70 DISCOVERY REPORTS 



THE MARIN DARBEL ISLANDS 



This group of small islands and rocks lies a few miles south-west of Cape Bellue at about long. 

 66" 20' W., lat. 66 00' S. In a brief note accompanying the specimens, in which the above location 

 is given, they are wrongly allocated to the Biscoe Islands, which form a long chain of islands north-east 

 of Adelaide Island. The above-given latitude and longitude are those of the Marin Darbel Islands. 

 I have been able to find no previous reference to the geology of these islands. 



The specimens collected are stated to come from a small uncharted island lying to the south-west 

 of Cape Bellue. This island, like all those in the vicinity, consists of an ice-worn mass of igneous rock. 

 Two large specimens of this rock (norite) were taken ; five others represent dikes penetrating it. 



The main rock of the island is a coarse plutonic type of a mottled, greenish grey tint, consisting 

 of white feldspars and greenish black ferromagnesian minerals. In thin section the appearance of 

 coarse grain is seen to be illusory, for the rock consists of large areas of fresh labradorite (An55) in small 

 crystals, alternating with larger and more isolated crystals of hypersthene, augite, and magnetite. The 

 hypersthene is mainly fresh and distinctly pleochroic, but some crystals are in process of alteration 

 to a pale green fibrous bastite mineral, and a few to brown biotite, both modes of alteration being 

 accompanied by the disengagement of magnetite. There is also some primary iron ore. The hypersthene 

 is apparently slightly preponderant over the pale diopsidic augite, and the periods of crystallization 

 of the two minerals appear to overlap. Thiis the rock is a norite or more exactly a hyperite, since the 

 hypersthene is accompanied by a notable amount of monoclinic pyroxene. In another specimen the 

 hypersthene has gone over completely to bastite. 



Three of the dike rocks are dark, greenish grey, aphanitic types in which numerous micro- 

 phenocrysts of serpentinized olivine and feldspar can be made out with the lens. In thin section they 

 turn out to be olivine-basalts with very numerous micro-phenocrvsts of bytownite (Augo) and almost 

 equally numerous olivines which are perfectly euhedral but completely altered to pale green serpentine. 

 The ground-mass is very minutely crystalline, and consists of microlites of plagioclase, augite, and 

 magnetite. Numerous spherical steam cavities are present which are usually filled with fibrous, 

 radiating, pale green delessite. A fourth specimen is much coarser and is highly carbonated. It appears 

 to represent a coarse basalt or dolerite. 



That part of the Graham Land peninsula and the Palmer Archipelago which lies between lat. 

 64°-67° S. and long. 62 "-66° W. seems to be rich in gabbroic intrusions and basic dikes. Thus 

 Pelikan {op. cit. siipro) described gabbros and dolerite dikes from Anvers Island, Bob Island (off 

 south coast of Wiencke Island), and Cape Anna (Danco Land). Gourdon, likewise {op. cit. supra) 

 described basalt dikes from Wiencke Island and Doumer Island, diabase dikes from Booth Island 

 (Wandel I.), diabase and gabbro from Petermann Island and Cape Tuxen. From the Andvord Bay 

 region the writer {op. cit. supra) described basalt dikes and an intrusion of fresh olivine-gabbro (Bruce 

 Island). Barth and Holmsen {op. cit. supra) commented on the abundance of basic dikes in the region 

 between Victor Hugo Island and Port Lockroy (i.e. along the line of lat. 65° S.), and described 

 eucrite and anorthosite from Victor Hugo Island. 



ADELAIDE ISLAND 



Adelaide Island is a large island off the coast of Graham Land at about lat. 67" S., long. 69° W. 

 Geologically nothing is known of the main island, but the French Expedition of 1903-5 collected rock 

 material from three small islands, Jennv, Leonie, and Webb Islands, off its south-eastern coast. 

 Gourdon {op. cit. supra) described them as consisting of gabbro cut by numerous dikes of basalt, 

 diabase, and andesite, and has given no fewer than ten analyses of these rocks. 



