ELEPHANT AND CLARENCE GROUP 85 



interwoven flakes of sericite, and folia made up of large crystals of green pleochroic chlorite with 

 ' ultra-blue ' polarization colours. In some of the intervening folia of quartz are remarkable ' trails ' 

 consisting of small euhedral crystals of zoisite, strung out as a line of separate crystals, or occurring 

 in small clots. Both the slide and hand specimen of this rock show that it has been permeated by 

 vein quartz which has separated and isolated the individual folia. 



A third type is rich in epidote. It shows alternating folia consisting (i) largely of quartz with 

 subordinate albite and calcite, but carrying films or thin folia of chlorite and epidote, and scattered 

 crystals of the same two minerals, and (2) mainly of chlorite flakes interwoven with epidote grains. 

 Sericite may form a notable constituent of these folia, but quartz only occurs as scattered fragments. 



The most feldspathic type is a comparatively coarse schist consisting of more or less rounded 

 grains of albite, intermingled with smaller grains of quartz and patches of calcite, forming a mosaic 

 through which wind streams of flakes of chlorite and sericite, together with grains of epidote and 

 zoisite, and interwoven folia of these minerals. The albite is fresh and water-clear and is mostly 

 untwinned, but a few crystals show simple twinning or the more usual albite twinning. Many of the 

 albites contain curving lines of inclusions of the above minerals, suggesting their growth by accretion 

 during shearing as in the well-known case of ' snowball ' garnets. This rock closely resembles the albite 

 schists of the south-western Highlands of Scotland.^ 



As a whole the series of schists from Gibbs Island closely resembles those of Elephant Island and 

 Clarence Island, especially those of Minstrel Bay, but they are coarser, somewhat more highly 

 metamorphosed, and do not possess the abundant carbonaceous matter of those rocks. 



Dtmite-serpentine. The least altered rock and the only one that contains unaltered olivine, is the 

 specimen which was collected from the south coast of Narrow Island. All of the serpentine rocks 

 collected show signs of intense shearing. They are, in fact, serpentine-schists of apple-green and 

 malachite-green colours and ornamental appearance. Some of the specimens show opaque patches, 

 streaks and veins of a black metallic mineral which turns out to be magnetite. 



The Narrow Island rock must have consisted almost entirely of olivine crystals, but it is now made 

 up of olivine fragments in a mesh of serpentine. The only other mineral is magnetite, a little of which 

 may be primary but, for the main part, is undoubtedly of secondary origin. The olivine is a highly 

 magnesian chrysolite with 21=90" and positive sign, and therefore with a FeO content of about 13 per 

 cent. About half of it has been transformed to serpentine or allied substances. The alteration proceeds 

 as usual along the fissures and from the peripheries of the crystals. The first effect of alteration is to 

 produce a pale brownish yellow uncleaved mineral which is of very low birefringence or sensibly 

 isotropic (delessite?), shot through with colourless fibres of positive elongation which may be chrysotile. 

 These areas of delessite(?) and chrysotile roughly outline the original hexagonal forms of the olivine 

 crystals, and enmesh fragments of them. The next stage of alteration produces colourless antigorite 

 in irregular sheaves of platy crystals with negative elongation, which can be seen to be growing at the 

 expense of the areas of delessite(?) and chrysotile, with the liberation of iron oxides in the form of 

 ragged grains of magnetite. 



In the remaining specimens of serpentine, all from the south coast of Gibbs Island, the alteration 

 is completed. Not a trace of olivine is left, nor of delessite (?) and chrysotile. The whole rock consists 

 of antigorite in closely woven felts of plates and prisms, with irregular ragged strings of magnetite 

 which have sometimes segregated into definite secondary veins about i mm. thick. The shearing to 

 which the rocks have been subjected has caused the reformation of the antigorite along the major 

 lines of movement, often with a superposed cross-lamellation. With a more severe crushing stress, 



1 A. Harker, Metamorphism, 1932, p. 213. The rock figured on this page (fig. 95 A) strongly recalls the microscopic 

 appearance of the Gibbs Island rock. 



