180 DR. C. E. TILLEY ON THE PETROLOGY OE [vol. lxxix, 



abnormal interference-tints (indigo blue to brown). There is no 

 reason to believe that any of this chlorite is secondarily derived 

 from metamorphic biotite in situ. The stage of typical biotite 

 development in the argillaceous rocks of this area has not been 

 reached. 



Usually albite is sparingly developed in these rocks, associated 

 with quartz, and distinguished by its cleavage and refraction, and 

 biaxial optically positive character. When it is present in abun- 

 dance, there is clear evidence (from other considerations) that 

 a rock of composite origin is involved. 



Albite figures in a remarkable mass of mica-schist in the Bolt 

 area near Starehole Bay, and Bolt Head. At this point the mica- 

 schist is found to contain knots of a black well-cleaved mineral, 

 forming porphyroblasts in a quartz-chlorite-muscovite ground- 

 mass. Thin slices of these rocks show that these porphyroblasts 

 are albite, the blackness being due to carbonaceous inclusions. 1 

 These carbonaceous inclusions form zonal bands in the albite, and 

 are associated with minute highly-refringent prisms of rutile. 

 The rutile is often abundant in the porphyroblasts themselves, 

 and good examples of the sagenite web occur. Albite-twinning 

 is developed in broad but irregular bands. The zonary structure 

 induced by the carbonaceous swarms is not infrequently irregular, 

 sinuous, and contorted, and the carbonaceous particles are also 

 found in the muscovite- chlorite layers. 



The remaining minerals of the mica-schists are developed as 

 accessories. Titanite in minute wedge-shaped grains is not 

 uncommon, and a like remark applies to tourmaline, usually 

 showing pleochroism in brownish-green to brown tints. The 

 iron -ores include magnetite, often showing alteration to haema- 

 tite, secondary limonite, and pyrites. Porphyroblasts of ilmenite, 

 now largely leucoxenized, occur in a quartz-mica-schist from Start 

 Cove. Zircon may occur with pleochroic haloes in chlorite. 



In the normal mica-schists garnet is a rare constituent; but 

 its presence is noted in a quartz-mica-schist from Seacombe Sands, 

 where it is developed in association with chlorite and muscovite in 

 idioblastic dodecahedra. This mineral is, however, not uncommon 

 in those schists of composite origin which are described below. 



IV. The Green Schists. 

 (a) General Description. 



The distribution and the stratigraphical position of these highly 

 interesting rocks have been dealt with in a previous section of this 

 paper. The lithological and petrographic characters of the rocks 

 remain to be touched upon. 



They are characterized by an almost universal green colour, 

 showing gradations from a yellowish green to a strong deep green. 

 At their junctions this green coloration is often replaced by red 

 and brown tints. This is clearly due to the presence of films of 



1 I believe this mineral to be that mentioned in Q. J. G. S. vol. xliii (1887) 

 p. 724, and there figured (fig. 2) and identified as kyanite. 



