ADELAIDE ISLAND 7, 



Among the first set of Discovery II material sent me I found a box of stones dredged from St. 599, 

 off the west coast of Adelaide Island at a depth of 203 m. The exact position of the Station is lat. 

 67 08' S., long. 69 o6|' W. Forty-six of these stones were examined and thin sections made. They 

 ranged in size from boulders 9 in. in greatest diameter to pebbles less than i in. across. As these 

 dredgings were taken only a few miles oif the western coast of Adelaide Island near the central point 

 of the western coastline, it is likely that many, if not all, were derived from this geologically unknown 

 land. 



Ten of the stones belong to the granite family, including ordinary granite, granophyre, granodiorite, 

 and tonalite. Eight are quartz-diorites, three dioritic lamprophyres, and one quartz-gabbro. No fewer 

 than fourteen of the specimens are quartz-porphyries or allied rocks, all of which show signs of 

 crushing and brecciation, in extreme cases reducing them to ' porphvroids ' and even to types which 

 might be regarded as metamorphic quartzite. Five of the stones are lavas, including rhyolite, dacite 

 (or dellenite), and andesite. Finally, the collection includes five andesitic breccias similar to those 

 which have been described from other parts of the Antarctandes. 



One of the two true granites consists of a coarse-grained allotriomorphic mixture of quartz, micro- 

 perthitic orthoclase, and somewhat less abundant albite-oligoclase which is much more heavily dusted 

 with clayey alteration products than the orthoclase. The sparse ferromagnesian constituents are 

 mainly chloritized biotite, and there are a few crystals of fibrous hornblende. 



The second granite, like the first, is of a pale flesh-pink colour, but is of finer grain and obviously 

 richer in dark constituents. The feldspars consist of micro-perthitic orthoclase and oligoclase (Ab,o) 

 in roughly equal quantity. The oligoclase frequently forms well-shaped crystals which are enclosed 

 in the larger plates of orthoclase. Both feldspars tend to be poikilitically enveloped in a mosaic of 

 large grains of quartz, and both exhibit coarse intergrowths with quartz. The chief ferromagnesian 

 constituent is biotite which is mostly chloritized. With abundant magnetite, sphene, and apatite, the 

 chloritized biotite mainly occurs in small clots or segregations which appear to be of cognate origin. 

 Both these granites are, strictly speaking, adamellites, as plagioclase occurs to the extent of more 

 than one-third of the total feldspar. 



One of the pebbles is a good granophyre consisting almostentirely of a fine micro-graphic intergrowth 

 between quartz and very turbid orthoclase. This encloses a few larger crystals of rounded and embayed 

 quartz. The original ferromagnesian minerals appear to have been biotite, now chloritized, and a few 

 flakes of muscovite; but a later mineralization has brought in some large aggregates consisting of 

 calcite, radial sheaves of muscovite, and irregular masses of pyrites. 



Next comes a granitoid rock which bears a considerable resemblance to the second adamellite 

 described above, as it carries the same clots of chloritized biotite, but with epidote and pyrites instead 

 of sphene and magnetite. It differs, however, in its more richly ferromagnesian character, and 

 especially in the relation between the feldspars. In this rock oligoclase occurs in distinctly superior 

 amount to the orthoclase. It is therefore to be classed as granodiorite. Another stone is a porphyritic 

 micro-crystalline variety of this type, and may be called granodiorite-porphyry or porphyritic micro- 

 granodiorite. 



Five stones belong to the tonalite group. Tonalite, in the author's opinion, is a granitoid rock inter- 

 mediate between granodiorite and quartz-diorite, distinguished by its abundant plagioclase relative to 

 orthoclase while retaining an amount of quartz sufficient to exclude it from the quartz-diorite group. 

 Its ferromagnesian constituents are mainly hornblende and biotite. They are more abundant than in 

 the granites and less abundant than in the quartz-diorites. 



Each of the five stones assigned to this group conform more or less closely to the above definition. 

 Two of them contain biotite, mostly altered to chlorite and epidote, as their sole ferromagnesian 



