part 4] study or the st. aiistell granite. 555 



mass is 32'6. If boundary-lines are now drawn between those 

 localities with quartz in excess of this amount and those with 

 values below it, it will be found that the quantities are not dis- 

 tributed in haphazard fashion, but fall into very definite groupings 

 shown in the map, fig. 2 (p. 548). The boundaries are drawn in 

 thick chain-dotted lines, and there are only four exceptional 

 places. Of these Nos. 19, 25, and 50 cannot be regarded as real 

 exceptions, since the differences are well within the limits of 

 sampling ; while No. 6 on Helman Tor would probably conform 

 if allowance had been made for the presence of phenocrysts of 

 orthoclase. 



Now, none of the areas so delineated show any relation to the 

 boundaries of the granite-outcrop. The mass is divided into a 

 series of broad belts, exhibiting a rough parallelism with axes 

 directed more or less west of north. There are three positive areas 

 alternating with negative areas. The first positive zone occupies 

 the Retew area, the second lies at the western end of the Hens- 

 barrow area, while the third occupies the eastern end of this area 

 and the south-western part of the Luxullyan area. The whole of 

 the St. Stephen's area is a negative belt. 



Orthoclase. — Several kinds of monoclinic felspar are probably 

 present in the granite ; but the dominant type is perthite, of which 

 the large phenocrysts are formed. In the Luxullyan area the 

 albite patches are relatively large, and the albite lamellae may be 

 distinguished without difficulty. In the Hensbarrow district, 

 where not obscured by alteration, the albite patches are finer, 

 while in the St. Stephen's area the orthoclase often appears to be 

 more homogeneous. The orthoclase includes and moulds most of 

 the other minerals, and a long period of crystallization is indicated. 



Greenish anorthoclase was discovered in the granite of Gready 

 by Mr. T. C. F. Hall. 1 I have not found this mineral in speci- 

 mens from other quarries. At St. Mewan Beacon, however, 

 associated with the perthite is a felspar showing the nearest 

 approach to the cross-hatching of microcline that I have found in 

 the area. The sections of this mineral are much more irregular in 

 shape than the perthite, but the cross-hatching is of finer grain 

 and less regular than that in good microcline, although more so 

 than that in the anorthoclase at Gready. 



The orthoclase, even apart from surface-weathered specimens, 

 has always suffered some degree of alteration. In the Luxullyan 

 area this does not amount to more than a slight cloudiness. In 

 the Hensbarrow area, on the other hand, the orthoclase is almost 

 opaque by ordinary light, and snow-white by reflected light, on 

 account of extensive alteration. In the St. Stephen's area the 

 orthoclase may sometimes be fairly fresh : it is often, however, ex- 

 tensively altered, as Dr. Flett noted, sometimes to kaolin, but more 



1 'Report of an Excursion to the St. Austell District (Cornwall)' Proc. 

 Geol. Assoc, vol. xxvi (1915) p. 40 & pi. v, fig. A. 



