CHAPTER VIII. 

 MADIGAN NUNATAK. 



The Madigan Nunatak is situated in Lat. 67 8j' and Long. 143 20', about 30 miles 

 distant from Cape Denison. It lies on a ridge which slopes away to the north, reaching 

 sea level at Cape Gray, 18^ miles distant. Its appearance is that of a small rock island 

 rising above the ice plateau at 2,400ft. above sea level, and it forms a small jagged ridge 

 of rock running north and south. It is 160yds. long and about 50yds. wide in the widest 

 part, and it rises from the level of the ice sheet at the southern end to a height of about 

 60ft. at the northern end. Views of the Nunatak are given on Plate XXIV., figs. 1 and 2. 



It is composed of gneissic rocks whose foliation strikes approximately north and 

 south, coincident with the direction of the ridge. There is a steep anticlinal fold at 

 the southern end (Plate XXVII., fig. 3), pitching slightly to the north. In contrast 

 to the freshness of the rock exposures on the coast at Cape Denison, Cape Gray, etc., 

 there is found considerable surface weathering. The surface is frequently brown and. 

 iron stained, and the felspars may lose their transparency. Frost action is prominent, 

 and many of the cracks and joint planes are filled with moderately fine disintegrated 

 material. There is no sign of recent glaciation and no glacial erratics or ice striae are 

 found on this area. 



Two rock types are found on this area. One is a black massive plagioclase pyroxene 

 gneiss or pyroxene granulite whose relation to the second type is not obvious in the field. 

 It was noted that it seemed to form either a band whose trend cut at right angles across 

 the foliation, or a band that may have been conformable with the anticline. Its 

 boundary on either side was indefinite or obscured by the angular blocks tumbled about 

 by the frost action. The second type is the more abundant acid gneiss, containing blue 

 quartz and hypersthene. In the neighbourhood of the anticline it has a banded 

 character, but in other parts the gneissic character, though evident, is less prominent. 



PLAGIOCLASE PYROXENE GNEISS (PYROXENE GRANULITE). 



The fresh specimens of this rock are black and massive with moderately fine and 

 even granularity. Macroscopically felspar and pyroxene are visible. The weathered 

 surface is discoloured by brown iron staining. So long as the felspar is sufficiently 

 fresh to be transparent the dark colour of the pyroxene dominates the colour of the rock. 

 When, however, the transparency is changed to translucency in the early stages of 

 weathering, the whiteness of the felspar is noticeable, and the rock assumes a grey colour. 



