268 AUSTEALASIAN ANTAKCTIC EXPEDITION. 



and is due to the presence of the golden-brown mica on the schistose surface, such as 

 is characteristic of the lawsonite-amphibolites at Cape Denison. The ferromagnesian 

 constituents include pyroxene, hornblende, biotite, and chlorite. The pyroxene, 

 which constitutes 5 per cent, of the rock, has the granular character and pale colour 

 common in the plagioclase-pyroxene-gneisses. In some cases it passes over directly 

 into chlorite, but its alteration to green hornblende is more common. Green hornblende 

 forms about 14 per cent, of the rock, and the remaining four-fifths of the rock consists 

 chiefly of biotite with lawsonite and chlorite, saussuritised felspar, and clear acid 

 plagioclase. Biotite possesses the curious reddish-brown colour previously seen in the 

 lawsonite amphibolite (No. 635), occurring in situ at Cape Denison, and shows various 

 stages of decolouration in its alteration to chlorite. Colourless lobate lenticles of 

 lawsonite are interlaminated with both biotite and chlorite. Lawsonite is also well 

 developed in the chloritic and saussuritised areas. The saussuritisation of the plagioclase 

 has been fairly complete. The albite molecules have reappeared as clear acid plagioclase, 

 while the anorthite molecules have combined with water and produced lawsonite and 

 granular epidote. Fibrous chlorite is also abundant among these aggregates, and 

 there is a little quartz and white mica. Sphene, ilmenite, and pyrite are important 

 accessory minerals, and isolated grains of allanite and apatite are present. 



Lawsonite, chlorite, and epidote indicate the relationship of this rock to the Epi 

 division. The presence of pyroxene indicates that the rock was previously subjected 

 to Kata zone metamorphism, being probably a plagioclase-pyroxene-gneiss. These 

 conditions are superseded by the hornblende-producing conditions of the Meso zone, 

 which were finally superseded again by the Epi zone conditions of recrystallisation 

 which now characterise the specimen. 



GABBRO SCHISTS AND GNEISSES. 



In addition to the typical amphibolites, the collection contains a number of 

 handsome hornblendic rocks of apparent coarse grain. They possess the same 

 mineralogical variety and composition as the amphibolites. In many cases they 

 possess the same average grain size, but the minerals are grouped in clusters, so that 

 the hand specimen assumes a mottled black-and-white character, with the appearance 

 of a coarse-grained rock. In these cases they are amphibolites, with the glomero- 

 plasmatic structure described by Loewinson-Lessing, and Holmes. 1 The mineral 

 grouping indicates either (1) that each cluster has been derived from an original 

 crystal unit, or (2) that each cluster is a minute metamorphic differentiation. The 

 average regularity of the areas throughout the hand specimens renders the first 

 explanation the more probable. In some cases this explanation is confirmed where 

 the white areas have retained the form of felspar crystals, and the rocks are metamorphic 

 representatives of gabbros. Some of the types may have been placed in order among 

 the amphibolites, but the " relic " gabbro structure of the hand specimens has favoured 

 their consideration under the heading of gabbro schists and gneisses. 



1 Trav. Soo. Nat. St. Peterab., vol. xxv (1900), p. 208; A. Holmes, Q.T.G.S. Ixxiv, 1918, p. 55 



