288 ME. T. 0. BOSWORTH OX THE [Jlllie I912, 



farther confirmatory observations which seem desirable.) — This 

 occurs in crystals so minute that they remain in suspension for 

 more than ten minutes, the materials settling before that time being 

 aggregations of these crystals. When dried, this sediment is an 

 exceedingly fiue powder which, if it be handled, clogs np the pores 

 of the skin. If w^ater be added it forms a plastic paste. 



Tlie mineral is in the form of minute laths, some of which are 

 curved. Straight specimens are clearly defined, have rectilinear 

 edges, and give a low grey interference-colour, with straight ex- 

 tinction. Some of the laths which are clearly single crystals 

 measure -004- x '0008 mm., but the majority are much smaller. 



The refractive index is so near that of Canada balsam that, 

 except between crossed nicoLs, the mineral can hardly be discerned. 

 Mounted iu water it is more visible. The laths are best seen 

 through a sclenite-plate and crossed nicols, in which case they are 

 rendered conspicuous by their higli double refraction. The shorter 

 axis shows the greater elasticity, and so great is the difference that, 

 despite their extreme thinness, the laths change colour from blue to 

 orange on rotation. 



The bulk of the material, however, is in the form of irregular 

 grains or laminae, with fibrous texture, which readily break up 

 into the laths above described. 



The density was determined only after much difiiculty ; but at 

 length a specific-gravity bottle method was devised, which yielded 

 consistent results about 2-48. The mineral is somewhat hygro- 

 scopic, and absorbs water during weighing. A correction on this 

 account reduced the value to somewhere near 2*40. 



Result of chemical analysis (kindly carried out for me by Mr. L. 

 V. Wright, M.A., E.I.C.) of the finest detrital residue : — 



Water 4-98 



Silica 58-99 



Alumina 17-02 



Ferric oxide T'OT 



Lime 0-98 



Magnesia 2*73 



Potash 6-64 



Soda 1-07 



Total 99-48 



This corresponds fairly well with a composition represented by the formula 

 2(H : K : Na), SiOg Al, (SiO-^g (Fe : Mg : Ca) SiOy. 



Thus the mineral appears to have tlie following properties: — 



Chemical composition Silicate of aluminium, potassium, iron, 



and magnesium. 



Specific gravity About 2-40. 



Eefractive index About equal to that of Canada balsam. 



Double refraction Very high; elasticity greater along the 



shorter axis. 



Shape Lath-like. 



Extinction Straight. 



