318 AUSTRALASIAN ANTARCTIC EXPEDITION. 



A thin vein of chlorite, and some flakes of calcite suggest that in the very late 

 history of the rock it has been subjected to a very slight metamorphism of a retrograde 

 nature. 



The rock is a little higher grade than No. 974. The hornblende-biotite masses 

 are more coherent, and the granules of pyroxene are much more numerously developed. 

 The greater abundance of pink garnet and the presence of some sphene indicate the 

 greater degree of metamorphism the rock has undergone. 



No. 973. Plagioclase Amphibolite. 



This is another of the amphibolites which were found in the dykes which cross 

 the gneiss on Stillwell Island. It is a little more schistose than is usual with this suite 

 of rocks, but preserves the main characteristics, medium-grain size, black, dense, and 

 hard. 



The rock differs completely from those described by Stillwell from this source, 

 but is much like certain plagioclase amphibolites from the moraines at Cape Denison 

 which have come under notice, described by the present writer. 



The ferromagnesians present are almost entirely confined to green compact 

 hornblende whose pleochroism is strong and which has been described before in the 

 place referred to above. The grains are nearly equi-dimensional, they are arranged 

 in the decussate manner, usually they show the two amphibole cleavages quite well 

 but where prismatic sections are seen only one cleavage is, of course, to be perceived. 



Some brown biotite, passing into chlorite is present. There is also some green 

 somewhat fibrous biotite, but here the alteration to chlorite is not quite so strongly 

 developed. The only other dark mineral component is magnetite which occurs in fairly 

 well-formed crystals, but irregular outlines are presented by some grains where 

 neighbouring particles have coalesced to make a larger individual. Occasional skeletal 

 crystals of this mineral occur. 



The felspar which is the mineral second in abundance is andesine with Ab 66 An 35 . 

 It is quite clear and usually gives sharp extinction. There is no doubt of its secondary 

 nature. Its pellucid nature and the absence of every trace of original structure are 

 conclusive evidence of this. It exhibits both albite and pericline multiple twinning, 

 the latter sometimes appearing in very delicately defined lines which are unusually 

 well-developed. A little quartz is associated with the plagioclase, but its amount is 

 remarkably small. 



Apatite occurs quite freely in small well-shaped rectangles, hexagonal basal 

 sections, and prism sections. It is also found in the felspar as acicular rods which 

 suggest that they have been able to resist the changed conditions which have so 

 considerably affected the rest of the rock. 



