part I] STUDY OF THE ST. AUSTELL GBANITJE. 567 



(h) L ithio n it e- granite. — If we except a little secondary 

 muscovite, the only mica present is pale lithionite. In thin section, 

 its colour varies from a medium reddish-brown to colourless. The 

 apparent axial angle is variable and generally large, but often 

 much smaller, approaching that of the biotites. Marginal 

 bleaching occurs, but the mineral is never chloritized, while the 

 pleochroic haloes are much less numerous than in the biotite of the 

 Luxullyan area. The microperthite is altered. The plagioclase 

 has extinction-angles indicating a composition not far removed 

 from albite, but with some content of lime-felspar. It is never 

 zoned. Topaz is abundant, especially near the granite margins, and 

 brown tourmaline is present in most localities. Apatite occurs in 

 small grains, often clear, but sometimes crowded with inclusions. 

 It is found embedded in plagioclase rather than in biotite. 



The whole of the Hensbarrow and Retew areas are occupied by 

 this type, and the rock exposed on Belowda Beacon is similar. 

 The grain is of medium size, passing into more finely granular, 

 and even into microgranitic types towards the margins. 



The granite of Hensbarrow Beacon itself has been mapped by 

 the officers of the Geological Survey as a separate (ine-grained 

 intrusion, analogous to the Hue-grained later intrusions in other 

 Cornish granites. They were unable to trace the boundaries 

 exactly. My further examination of the locality reveals a rock 

 that does not differ quantitatively or qualitatively from those of 

 immediately neighbouring localities, but which (in grain) gradually 

 passes into that of the central rock. Moreover, unlike the fine- 

 grained intrusions of (for example) Carmenellis, it occupies a 

 marginal situation, and is similar to other marginal types met 

 with near Trenance and elsewhere within this area. I, therefore, 

 consider it to be a fine-grained marginal facies. 



(c) Grilbertite-granite. — This type is found between the 

 boundaries which I have laid down as defining the St. Stephen's 

 area, and which practically coincide with Mr. J. A. Howe's limits 

 for the occurrence of ' china-stone '- 1 This granite is, in fact, the 

 china-stone rock for which J. H. Collins proposed the name of 

 ' petuntzyte '. 2 The mica exhibits the microscopic characters of 

 gilbertite, is colourless but strongly absorptive in thin section, and 

 has a smaller apparent axial angle than muscovite. The orthoclase 

 is perthite, but often considerably altered. The plagioclase gives 

 the extinction-angles of pure albite — a determination confirmed by 

 the mineral composition calculated from analysis (see below). 

 The apatite occurs in large grains, is almost opaque with inclusions, 

 and crystallized out at a relatively late stage. Fluorite in irregular 

 grains lias a mean value greater than 1 per cent, in all localities. 

 Topaz is generally present, but primary tourmaline always absent. 



1 See map, fig 1 . 2. ' Handbook to the Collection of Kaolin, &c.' Mem. Geol. 

 Surv. 1914. p. 11. 



2 ' The Hensbarrow Granite District' Truro, 1878. 



Q. J. G. S. No. 310. 2 q 



