ADELAIDE ISLAND 73 



deformation of the same kind as that described by Quensel from the ' porphyry formation ' of Patagonia 

 and Tierra del Fuego.^ Sixteen stones belong to this group. 



Three specimens appear to belong to the rhyolite-dacite group. One is a dense whitish rock mottled 

 with pale green streaks which exhibit a rough parallelism. In thin section it becomes clear that this 

 is a coarse and even contorted flow-banding of alternating lighter and darker streaks, more obvious 

 when the slide is held up to the light than when it is viewed through the microscope. The rock consists 

 of a quartzo-feldspathic paste of variable but always fine grain, mingled with varying quantities of 

 sericite and a colourless to palest green, almost isotropic mineral of higher refractive index than quartz 

 or Canada balsam. This mineral occurs in reticulated areas with a flaky, fibrous, or vermiculate structure 

 under polarized light. These properties may serve to identify it tentatively as a variety of kaolinite. 

 Sericite and kaolinite are much more abundantly developed in the darker bands, although they are 

 not absent from the lighter streaks. The only other identifiable mineral is some secondary pyrites. 

 The rock is intersected by thin, thread-like, discontinuous veins of secondary quartz. The flow 

 structure may be primary and the rock therefore a rhyolite; but there is the possibility that it is a 

 pseudo-flow structure like that of the quartz-porphyries or porphyroids described later, and due to 

 cataclastic deformation. The facts that some of the larger quartz grains show undulose extinction, 

 and the considerable development of sericite, may perhaps be regarded as in favour of this view. 



Another rock appears to be the same as that described by Quensel- from Patagonia as 'felsite- 

 porphyry'. This shows small phenocrysts of bipyramidal quartz, orthoclase, and oligoclase, in a 

 largely cryptocrystalline, quartzo-feldspathic ground-mass. There is, however, a large amount of 

 recrystallized quartz forming irregular areas which carry inclusions of ground-mass material, and 

 which impregnate feldspar phenocrysts in their vicinity. Both quartz and feldspar phenocrysts are 

 euhedral, and the latter enclose large, well-developed crystals of epidote and zoisite. The only coloured 

 minerals present are a few areas of leucoxene representing altered ilmenite, and some secondary 

 pyrites. Veins of secondary quartz traverse the rock and cut through some of the feldspar phenocrysts, 

 but appear to merge into the areas of recrystallized quartz in the ground-mass. This rock is a quartz- 

 felsite or quartz-porphyry which differs from those later described in its comparative lack of alteration 

 and in its much smaller proportion of phenocrysts to ground-mass. Its mineral composition roughly 

 corresponds to that of adamellite or granodiorite, and it might therefore, if a lava, be styled dellenite. 



A third member of this group is obviously a fragmental rock of composition similar to the above 

 except that plagioclase feldspar is much more abundant. It contains numerous angular chips of 

 rhyolitic or dacitic composition in a uniform cryptocrystalline ground-mass of quartzo-feldspathic 

 composition. The rock has been heavily impregnated with secondary pyrites which has stimulated 

 local silicification of the ground-mass. It is best regarded as a dacitic tuff. 



Next come three rocks interpreted as coarse tuffs or igneous breccias consisting mainly of fragments 

 and fine comminuted debris of the rhyolite and quartz-felsite (dellenite) just described. One of them 

 consists mainly of fragments similar in composition and structure to the above rhyolite, but in general 

 of coarser grain. There are nevertheless rapid variations in grain size across barely visible boundaries 

 between adjacent fragments. In fact it was only possible to identify the rock as a rhyolitic breccia 

 through the occurrence of a few angular fragments of a coarse feldspathic type apparently belonging 

 to the granite-porphyry described later. Some of the coarse-grained material may be due to secondary 

 silicification. The two remaining rocks of this group are clearly igneous breccias consisting mainly of 

 fragments of the dellenite above described. 



1 P. Quensel, ' Die Quarz-porphyr- und Porphyroidformation in Siidpatagonien und Feuerland ', Bull. Geol. Inst. Upsala, 

 xn, 1913, pp. 9-40. 



2 Op. cit. supra, p. 14, and fig. 10, p. 27. 



