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



lamellar twinning: these are of a very deep red colour. Bluish- 

 green hornblende and various pyroxenes, probably including both 

 rhombic and monoclinic varieties, are frequent. Only one undoubted 

 crystal of apatite was found: this is of the usual prismatic form, 

 showing high refractive index, weak birefringence,straight extinction, 

 and negative optical character. The surface shows the usual speckled 

 appearance of this mineral. The presence of several other minerals 

 was suspected though not definitely proved, including anatase, 

 sillimanite, and barytes. Only one crystal of each was seen, and the 

 determinations could not in any case be made with certainty. The 

 horse-shoe magnet extracted nothing from this sample, so that 

 magnetite is absent and even ilmenite is by no means abundant. No 

 garnet was found. 



"When compared with specimens from localities to the west of 

 Cambridge all these samples from Castle Rising are notable for the 

 much larger size of the heavy mineral grains, which also show more 

 pronounced angularity. 



3. Ely. 



The Lower Greensand covers a considerable area around the city 

 of Ely, forming the capping of the higher ground of the "island". 

 The beds are certainly very thin, perhaps not more than 9 or 10 feet, 

 although it is not certain that this is the whole original thickness, 

 since part may have been removed by recent denudation. Eor the 

 most part they consist of a rather ferruginous sandstone or pebbly 

 grit, which in places become conglomeratic. This is the general 

 character of the rock as used for building stone around the cathedral 

 and elsewhere. 



The specimens selected for examination came from the well-known 

 exposure in Roswell Pit. This particular band is in a highly inclined 

 position and has certainly slipped. It is in places a good deal 

 weathered and decomposed to soft and crumbly sandstone, but the 

 fresher portions form an unusually good example of a calcareous 

 sandstone, with a cement of crystalline calcite. 



A sample of the soft weathered sandstone, when broken up and 

 washed, is found to consist of fairly large and rather angular grains 

 of quartz, with a large number of grains or small pebbles of 

 ferruginous and cherty rocks, lydianite and other allied types. The 

 quartz grains are estimated roughly to, constitute about one-half of 

 the total. 



The heavy minerals in separated samples do not show a great 

 amount of variety, but there are one or two special points of some 

 interest. The characteristic, and in fact almost the only, constituents 

 are zircon, tourmaline, kyanite, and staurolite, with a smaller 

 amount of rutile. The zircon crystals, which are the most abundant 

 mineral, present no features of any special interest, beyond the fact 

 that most of them are broken : doubly terminated crystals are very 

 rare. Tourmaline is common, the larger crystals being usually 

 bluish, while the smaller ones are brownish or olive-green and much 

 more rounded. Kyanite is quite abundant, both as large angular 

 flakes and as smaller and more rounded grains. Staurolite also occurs 



