part 4] study or THE ST. AUSTELL G-BAttlTE. o6i 



at St. Denis, where it occurs in small grains which arc always 

 pleochroic. Some corclierite is present, and this is apparently a 

 new locality for andalusite, 



Cordierite of the well-known habit now associated with this 

 district is confined to the Luxullyan area. There is no fresh 

 cordierite, nor any similar pseudomorphs, outside the limits chosen 

 for this area ; hut it is present at every locality within the area. 

 In the Hensharrow area, however, there occur small six-sided or 

 rectangular grains of cryjrtocrystalline micaceous material, re- 

 sembling pinite from the Vosges and elsewhere. These pseudo- 

 morphs are not similar in shape to the topaz-grains, nor is the 

 structure that of mica-replacements after topaz. These small 

 pseudomorphs, therefore, have been somewhat doubtfully measured 

 as ' cordierite ' ; but their amount is generally quite small. 



Pneuinato lytic minerals. — The tourmaline found in the 

 slides has the characters and relations of the type generally 

 believed to be original. There is, in addition, a little obviously 

 secondary tourmaline, in the form of prussian-blue needles and 

 grains. Colour-zoning is sometimes observed, either as different 

 tints of the same colour, or occasionally as zones of brown and 

 blue. The sequence of the bands follows no general law, and 

 shows no rhythm. The crystals are rarely idiomorphic, and then 

 only towards quartz. Prismatic sections, whether of primary 

 prism or of secondary needles, always show a rough basal parting. 

 When secondary tourmaline is replacing mica, this parting bears 

 no relation to the direction of the mica-cleavage, nor is there any 

 structure present, such as Mr. D. A. MacAlister observed at 

 St. Agnes Head, representing it. 1 Haloes are often present in 

 the tourmaline, but are far less numerous than in the associated 

 coloured mica. 



1*8 per cent, of tourmaline is present in the general average. 

 It is distributed in belts similar to those of quartz and orthoclase. 

 Primary tourmaline is totally absent from the St. Stephen's area, 

 although there are a few occurrences of secondary needles. The 

 Hensharrow area is occupied by a positive belt ; but in the Luxul- 

 lyan area the tourmaline is generally below the average. 



The percentage amount of topaz in the general average is 1"4, 

 and its distribution presents a distinct relation to the granite- 

 margin, for the positive groups are arranged as small festoons 

 along the outcrop. However, there is not a complete marginal 

 zone, for negative values occur in places along the margin (fig. 7, 

 p. 558). In the Luxullyan area topaz is practically absent, but 

 high values are found in all the other areas. 



F 1 u o r i t e, except in traces, is confined to the western part of 

 the St. Austell granite, and in the general average amounts to 

 U - 4 per cent. Now, all localities containing over 1 per cent, of 

 tluorite, with the single exception of No. 30, are to be found within 



1 ' The Geology of the Country near Newquay ' Mem. Geol. Surv. 1906, 

 p. 138. 



