90 AUSTRALASIAN ANTARCTIC EXPEDITION. 



Diablastic structure is common. That this vermicular interlocking of quartz 

 and felspar is part of the metamorphic character is evident, because it is most common 

 in the areas with marked cataclasis, and it appears wholly enclosed within felspar crystals- 

 In other cases it has developed as a partial fringe to the plagioclase whose original outline 

 is quite evident, or it may transgress as a bight into the side of a crystal. These features 

 distinguish this intergrowth from the pegmatitic intergrowth of igneous rocks which is 

 the crystallisation product of a eutectic mixture, and which is the last to crystallise 

 in the consolidation of a rock magma. The diablastic structure does not have the 

 character of a final product, but it has arisen contemporaneously with the other meta- 

 morphic minerals and structures. 



Small crystals of ragged brown biotite are evenly distributed throughout the 

 slide, and epidote may be associated with it. Odd grains of allanite are present, though 

 no crystal comparable in size with the large examples exists in this slide. The 

 development of allanite is, however, quite a feature of this locality. Macroscopically 

 it has a black, pitchy lustre, and, in some cases, is surrounded by a reddish-brown zone. 

 In thin section the allanite is found in reddish-brown pleochroic crystals. When 

 associated with biotite it is surrounded by pleochroic haloes. They are biaxial with 

 oblique extinction. The double refraction in many cases is high, and there may be 

 a small amount of zoning. In such cases clinozoisite seems to be developed along its 

 sides. In other cases it alters to a brownish-yellow amorphous gum-like mass. The 

 allanite proved to have a refractive index greater than monobromnapthalin (1-648) 

 and less than iodmethylene (1-740). When equal proportions of these two oils are 

 mixed, part of the crystal had a refractive index greater than the mixture and part less. 

 The mean refractive index is, therefore, in the neighbourhood of 1-68, a value which 

 is on record for allanite. These characters are sufficient to render the identification 

 fairly certain. 



Since the cerium metals are present, and apatite is present as an accessory, it is to 

 be expected that monazite should be found. Grains are found with a heavy dark border 

 and with high polarisation colours and with marked similarity to zircons. Oblique 

 extinction has been noticed, and these small crystals are, therefore, considered to be 

 monazite. Pleochroic haloes around monazite in biotite are strong. Accessory grains 

 of magnetite and reddish hematite are fairly common, while clear apatite is less so. 



The rock corresponds closely to the family of Glimmerarme meso alkali felspar 

 gneisses in Group I. of Grubenmann's classification. The development of microcline and 

 perthite and the diablastic structure rather signifies the meso zone characteristics. It 

 may, therefore, be described as an alkali felspar gneiss, poor in mica, or an aplite 

 gneiss developed from an aplite vein under conditions approximating to those of the 



meso zone. 



No. 150. An example (No. 150) from the pegmatite bosses is very similar in most 

 respects to the example (No. 10A) collected from a vein. It shows variation in its larger 

 grain size, and its more massive texture, and in general, it shows stronger epi zone 



