CHAPTER VI. 

 THE MACKELLAE ISLETS. 



FIELD NOTES*. 



The Mackellar Islets are a group situated nearly due north of Cape Denison in 

 Commonwealth Bay (Plate XXXII.). The smaller members are normally covered with 

 a thick ice cap built by the frozen spray, which is swept from the surface of open water 

 by the incessant winds. The largest island is practically ice free, and consists of a low 

 plain whose highest point is about 40ft. above sea level (Plate XXXIII., fig. 1). The 

 surface of the islets is, generally speaking, flat, and forms a contrast to the miniature 

 mountain area of Cape Denison. Where prominences are seen they are well rounded. 

 No polished areas or striae were found, but the surface is very smooth in places, and 

 appears to have only been recently roughed up by frost action or other disintegrating 

 agencies. The surface is everywhere covered with saline matter blown off the sea, 

 and this may assist the disintegration. 



No moraines of truly foreign boulders occur on the islets, but patches of roughly- 

 rounded boulders of gneiss, similar to the " lower moraines " on the mainland at Cape 

 Denison, are found. In fact the general appearance is similar to the lower rock belt at 

 Cape Denison. Detrital gravel patches appear in a few sheltered spots ; their 

 appearance and situations suggest that they are probably submarine accumulations, 

 and, if so, they supply some evidence of relative uplift. 



The dominant rock is a grey gneiss very similar to that at Cape Denison, but it is 

 more uniform in character and more granitic. At intervals there are finer-grained 

 darker patches. Irregular fine-grained black patches and streaks (amphibolites) are 

 also moderately frequent, but they are not so conspicuous as on the Cape Denison 

 outcrop. The foliation is sometimes almost horizontal, and at other times nearly 

 vertical. At the north end the gneiss is particularly massive and granitoid. The 

 trend of the rock bars through the Mackellar group is in a direction between N.N.W. 

 and N. by W. Transverse to this dominant structure is a fracturing in a direction 

 W. by N. (nearly W.N.W.) which has led to the development of cross gullies and ravines. 



DESCRIPTION OF ROCK SPECIMENS. 



The rock specimens from the Mackellar Islets consist of three specimens of a 

 granitic gneiss and one of amphibolite. 



* The field notes are supplied by Sir Douglas Mawson. 



