272 R. H. JRastall — Minerals of Lower Greensand. 



an immense number of closely set twin-lamellse : the law in this case 

 is indeterminable. 



Among tlie rare minerals may be mentioned a single subangular 

 grain of garnet, with unusually rounded contours and smooth surface. 

 This must have undergone an immense amount of attrition during 

 a prolonged period and has doubtless been derived ready-made from 

 some older rock, since such a degree of smoothness is very unusual in 

 this mineral. Two good-sized grains of a brown mineral are of much 

 interest, since they are probably cassiterite. The most notable 

 characters are the pleochroism, red to greyish brown, and the very 

 numerous twin-lamellae, so characteristic of the cassiterite in Devono- 

 Cornish granites. Sphene was not found, though a prolonged search 

 was made for it during several hours. 



Summary and Conclusions. 



Although it is obvious that much more detailed work would be 

 required before any positive deductions could be drawn from the facts 

 here set forth, nevertheless certain interesting features present 

 themselves and may be briefly summarized here. In the first place, 

 although the general mineral assemblage shows great similarity 

 throughout the whole stretch of country yet examined, being 

 ■characterized everywhere by zircon, kyanite, staurolite, tourmaline, 

 and rutile, there are certain distinct local differences. In Norfolk 

 we find in fair abundance garnet and blue soda-amphibole, and these 

 appear to extend as far as Ely. Further west than this the 

 amphibole is non-existent and garnet is exceedingly rare, having 

 been observed at two localities only in the minutest quantity. Green 

 mica was also observed only in the Sandringham Sands. Again in 

 Norfolk most of the heavy minerals are distinctly larger than further to 

 the west and some of them more angular. These facts taken together, 

 and especially the presence of blue amphibole, suggest derivation 

 from the north-east, possibly from Scandinavia. 



Further to the west a different assemblage of accessory minerals 

 appears to set in, such as colourless pyroxene and green epidote, 

 with curious pinkish zoned crystals usually attributed to zircon. 

 Here also deep-blue tourmaline seems to be more common, though 

 it is not confined to this neighbourhood. But perhaps the most 

 noteworthy mineral of the western area is sphene ; this is local in 

 its distribution : although it was observed at Great Gransden, it is 

 really common only at Aspley Guise. It is also of some interest to 

 note that some grains believed to be cassiterite were seen only in 

 the extreme west, near Brickhill. If the presence of this mineral in 

 any quantity could be established with certainty a south-western 

 origin for the material could be regarded as established. This point 

 needs further investigation. 



Thanks are due to Professor Bonney for the kindly interest that he 

 has shown during the progress of the work and for opportunities of 

 examining specimens in his collection for purposes of comparison ; 

 also to Mr. W. H. Wilcockson for help in the field and laboratory 

 in 1914. 



