GNEISS AND GRANITE. 
125 
interlocking grains of clear quartz and slightly altered oligoclase and orthoclase, with 
biotite and hornblende in strings marking the foliation. Both quartz and orthoclase in 
the mosaic occur in very irregular interpenetrating patches, which in ordinary light 
appear to belong to individual crystals, but break up between crossed nicols into a 
number of differently orientated grains. Cataclastic structure of this kind is well marked 
in the augen-gneiss 727 (see Plate IX, Fig. 1) from D 4 in the Kukri Hills (pp. 30 and 38). 
In this rock the interspaces between the large “ eyes ” of orthoclase are occupied partly 
by a quartz-mosaic showing marked undulose extinctions, and partly by shattered 
felspars crowded with small blebs of quartz (“ quartz de corrosion,” as described by 
Lacroix in the case of the charnockites of India*), or forming with quartz a micro- 
pegmatitic intergrowth (see Fig. 70). 
The red augen-gneiss (716) from Cathedral Rocks has somewhat similar characters, 
but the quartz-felspar-mosaic between the large “eyes” of pink orthoclase shows less 
pronounced cataclastic structure, and the quartz and 
felspar are in more distinct grains ; effects of pressure, 
however, are evident in the bent twin-lamellse of the 
oligoclase, and quartz of corrosion is also present. 
This red granite-gneiss appears to be intrusive in 
the gray, just as in Greenland the fine-grained red 
gneisses are intrusive in the gray mica- and horn- 
blende-gneisses. 
O 
From Cathedral Rocks comes a hornblende- or 
diorite-gneiss (724) consisting of a granular aggregate 
of quartz and plagioclastic felspar with much pleo- 
chroic (green to brownish-yellow) hornblende. The 
quartz is not in large amount, and the felspar is more basic than the oligoclase of 
the other gneisses, since it shows symmetrical extinctions of 18°-21°. 
Associated with the gneisses are hornblende-schists (730, 704) which help to give 
the dark streaky appearance to the rocks referred to on p. 28. 
Granites and Diorites. 
The granites of South Victoria Land are for the most part typical hornblende- 
biotite-granites similar to those from Cape Adare brought back by the ‘ Southern Cross ’ 
Expedition. 
Of these rocks the more noteworthy will be considered under the particular 
localities from which they come, and as far as possible in the order in which they are 
mentioned in the Report on the Field-geology. 
Granite Harbour . — The gray biotite-granite (129) which forms the greater part of 
the boss at Granite Harbour (see p. 33) is somewhat gneissic in character, and shows 
signs of parallel structure in the arrangement of the shreds of biotite. It consists of 
* Lacroix, Rcc. Geol. Surv. Ind., 1891, vol. xxiv, p. 168. 
Eig. 70 . — Micropegmatite surrounding 
Felspar in Augen-gneiss (727) from 
the Kukri Hills. (Magnification, 25 
diam.) 
