Yd. 56.] CONSTITUENTS OF THE CRAG. 741 



In this sample quartz makes up about half of the total weight of 

 the Crag. 



Very much larger quantities were afterwards separated by means 

 of Klein's solution (borotungstate of cadmium). 



The fraction of a density above 3-28 contained zircon, rutile, 

 cyanite, andalusite, corundum, garnets, ilmenite, and leucoxene. 



The zircon occurs principally as small grains, averaging 

 •2 millim. in length. The majority of the crystals have 

 sharp clear outlines, but others are rounded or oval in 

 form. This, however, cannot be regarded as the result of 

 abrasion by subaerial agents, as precisely similar rounded 

 grains are found enclosed in the mica which comes down 

 with a lighter fraction. The zircon itself usually contains 

 many inclusions — long brown needles — hexagonal in sec- 

 tion, and frequently oriented so that their long axes are 

 parallel to the pyramidal faces. 



Dark-brown or reddish crystals of rutile are also common. 

 Most of them are very small, only a little larger than the 

 zircons. Occasionally larger grains are found, having a 

 diameter of *5 millim. Usually the grains are angular and 

 show crystalline faces, but rounded forms occur among 

 the larger grains. 



Cyanite is fairly plentiful, both as rounded and angular 

 grains. The latter are mostly long rectangular prisms 

 with strong cleavages, and often crowded with inclusions 

 of zircon and rutile. The angle of extinction, measured 

 from the prism-faces, is about 31° 30'. Some specimens 

 exhibit a fibrous structure, and appear faintly blue under 

 the microscope. The inclusions usually lie in lines which 

 show extinction when parallel to the cross-wires of the 

 microscope. 



Andalusite is not common. It occurs as long, clear rect- 

 angular crystals, showing straight extinction and having 

 a bright shagreen appearance. 



Corundum occurs as brown or yellow crystals, mostly well 

 rounded. 



Garnets are so plentiful in the heavy fraction that the 

 material in the mass has a strong pinkish colour. The 

 grains are nearly always well rounded, and range up to 

 and above *5 millim. in diameter. They contain many 

 inclusions. 



Ilmenite occurs both as angular and rounded grains. It 

 shows the characteristic greyish-black metallic lustre by 

 reflected light. 



Leucoxene accompanies the ilmenite as large, white, opaque 

 grains. Sometimes the centre of a grain is black with a 

 white border. 



The material separated out by a solution having a specific gravity 

 of 3 consisted almost entirely of 



