72 DISCOVERY REPORTS 



mineral, with magnetite and apatite as accessories. In one of these rocks the biotite is interleaved with 

 narrow lenticles of a colourless mineral of high refraction and birefringence, straight extinction, 

 and good cross-fracture, which is doubtfully identified as sillimanite. The remaining three tonalites 

 have a considerable amount of green hornblende in addition to biotite, and sphene is a rather abundant 

 accessory. One of these rocks, however, has a well-marked granulose structure, and the irregular 

 grey-green plates of hornblende are spotted with rounded inclusions of quartz and feldspars. This is 

 the 'sieve structure' which is often taken as a sign of hybridism. 



The diorite family is represented by eight rocks of which six are typical quartz-mica-diorites, 

 consisting of plagioclase (oligoclase to andesine), hornblende, and biotite, with a small residuum of 

 quartz and occasionally a little orthoclase. Magnetite and apatite are the most important accessory 

 minerals, and the apatite often occurs in some abundance as comparatively large crystals. Pyrites, 

 epidote, and chlorite occur as secondary minerals, the two last-named being the products of alteration 

 of feldspar and biotite respectively. The six quartz-diorites vary among themselves within narrow 

 limits in the proportions of dark to light minerals, and in the relative amounts of hornblende and 

 biotite. 



The seventh quartz-diorite is distinguished from the above-described by containing a notable 

 amount of colourless augite, which occurs in small clots or segregations with hornblende, biotite, 

 magnetite, and apatite. It is therefore a quartz-mica-augite-diorite of a type approximating to 

 Stelzner's 'andendiorit' from the Argentinian Andes. The eighth rock assigned to the diorite group is 

 a micro-diorite of very fine grain and uniform, allotriomorphic granulose texture, consisting of andesine 

 and green hornblende in about equal quantity. A small amount of biotite is involved with the horn- 

 blende as well as a notable quantity of apatite and magnetite, and there is also a small residuum of 

 quartz. This rock may be regarded as a mesocratic quartz-micro-diorite which shows affinity to the 

 malchite of Osann.^ 



Only one of the stones in this collection falls in the gabbro family. It is a medium-grained rock 

 consisting of plagioclase, probably labradorite, but now intensely altered with the production of 

 aggregates of epidote and unidentifiable turbid matter; pale augite, and an almost equal amount of 

 faintly pleochroic hypersthene which is largely altered to chlorite. A little brown hornblende occurs 

 as an alteration product of the augite. Magnetite and apatite constitute the only accessory minerals, 

 together with a small residuum of quartz. This rock may therefore be described as quartz-hypersthene- 

 gabbro or quartz-hyperite. It is probably to be correlated with the quartz-gabbros of the Jenny 

 Island group off the south-eastern coast of Adelaide Island. - 



The three lamprophyres in the collection all belong to the spessartite group, and consist essentially 

 of green hornblende and andesine with typical panidiomorphic texture. The hornblende is somewhat 

 in excess of the plagioclase. One of the rocks contains numerous phenocrysts and crystal aggregates 

 of hornblende in the lamprophyre ground-mass. Another contains patches of a pale bleached biotite 

 and of pale green chlorite, with a few micro-phenocrysts of feldspar. The third has much chlorite 

 and magnetite, and its hornblende is mostly of the brown variety. All these rocks carry a small 

 residuum of quartz. This group of lamprophyres appears to be abundant in the Graham Land 

 peninsula and the adjacent archipelagos. 



We now come to the most interesting and important group of stones from Adelaide Island, namely, 

 the acid volcanic rocks, including rhyolite, dacite, and igneous breccias which contain a variety of acid 

 types. The breccias consist mainly of quartz-porphyry fragments which have suffered cataclastic 



1 Osann-Rosenbusch, Elemente der Gesteinslehre, 4th ed., 1922, p. 321. 



2 E. Gourdon, ' Sur la constitution mineralogique de I'lle Jenny (Antarctica)', C.R. Acad. Sci., Paris, 159, 1914, 

 369-71. 



