554 



ME. VT. A. RICHAKDSOX : A MICJROMETRIC [vol. lxxix, 



large as 0*02 mm. in longest diameter. The larger inclusion- 

 cavities do not appear to favour definite areas. In the Luxullvan 

 area rutile-needles were occasionally met with, though not detected 

 with certainty elsewhere. More numerous are minute bipyramids 

 of zircon, commonest towards the east, but present in most 

 localities. Various modes of arrangement of the minute inclusions 

 are found. Generall} T they are linear, as in most granitic quartz. 

 In some areas, especially St. Stephen's, they are present uniformly 

 distributed as dust. More rarely, the} T are segregated into wide 

 dense bands, which divide the quartz into irregular polygons. 



Table V. — Mean Mineral Composition of Granites. (Weight pee cent.) 



Mineral. 



I 



II 



Ill 



IV 



V 



VI 



VII 



Quartz 



32-6 



317 



33-8 



29-9 



31-9 



24-4 



22-6 



Orthoclase . . . 



34-5 



37-3 



34-1 



337 



10-2 



362 



197 j 



Microcline , 



tr. 



— 



- 



- 



20-5 



- 







Plagioclase ... 



19-2 



16-5 



18-5 



22-0 



20-2 



33-6 



46'8 



Biotite 



H 



7-6 



— 



— 



11-9 



5-8 



5-8 



Lithionite . 





8-8 



- 



— — 



— 



Muscovite 



4-5 



3"4 



- 



97 



4-7 — 



— 



Hornblende ... 











2-9 



Magnetite 



0-3 



0-4 



0-3 



o-i 



0-4 — 



2-2 



Apatite 



0-3 



0-4 



0-3 



0-4 



0-2 



— 



tr. 



Coi'dierite 



0-3 



1-4 



o-i 



0-2 



— 



— 



Andalusite ... 



tr. 



tr. 



— — 



— 



— 



— 



Tourmaline ... 



1-8 



1-2 



2-4 



0-8 



— 



— 



— 



Topaz 



1-4 



o-i 



1-6 



17 



- - 







Fluorite 



0*4 



tr. 



o-i 



1-5 



. 



— 



I = St. Austell Gi'anite ; mean of all localities and types. 

 II = St. Austell Granite ; mean of biotite-muscovite type. 



III = St. Austell Granite; mean of lithionite type. 



IV = St. Austell Granite ; mean of gilbertite type (petunzite). 

 V = Alkali-granite, Rnbislaw, Aberdeen. 



VI = Biotite-granite, Wasdale Head, Shap (A. Holmes, ' Petrograpliic 

 Methods & Calculations' 1921, pp. 394-95). 

 VII = Hornblende-granite, Mountsorrel. 



The number of inclusions varies enormousby. Xo numerical 

 estimate was made ; but, speaking generally, the quartz is most 

 crowded with them in the St. Stephen's area, and least so in the 

 Luxullvan area. 



The mean percentage weight of quartz for the whole granite* 



