicuous 

 inclusions of various kinds, including both crystals and spherical or 

 spheroidal bodies, which Krushtchov believes to be glass, while Fig. 3 

 shows zoned crystals of zircon or xenotime, one of them being very 

 much rounded by attrition. 



