66 



AUSTRALASIAN ANTARCTIC EXPEDITION. 



granularity has been produced by secondary or metamorphic crystallisation, and that 

 they are indeed a part of the dyke series. Hence a revised study of the field relations 

 would have been profitable had circumstances permitted it. Small examples of basic 

 schlieren can be seen in the illustrations of polished rock (Plates XV1IL, fig. 1 ; XXII., 

 fig. 2). 



Petrographical Characters. 



We deal in detail with three examples of this type which bear the field numbers 

 of 9, 13, and 10. No. 9 belongs to the massive type, and Nos. 13 and 10 to the schistose 

 types. These specimens were collected from hornblendic patches which passed by 

 transition into normal gneiss. Rosiwal measurements have been made of thin sections 

 of these rocks in the same manner as before, with the following results : Columns 

 13A and 10A are the recalculated compositions of Nos. 13 and 10 when the quartz has 

 been disregarded. 



No. 9. The specimen was collected near the magnetograph house. It is dark, 

 massive, coarse grained, showing abundant platy hornblende and dull felspar. Grains 

 of pyrite and sphene are occasionally seen. 



In thin section the rock is coarse and granoblastic. The average absolute grain 

 size of the hornblende is approximately l-5mm. ; but in other specimens from the 

 same locality the hornblende crystals are as much as 4mm. and 5mm. broad. Horn- 

 blende, which forms nearly two-thirds of the rock, is found in granular crystals without 

 terminal faces. The prism faces and cleavage are well developed as usual. Its colour 

 scheme is X greenish yellow, Y bright green, Z bluish green. It contains abundant 

 inclusions of biotite, sphene, ilmenite, and epidote. Parallel strings of small sphene 

 inclusions are common in sections parallel to the cleavage. 



The 23-1 per cent, of felspathic material forms the colourless constituents of the 

 rock, and consists partly of turbid saussuritised felspar and partly of clear felspar. The 

 saussurite yields a brightly polarising aggregate which, under close examination, opens 

 up into mica, epidote, chlorite, and clear felspar. There are no traces of cataclasis. 

 The clear felspar is less in amount than half the total felspar. Part is untwinned and 



