566 MR. W. A. RICHARDSON : A MTCBOMETRTC [vol. lxxix, 



Little difference in the order of separation can lje detected 

 between the different areas. The same order is general throughout 

 the mass, except in the case of apatite, and, perhaps, zircon. In 

 the St. Stephen's area the apatite occurs in large irregular grains, 

 and is more abundant than is usual in granites. These plates are 

 found moulding felspars, or gathered into irregular aggregates 

 formed later than the felspars. Zircon does not appear to be so 

 common in this area, and is oftenest found in quai'tz. 



Slight strain-effects are shown in practically every slide. The 

 quartz always has some strain-shadows, except, as stated above, 

 when enclosed in orthoclase. In this case the host doubtless 

 protected the inclusion from pressure. Moreover, quartz-grains in 

 aggregates commonly have suture-structure developed. Secondary 

 quartz, however, is always water-clear, and never shows these 

 phenomena. Finally, mica-flakes are occasionally bent. It is 

 difficult to estimate the strength of the strain-effects ; but, by 

 comparison of slides, the amount of strain appears to increase 

 towards the margin and to be least in the Luxullyan area. 



VI. The Rock-Types and their Distribution. 



Earlier in this paper the granite-mass was divided into four 

 areas, which had been found to correspond qualitatively and 

 quantitatively to different rock-types. This will have been more 

 or less apparent in the preceding discussion, and it is now 

 necessary to consider these rock-types and their relations. The 

 rocks have been primarily separated by means of the dominant 

 types of mica present into (a) biotite - muscovite - granite, 

 (b) lithionite-granite, and (c) gilbertite-granite. The mean com- 

 position of these types will be found in columns II, III, & IV 

 respectively of Table V (p. 554). 



(a) Biotite-muscovite-granit e . — This type is characterized 

 by a biotite, with deep red-brown colour, strong pleochroism, and 

 small axial angle. It is somewhat chloritized, and always inter- 

 grown with muscovite. In the main granite-mass this type is 

 confined chiefly to the Luxullyan area. The apatite occurs in 

 small, clear, idiomorphic grains. The plagioclase is an acid oligo- 

 clase in large strongly-zoned crystals with intensely altered centres, 

 while the perthite is fresh. Brown tourmaline is present in some 

 abundance, but there are only traces of topaz and fluorite. 

 Contact-minerals are represented by abundant cordierite, associated 

 with a little andalusite. The rock is coarse-grained, coarsely por- 

 phyritic, and is noteworthy for crowded pleochroic haloes around 

 zircons enclosed in the biotite and, to a less extent, in the tourmaline. 



Besides the Luxullyan l-ock there are two very small areas of 

 biotite-granite within the Hensbarrowarea : namely, at St. Mewan 

 Beacon and Carnsmerry. These rocks are of finer grain and 

 practically non-porphvritic, 



