THE METAMORPHIC ROCKS OF ADELIE LAND. 8TILLWELL. 73 



transition from the latter into the biotite gneiss is fairly rapid. There is no variation 

 in grain size or in structure or configuration of the crystal grains across the apparent 

 junction. The zone of biotite gneiss consists of abundant brown biotite set among 

 grains of clear felspar, cloudy felspar, and quartz. The character of the plagioclase 

 is the same as in the gneiss. There is less quartz in the biotite zone than in the grey 

 gneiss, and plagioclase occupies a greater percentage of the colourless material. Epidote 

 is practically absent. 



The transition from the zone of biotite gneiss to amphibolite is more gradual than 

 its passage into the granodiorite gneiss. In the amphibolite the hornblende largely 

 replaces the biotite, but the relative amount of biotite is probably sufficient to call 

 the rock biotite amphibolite. The hornblende has, on the average, a larger grain size 

 than the biotite, but its character is quite similar to the hornblende in the amphibolite 

 dykes. Quartz still appears in small pieces in the hornblende area ; but by far the 

 greater portion of the colourless constituents consists of saussuritised felspar. A little 

 lawsonite is intergrown with the biotite. 



In this case it is perfectly clear that a zone of biotite gneiss has developed along 

 the contact between granitic gneiss and amphibolite. But it has not been produced 

 by simple contact metamorphism, nor by assimilation, and we believe that it is another 

 example of a metamorphic hybrid produced by solid diffusion. 



No. 160 is another specimen from the moraines (Plate XII., fig. 6) which is 

 diagrammatic of the manner in which the black amphibolites cut the granitic gneiss. 

 In this case the gneiss is more basic than the Cape Denison granodiorite gneiss. Though 

 there is still abundant quartz in it, labradorite has been identified among the plagioclase 

 and hornblende is much more abundant than biotite. It may be distinguished as a 

 hornblende gneiss. The amphibolite consists of hornblende and plagioclase with a little 

 biotite. Though the granoblastic structure is noticeable on both parts of the rock, 

 the junction is a line of interlocking crystals and is comparatively sharp. 



Composite Gneiss. 



We have now to consider areas at Cape Denison in which the amphibolite bands 

 seem to open out into a series of thin parallel threads interwoven with the granodiorite 

 gneiss. The boundaries of the threads are often indefinite, so that some doubt existed 

 in the field as to whether they were related to the amphibolites. These are areas of 

 composite gneiss. A specimen (No. 144) from one of these thin interwoven bands is a 

 dark-coloured massive rock, a little coarser than the normal amphibolite. The hand 

 specimen shows abundant glistening biotite and small felspar porphyroblasts are 

 distributed through it. 



In section it is found to be a crystalline aggregate similar in type to the biotite felspar 

 gneisses, No. 630A and No. 10. It consists chiefly of biotite and felspar in much the 



