THE MBTAMORPHIC ROCKS OF ADBLIE LAND. STILLWELL. 69 



show strain polarisation. The grains are interlocking, and not infrequently possess 

 a lenticular shape due to solution at the points of greatest pressure and simultaneous 

 deposition at points of minimum pressure. It is quite different in character to the 

 quartz in the granodiorite gneiss. Apatite is an accessory mineral. Reddish hematite 

 occurs among the saussurite, but grains of magnetite are scarce. The rock may be 

 named an hornblende gneiss. 



The quantitative expression of this mineral composition in Table II. emphasises 

 the difference between No. 13 and No. 9. Marked as this difference is, the microscopical 

 description brings forward points of resemblance. The hornblende and mica are similar 

 in both cases, and the saussuritised felspar is quite abundant considering the high 

 silica percentage of the rock. If we neglect the quartz and recalculate the mineral 

 composition we obtain 31-4 per cent, felspar, 21-1 per cent mica, 45-1 per cent, horn- 

 blende, and 2-4 per cent, epidote. Then we find that the proportion of felspar (saussurite) 

 to the ferromagnesian (hornblende and mica) is very similar to that of the typical 

 amphibolites. Yet, in appearance and in the abundant quartz it possesses some likeness 

 to the granodiorite gneiss. The examination of this rock, therefore, provides micro- 

 scopical evidence to support the field observation that there is a gradual passage from 

 this basic patch into the enveloping granodiorite gneiss. 



No. 10. Specimen No. 10 is a different type collected from the same small area 

 as No. 13. Glistening biotite is abundant on the cleavage surfaces, but nevertheless 

 the rock has a tendency to a massive texture as a result of its very fine-grained character. 



In thin section the rock consists of biotite, saussuritic felspar, clear felspar, quartz, 

 epidote, apatite, magnetite, and zircon. The biotite is the pale brown variety and has 

 a noticeable parallel arrangement. Scattered patches of chlorite may be found which 

 are often accompanied by iron ore. The felspar consists of twinned and untwinned 

 felspar and cloudy saussurite. The quartz shows considerable cataclasis and strain 

 polarisation effects. It appears as parallel layers in the section conformable with the 

 layers of saussurite and biotite. The grains are clear except for occasional inclusions 

 of apatite, and there is a noticeable absence of the linear inclusions that appear in the 

 quartz of the granodiorite gneiss. Epidote is relatively abundant and sometimes forms 

 large well-shaped individuals and sometimes it is finely granulated. It is frequently 

 included in the biotite, but the finely granular epidote may form a rim to a biotite 

 crystal. Sphene and apatite are accessory minerals and ilmenite is present in occasional 

 large crystals. Hornblende is absent. The rock may be described as a biotite gneiss. 



The quantitative mineral composition is expressed in Table II. In the large 

 percentage of quartz it resembles No. 13 ; but this percentage is lower than that of 

 No. 13. It is not expected that these percentages would show any other similarity 

 than correspondence between two extremes. The ferromagnesian total, however, is 

 not much different. We notice again the sympathetic variation of the percentages of 

 biotite and epidote. 



