R. H. liastall — Minerals of Lower Greensand. 219 



This specimen also contains several interesting forms of rutile, 

 especially those showing twin-lamellae. The colour of the crystals 

 of this mineral varies from yellow to orange-brown and deep red. 

 One conspicuous elbow twin on the common 101 law is of a quite 

 pale yellow colour, but most are darker than this. Fig. 4a shows 

 a deep-red, rounded crystal with twin-lamellae making an angle of 

 61° with the prism zone, while Fig. 4b shows a remarkable resem- 

 blance to a crystal figured by Dr. Teall from the Bagshot Sands of 

 Hampstead Heath. 1 The arrow-head twins there figured have, 

 however, not been observed in the Greensand. It is clear that rutile 

 varies very widely in many of its physical characters, especially in 

 colour and crystallographic development, and its discrimination from 

 the other Ti0 2 minerals is not always easy, especially in the case of 

 brookite. The latter mineral, however, does not appear to form 

 twins, which are so common and characteristic in rutile. 



The other minerals from this locality do not show any noteworthy 

 features. 



5. Gamlingay. 



A few years ago the large brick pit at Gamlingay afforded a very 

 good section showing the unconformity of the Lower Greensand on 

 the Ampthill Clay, but it is now a good deal overgrown and obscured. 

 The sands are somewhat ferruginous, fine, and very uniform in 

 texture, without pebble-beds. 



Fig. 4.— Crystals of Rutile, Great Gransden. x 200. 



From an examination of washed but unseparated samples it is 

 clear that heavy minerals are unusually abundant here, since 

 specimens mounted without any concentration show a large number 

 of grains of most of the characteristic species. In samples from 

 most other localities it is rare to find more than half a dozen grains 

 in any one slide. The quartz grains vary much in size and shape : 

 some are quite angular, but the majority are more or less rounded, 

 while a few are very round indeed. The characteristic heavy 

 minerals in separated samples are zircon, kyanite, staurolite, 

 tourmaline, and rutile. 



Zircon is found in many varieties; some are fairly large, having 

 the form of long prisms with the usual inclusions, but the majority 



1 Teall, British Petrography, London, 1888, pi. xliv, fig. 4. 



