THE METAMORPHIC ROCKS OF ADELIE LAND 8TILIAVELL. 75 



Specimens (No. 145) from the same locality are in the rock collection which show 

 the junction of the dark biotite threads with the grey granitic tongues. A dark line 

 of demarcation exists between them in some specimens, but no sharp junction exists. 

 In other cases (Plate XII., fig. 4), where the biotite felspar rock is seen on either side of 

 a tongue of grey granitic gneiss, the boundaries are quite indefinite. 



In sections of No. 145 there is little to indicate a junction. At one end of a section 

 we may find biotite and felspar, with but little quartz. At the other end there may be 

 less biotite and much more quartz, but the distribution of these constituents is not 

 regular. No change in the character of th'e plagioclase is observed throughout the 

 slide. The presence of large quartz crystals and the larger grain size of the quartzose 

 areas are the most noticeable features of the grey rock. We are therefore dealing with 

 a partially obliterated junction, and we can again consider, as in No. 372 and No. 10, 

 that we are dealing with a metamorphic hybrid product produced by metamorphic 

 diffusion. The biotite felspar gneisses are related to the amphibolites which have 

 been produced from an intrusive rock. A supply of potash and silica from the granitic 

 rock enables the ferromagnesian content to be expressed in biotite instead of in 

 hornblende, as in the normal amphibolite. As a dyke disappears into the thin sheets 

 the number of junction planes is considerably increased, and there is more opportunity 

 for the subsequent interdiffusion of material. With this opportunity there is a greater 

 development of the biotite felspar schist, and the intrusive features in the field become 

 correspondingly more indefinite. Hence the thin dark threads which appear in the 

 field to be connected with the amphibolite bands are so related, but the basic rock has 

 been modified by metamorphic diffusion. It is not to be assumed that only the 

 amphibolite undergoes change during the metamorphism. The granodiorite gneiss 

 may also be modified. It only so happens that the change from hornblende to biotite 

 is one that can be readily recognised. A change in the granodiorite, which involves 

 a decrease in silica and in alkalies, is one that cannot be so readily detected. 



No. 424. No. 424 is another example of a biotite felspar gneiss, which was obtained 

 from a schliere of dark rock in the granodiorite gneiss. The schliere is a few yards away 

 from a definite band. In addition to the brown biotite and clear felspar, the rock, 

 like No. 10, contains a good deal of quartz. Epidote is also moderately abundant, 

 while pyrite, allanite, apatite, and sphene are present. In these accessory minerals 

 there is a likeness to the dark amphibolites. The junction with the enclosing gneiss 

 is not sharp, and the quartz may again have entered by diffusion. In any such isolated 

 instance there is always a possibility that such a rock is the metamorphosed equivalent 

 of a primary basic segregation in the granodiorite ; but against this supposition there is 

 the symmetrical relation of the schliere to the planes of foliation of the gneiss. Knowing 

 the relations of other biotite gneisses, one would favour the inclusion of this lenticle 

 with the amphibolite series. 



No. 411. No. 411 is another example of porphyroblastic felspars which are in 

 this case set in a biotite amphibolite. It was found not more than 100yds. away from 



