326 AUSTKALASIAN ANTARCTIC EXPEDITION. 



These members of the second type of gneiss at the Madigan Nunatak are coarse- 

 grained rocks in which the gneissic structure can be detected (cf., Stillwell, p. 133). This 

 gneissic structure is much more prominent in some specimens than in others. For 

 instance, very definite leucocratic and melanocratic bands are visible in Nos. 779, 787, 

 but the texture of Nos. 798, 1226, 1227 is granulitic, and that of Nos. 778, 790, 791, 793, 

 1254, is best described as intermediate. The colours of the rocks taken as a whole also 

 form a series from light to dark with an intermediate group where neither colour 

 predominates. In the leucocratic group are Nos. 778, 790 and 1254, the intermediate 

 group consists of Nos. 779, 791, 793, 798, 1226, 1227, and in the melanocratic group is 

 No. 787. The macroscopically visible minerals are quartz (which is often blue, but 

 sometimes brown), felspar and hypersthene. Weathering of these minerals has 

 produced normally a brownish-red colouration, but more complete hydration has 

 sometimes resulted in the production of a remarkable mustard-yellow coating which is 

 particularly noticeable in No. 778. There is a considerable variation in the grain-size 

 of these minerals. Not only is this noticeable in the case of the hypersthene (cf., 

 Stillwell) but in the felspar and quartz as well. 



(b) Mineralogical Characters. 



These rocks consist of quartz, orthoclase and plagioclase as their most abundant 

 constituents, but always with important hypersthene, biotite and ilmenite. Accessories 

 are apatite, pyrites and zircon (or monazite). 



Most of the felspar is orthoclase, which is highly perthitic. The small inclusions 

 of plagioclase have a higher R.I. than their host. They are usually linearly arranged in 

 a great number of parallel lines which preserve their parallelism extraordinarily well. 

 The regularity of the size and spacing of the intergrowths is remarkable. No crystal of 

 orthoclase has been observed which is not perthitic. These intergrowths, as a matter 

 of fact, frequently follow a rectangular pattern, where, again, the regularity of size, 

 arrangement and orientation is remarkable. In rocks where the orthoclase 

 overwhelmingly preponderates (e.g., No. 790) there is occasionally some multiply twinned 

 acid plagioclase (albiclase, Ab 90 An 10 ) which has perthitic inclusions in the sense used 

 by Chudoba (Chudoba, translated by Kennedy, 1933, p. 18), that is, where the host 

 has a lower refringence than its inclusions. In other rocks, e.g., No. 1227, where more 

 plagioclase is present, large areas of antiperthite are frequently found. Where the 

 plagioclase is more abundant it shows multiple twinning badly defined, and has the 

 other properties of a little more basic variety, viz., oligoclase, with Ab 80 An 20 (approx.). 

 These differences in the composition of the felspar units through a single individual 

 give a distinctive appearance to the rock even when the nicols are parallel, but more so 

 when they are crossed. This appearance is enhanced by the effects produced by the 

 extreme cataclasis which the rock has suffered. 



