58 AUSTEALASIAN ANTAECTIC EXPEDITION. 



amphibolite containing saussuritised felspar, sodic felspar, hornblende, biotite, chlorite, 

 epidote, magnetite, sphene, pyrite, apatite, and rarely rutile and fluorite. These changes 

 have, at times, been accompanied by the addition or transfer of material, and, in some 

 cases, it is very important and leads to a theory of metamorphic differentiation. 



8. ORIGIN OF CERTAIN CLOTS IN THE DYKES. METAMORPHIC DIFFERENTIATION. 



Though we have determined the origin of the amphibolite dykes, there remains 

 for explanation the curious schlieren of biotite which were found in two dykes and were 

 mentioned in the field characters. These appeared like segregations in the dykes and 

 could be completely surrounded by the apparently normal dyke rock, though there 

 were no sharp boundaries. Schlieren of chlorite and epidosite were found in similar 

 circumstances (fig. 5). In each of these three cases the schlieren occur within portions 

 of sharply walled dykes. At first sight these seem to find explanation by postulating 

 primary magmatic xenoliths, composed possibly of augite or olivine, whose individuality 

 has been preserved throughout the metamorphism. 



We consider first the biotite schlieren. One of the biotite schlieren, No. 4 (Plate 

 II., fig. 6), has been found to possess the following mineral composition : 



Biotite 64-9 



Hornblende 32-2 



Quartz 1-7 



Muscovite 0-6 



Lawsonite 0-4 



Epidote 0-2 



Apatite, Sphene present 



The rock is highly schistose and shows a number of angular folds. The angle 

 made by the sides of the folds is 30. The biotite forms practically two-thirds of the 

 rock, while hornblende nearly completes the remaining third. The biotite is brown, 

 well crystallised with numerous pleochroic spots. Epidote is only rarely associated 

 with the biotite. The hornblende is intergrown in parallel position with the biotite, 

 and cross sections are idioblastic against the biotite. It has a more pronounced 

 prismatic habit than in the normal amphibolites. Its colour is different and appears 

 to follow the scheme X very pale yellowish green, Y green, Z bluish green. The 

 colour is not so intense as usual, indicating less iron in its composition. The hornblende 

 only rarely contains inclusions of quartz, apatite, or sphene. A small amount of biotite 

 is replaced by colourless muscovite^ and occasionally the biotite is intergrown with 

 lawsonite. Quartz is irregularly distributed, but rather seems to concentrate in the 

 axes of the miniature folds. 



