■360 MR. W. A. RICHARDSON : A MlCHOMETRIC [vol. Ixxix, 



pleochroic haloes. In order to obtain a quantitative idea of the 

 variation, I counted the numbers present in each slide, and, so as 

 to facilitate comparisons, these were reduced to a common basis 

 by expressing them as the number per square millimetre of biotite 

 (or lithionite). The values obtained are given in Table IV 

 (facing p. 552) as 'number of haloes 1 . As might be expected, 

 the distribution is somewhat irregular. When mapped they show 

 no relation to the mica-zones of fig. u' ; nor any striking corre- 

 spondence with depth of colour. They are most numerous in the 

 Luxullyan area, where the biotite is often so crowded with them as 

 to defy enumeration. On the whole, despite some high values, they 

 decrease steadily westwards, being absent in the St. Stephen's area. 

 In the Ketew area they reappear with the lithionite, and are most 

 numerous at Melangoose (No. 50). 



Iron -ores. — These are mainly magnetite, with occasional red 

 patches 'and stains of haematite. Very little pyrites was seen. 

 tn the Luxullyan area the magnetite is well shaped, and occurs 

 in large grains mainly associated with aggregates of biotite. In 

 other areas the grains are smaller, more widely distributed, and 

 less regular in shape. The average percentage is 0*3, and, when 

 the deviations from this are plotted, two high zones are found. 

 One includes the Avhole of the Luxullyan area and some of the 

 localities on the west, and the other is situated immediately west 

 of Hensbarrow Beacon. The St. Stephen's and Eetew areas are 

 deficient in iron-ores. 



Apatite. — Two varieties of apatite are present. There are the 

 usual clear grains of prismatic habit, associated with biotite-aggre- 

 gates in the Luxullyan area, and more commonly with plagioclase 

 in the other areas. In the St. Stephen's area a more uncommon 

 variety is met with : it occurs in comparatively large, irregularly 

 shaped masses showing indications of a cleavage, and has a dusty 

 appearance, or is almost opaque Avith inclusions. It appears to 

 have crystallized, or rather to have finished crystallizing, at a 

 somewhat late stage, since it may be found partly enveloping 

 felspar. Its optical properties agree with apatite, but this deter- 

 mination was confirmed microcheinieally. 



Apatite amounts to 03 per cent, in the general average, and 

 the mapped values indicate two positive areas. One is a long 

 narroAv belt stretching north-eastwards from Kestowrack (No. 40) 

 to Molinnis (No. 9), lying entirely Avithin the large positive mica- 

 zone (fig. 6, p. 558). The other is a small zone in the centre 

 of the Luxullyan area. Except for three isolated values, the 

 remaining localities are beloAv the aA^erage. 



Contact-minerals. — Andalusite occurs sparingly in the 

 Luxullyan area associated with the cordierite, and is described 

 by Dr. Flett. Outside this area it is not found within the main 

 mass. Andalusite is, however, abundant in the satellite mass 



