part 4] study of the st. austell gkaxite. 565 



the rock is porphyritic with a ground-mass of finer grain, this fact is 

 brought out at once by the frequency method (for two modes are 

 shown on counting), whereas an arithmetical mean would conceal it. 

 The quartz-grain so determined is tabulated for each locality in the 

 bottom row of Table IV (facing p. 552) as ' grain in mm.' 



When these values for the quartz-grain of the rock are mapped 

 (fig. 9, p. 55S), it becomes evident that the different areas have 

 different characteristics. Specimens from the Luxullyan area 

 show grain that may be more than twice the size of that noted 

 elsewhere. Its general value is about 2-5 mm., but marginal 

 specimens jdeld somewhat smaller values. In the Hensbarrow area 

 the grain is more A'ariable, and alwaj^s much smaller. Its general 

 value may be taken as 1 mm. ; but, especially towards the margin 

 of the granite-outcrop, it is still smaller. The St. Stephen's area 

 differs from the others, for the grain is sensibly uniform through- 

 out ; the marginal specimens do not differ from the more central. 

 The general value of the quartz-grain in this area may be taken 

 as 15 mm. The grain in the Retew area has a low value, and is 

 about 0*5 mm. The grain, then, is coarser in the central parts, 

 and finer towards the margin of the granite-mass. 



Several types of structure are to be distinguished. There is 

 the type which has been convenientl}' called ' granite-porphyry ', 

 found in the Luxullyan area, with large perthite-phenocrysts 

 and a coarse hypidiomorphic ground-mass. Secondly, we note 

 the usual granitic structure with small sporadic phenoerysts, 

 characterizing the larger portion of the remaining areas. In 

 addition to these, and always marginal, appears a porphyritic rock 

 with a microgranitic ground-mass : as, for example, at Chytane, 

 Hensbarrow Beacon, and near Trenance Viaduct. The phenociysts 

 comprise all the minerals, and are of the same grain as is found 

 in the central parts of the area ; while the ground-mass, consisting 

 chiefly of quartz and felspar, is much finer. Associated with this 

 type in the Trenance district are thin bands made up 'almost 

 exclusively of elongated perthite-phenocrysts, showing flow- 

 structure, and generally, but not invariably, arranged parallel to 

 the margin. 



The order of separation of the minerals has been elucidated by 

 Dr. Flett, and I have little to add to his notes (op. cit. p. 58). 

 So far as the normal minerals are concerned, it is (1) iron-ores 

 and apatite ; (2) micas ; (3) plagioclase ; (4) orthoclase ; and 

 (■') i quartz. The observer is, however, struck by the astonishing 

 degree of overlap in the periods of crystallization of the minerals. 



Dr. Flett also showed that the pneumatolytic minerals — tourma- 

 line and topaz — were at least largely primary, and exhibited long 

 periods of crystallization, beginning shortly after the biotite. The 

 bulk of these minerals appears to me to have separated just before 

 the main crystallization of quartz. Seven cases of idiomorphic 

 tourmaline were noted, and of these three were idiomorphic to 

 orthoclase and four towards quartz. Finally, the minerals of the 



ntact-group commenced shortly after the micas, 



CO 



