part 4] STUDY OP THE ST. AtTSTElt &RAKITE. 571 



the development of central positive areas that almost coincide 

 with the negative quartz-zones. The St. Stephen's and Hens- 

 barrow areas have zones, elongated in the direction of their 

 axes, of high plagioclase (fig. 5, p. 550), but the distribution of 

 the plagioclase in the other areas is somewhat indefinite. It 

 may also be noted that a marked positive region of tluorite 

 practically coincides with the positive plagioclase zone of the St. 

 Stephen's area. Lastly, the micas (fig. (j, p. 558) call for special 

 mention. The second-order grouping does not materialhy differ 

 from the first, but is remarkable for cutting across the other 

 zones, and showing a relation with the margins of the whole mass 

 rather than with its subdivisions. 



In order to present as clearly as possible the variation of the 

 St. Austell granite-mass, fig. 11 (p. 572) has been prepared by 

 a careful review of the lines and points of previous maps. The 

 district is divided into areas characterized by quantitative and 

 qualitative differences. There are, first, the three main areas with 

 different granite-types, about which much has been said already. 

 The quantitative subdivision relates entirely to the averages for 

 these types. Where the name of a mineral is written within a 

 boundary, that area contains the mineral in excess of the mean for 

 its type ; and, in addition, positive areas of quartz are shaded 

 by lines sloping differently for regions occupied by the different 

 granite-t} r pes. Zones with a high content of pneumatolytic 

 minerals are indicated by their axes, and a similar device is adopted 

 to show the narrow zone rich in apatite. Lastly, the two small 

 occurrences of biotite-granite are marked by the letters A (St. 

 Mewan Beacon) and B (Carnsmerry). 



The small Retew area of lithionite-granite shows a tendency to 

 have a central felspathic zone, surrounded by a quartzose zone ; 

 but the points are rather few. 



The larger Hensbarrow area of lithionite-granite shows a central 

 felspathic region, consisting mainly of plagioclase in its northern, 

 and of orthoclase in its southern part. The orthoclase-rich 

 portion extends down to the southern margin of the granite, where 

 thin bands of elongated phenocrysts occur. Surrounding this 

 felspathic zone is a marginal one rich in quartz. In addition, 

 there are two marginal bands rich in mica : the larger along the 

 north-western margin, and the smaller along the extreme south- 

 eastern margin of the area. Each of these includes the axis of a 

 high pneumatoPytic zone, generally parallel to its own direction ; 

 and, finally, the northern zone includes a long narrow strip rich in 

 apatite, elongated in the same general direction. 



The St. Stephen's area of gilbertite-granite, largely quarried for 

 china-stone, has a well-defined concentric arrangement. The 

 centre is occupied by a positive orthoclasc-zone, surrounded by a 

 quartzose one, while there is a very narrow but central area 

 extremely rich in plagioclase (albite) and fluoritc. Practically 



