51 



andalusite and sericite. It appears to be of quite different 

 origin from the ordinary biotite of the rock. 



(h) Cordierite Mica Schist. — There is a considerable 

 development of knotted schists at intervals to the west of 

 Rosetta Head. The knots are often very abundant, and may 

 be upwards of half an inch in diameter. The schists, too, may 

 be corrugated, and the transition from smooth to knotted or 

 corrugated schists is often quite sharp. Some of the knotted 

 varieties are, as shown above, andalusite-bearing, but in a 

 specimen collected about half-way round towards King Point 

 the knots prove to be cordierite. On the weathered face they 

 show up of a brownish-yellow colour in roughly elliptical 

 sections, with slight traces of an orientation parallel to the 

 schistosity. The knots are very numerous, occupyino^ about 

 half the surface on a face cut perpendicular to the schistosity. 

 In thin section the ground fabric of the rock is seen to 

 be of the usual type; there is well-marked schistosity indicated 

 by parallel layers of biotite separated by finely granular 

 quartz. The knots are really individuals and aggregates of 

 medium to large xenoblasts of cordierite, the aggregates com- 

 prising anything up to half a dozen intergrown or interlocking 

 grains, and measuring up to about 12 mm. in greatest 

 length. The external boundaries of the minerals are as 

 a rule indistinct, being frayed out into the ground fabric and 

 indented by biotite. Sometimes the schistosity folds round 

 the cordierite, and in one case it is much dented by the 

 porphyroblast ; here the enfolding biotite is thickly studded 

 with pleochroic haloes and has an unusual concentration of 

 iron ore. 



The grains of cordierite are crammed with inclusions, 

 most of them colourless, and so extremely tiny that their 

 identification is generally impossible. They seem to be mostly 

 quartz, but zircon is recognized by the characteristic yellow 

 pleochroic haloes; in addition there are inclusions of 

 muscovite, biotite (often bleached), and iron ore, and these 

 larger grains occasionally carry the schistosity direction 

 through the porphyroblast. The cordierite is stained yellow 

 in places, but strangely enough shows no trace of the very 

 usual alteration to pinite. 



As regards the ground fabric, in addition to biotite and 

 quartz it contains muscovite and chlorite, both of which are 

 in crystals larger than the biotite. They are mostly found 

 round about the cordierite, and sometimes associated with the 

 little veinlets of quartz containing small prisms of apatite. 

 Granules and tiny crystals of iron ore are fairly plentiful, but 

 are most abundant in the cordierite, suggesting their deriva- 

 tion by the dissociation of biotite. The rock is also 



