part 4] MICROMETRtC STUDY OE THE ST. AUSTELL GEA>ITE. 573 



the Avhole area, except a narrow strip along the western margin, is 

 covered by a positive mica-area. 



The Luxullyan area of biotite-muscovite-granite has a well- 

 marked central orthoclase-zone, as is shown by the distribution of 

 perthite-phenocrysts alone. This is surrounded by a marginal 

 area characterized by quartz and plagioclase ; but the actual 

 marginal specimens show an exceptional richness in biotite. Still 

 more detailed work is desirable in this area, and along its junction 

 with the rock on the west. 



The rock at St. Mewan Beacon, besides containing biotite and 

 microcline, is rich in plagioclase, which shows a distinct tendency 

 to zoning. The plagioclase of the muscovite-biotite-granite at 

 Carnsmerry also presents many of the features of that of the 

 Luxullyan area. At Stenalees, in the Hensbarrow area, a very 

 felspatbic occurrence of the lithionite-granite is quarried as for 

 china- stone — the only example outside the St. Stephen's area. 



Fig. 10 (p. 570) is a space-variation-diagram, or quantitative 

 ' section ' of the map from west to east, and shows the changes 

 over the district in another way. It is drawn by having regard to 

 the areas of the map and the values at adjacent points. The chief 

 features to notice are the gradual changes in passing from the 

 gilbertitc to the lithionite areas, and the marked tendency to 

 discontinuous change in passing over from the lithionite- to the 

 biotite-granite. 



The general quantitative arrangement in the areas, then, follows 

 much the same plan. There is a more or less central area rich in 

 orthoclase, always in large plates and often as conspicuous pheno- 

 crysts. In two of the areas there is a central core of plagioclase, 

 and in all an outer quartzose zone. Despite peculiarities, the 

 micas tend to concentrate at the margins, and in the western 

 portion coincide with zones which have a high content of 

 pneumatolytic minerals. The zonal axes are also parallel to the 

 direction of the china-clay pits and chief lodes : that is, parallel 

 also to directions of later pneumatolytic activity. 



VII. History of the Intrusion-Phenomena. 



Now, one feature demanding preliminary attention is the 

 peculiar microstructure of the marginal phase in the lithionite areas. 

 It has a porphyritic character, in which the phenocrysts include 

 all the minerals ; Avhile the ground-mass is always fine-grained, 

 and usually microgranitic or aplitic in texture. Moreover, the 

 minerals of the ground-mass are chiefly orthoclase and quartz. 

 A careful study oh' these types leads to the conviction that the 

 porphyritic elements had already crystallized before intrusion, and 

 bad drifted into position ; while the ground-mass represents the 

 rapidly chilled mother-liquor in which they floated. Thus two 

 periods of crystallization are indicated : one pre-intrusive, and the 

 other post-intrusive. 



The granite of the Luxullyan area is more nearly a normal 



