part 4] MlCEOMKtEtO STTTIir OF THE ST. AUSTfiLL GKA^ITE. 559 



and lletew areas. It is distinguished in general appearance from 

 the biotite by a paler colour, with a reddish-yellow rather than 

 reddish-brown tinge, passing at times into a pale yellow. Pleo- 

 chroic haloes are less numerous, and, further, the lithionite, even 

 in the most weathered surface-specimens, is never chloritized. 

 The colour in thin section shows every gradation from examples 

 with moderately deep tints to those that are completely colourless. 

 Sometimes merely a faint flush is discernible as we rotate the 

 polarizer, and in other cases the only signs of colour are pleochroic 

 haloes and irregular pleochroic areas. The flakes may be bleached 

 at the margins, or the colour may have a very patchy distribution. 

 Some examples are clear and limpid, while others have a dirty or 

 dusty appearance, whether because of minute inclusions or of 

 some kind of alteration I was unable to determine. The apparent 

 axial angle has a wide range of values, and appeal's to decrease 

 with depth of colour. The more deeply coloured flakes give an 

 angle of about 30°, which increases up to 54° in a variety colourless 

 in thin section ; while the pale varieties give angles between 40° 

 and 46°. The angle of lithionite, according to Dana, is 50°. 



The mica within the Hensbarrow area, except for small amounts 

 of secondary muscovite, is to be regarded as of one kind. The 

 distribution of the more deeply coloured types is not very regular, 

 and all that can be said is that the pale and colourless varieties 

 are more frequent near the margin : as, for example, at Hensbarrow 

 Beacon (No. 18), or at Trenance Viaduct (Nos. 24 to 28). 



True primary muscovite is practically confined to the Luxul- 

 lyan area, where it is intergrown with the biotite. A little occurs 

 associated with biotite at St. Mewan Beacon (No. 32), and also at 

 the contact in Dyer's Quarry, Meledor. 



The St. Stephen's area has a peculiar mica with silvery lustre 

 determined by Dr. Flett as gilbertite. It is colourless, and in 

 thin section no haloes were seen. This is partly, but not altogether, 

 due to the colourless medium, for high -power examination showed 

 that highly refractive inclusions were scarce. The apparent 

 axial angle was small when compared with that of muscovite. In 

 general most good cleavage-flakes gave values between 23° and 

 30°, but values as high as 45° were obtained in some flakes from 

 Tregargus Mill. 



The distribution of the types has already been considered, and, 

 owing to their variety, the total quantity of mica present without 

 regard to type has been plotted on the map (fig. 6, p. 558). On 

 grouping the values in relation to the general average of 9*2 

 per cent., 1 discovered one large positive belt, stretching from the 

 River Fal to Helman Tor, with two very small groups along 

 the south-eastern margin of the granite-mass. Only one excep- 

 tion appears within these groups : namely, No. 27 ; but its diver- 

 gence is not significant. Some relation to the boundary of the 

 granite exists, and the mica-zones are differently orientated from, 

 and cut across the belts of the minerals already mapped. 



Attention has been drawn to fluctuations in the number of the 



