part 4] STUDY OE THE ST. AUSTELL GRANITE. 557 



of andesine, varying outwards to that of pure albite. The bulk 

 composition of these zoned felspars is probably that of a somewhat 

 acid oligoclase. 



The second type of plagioclase occupies the remainder of the 

 granite-mass. It has a mean grain considerably smaller than 

 the Luxullyan plagioclase ; it is not zoned, and, when altered, the 

 products are distributed uniformly. The extinction-angles are 

 greater, and on the whole increase westwards. A felspar con- 

 taining but a very small proportion of lime is indicated, and in the 

 St. Stephen's area angles approaching those of pure albite were 

 frequently obtained. 



The chief alteration-product appears to be mica. The Lux- 

 ullyan crystals have cpiite fresh margins, and the mica-flakes can 

 lie detected in abundance in the central zone of intense alteration. 

 In the Hensbarrow area the plagioclase is generally cloud}', and 

 mica-flakes appear parallel to the directions of cleavage ; but the 

 plagioclase is much less affected than the orthoclase of the area. 

 In the St. Stephen's area the plagioclase is often very fresh, and, 

 when it is altered, mica again plays a part. 



Plagioclase amounts to 19"2 per cent, in the general average. 

 After grouping the values on a ma]) (fig. 5, p. 556) we find two 

 zones in which plagioclase is above the average. The westernmost 

 of these lies entirely within the St. Stephen's area, coinciding with 

 a belt of low quartz, and with part of a belt of low orthoclase. 

 The second zone forms a narrow elongated strip immediately east 

 of Hensbarrow. At St. Me wan Beacon (No. 32) the variation 

 is greater than the probable limits of sampling, and may be 

 significant. 



Micas. — The micas are, in some respects, the most interesting 

 minerals. They have been tabulated in two classes — coloured and 

 colourless — according to their appearance in thin section. There 

 are at least four species present, which will now be described under 

 the names biotite, lithionite, muscovite, and gilbertite. These 

 varieties (as will be shown below) have a definite areal distribution. 



In addition to their general characters, the apparent axial angle 

 (2 K) was determined in all suitable sections, and on cleavage-flakes 

 from the hand-specimens. The angles obtained were very variable, 

 even in good basal sections among the same types of mica ; but 

 each type had a fairly characteristic range of values. 



The biotite is confined to the Luxullyan area, with two 

 exceptions : namely, Carnsmerry (No. II) and St. Mewan Beacon 

 (No. 32). The biotite has very strong absorption, a deep reddish- 

 brown colour, and is crowded with pleochroic haloes. In every slide 

 there are always some crystals partly chloritized. The apparent 

 axial angle in the cleavage-flakes and sections varies from 12° up 

 to 15°. In the absence of chemical evidence it is, of course, not 

 denied that this mineral may be lithia-bearing : but its optical 

 characters most closely resemble those of ordinary granitic biotite. 



The lithionite type of mica is confined to the Hensbarrow 



