THE iMETAMORPHIC ROCKS OF ADELIE LAND. 8TILLWELL. 67 



part possesses both albite and pericline types of lamellar twinning. A simple twin 

 with lamallse in both halves was found to give extinction angles of 14 in one set and 15 

 in the other set. Hence we designate the clear felspar albite. 



The biotite is well formed and often appears as inclusions in the hornblende. It 

 is mainly the normal brown biotite, but it is often intergrown with a green biotite. 

 Some of the green appears to be chlorite, and normal chlorite with its low, anomalous 

 blue polarisation colour is present in the section. The brown biotite seems to be 

 developing from the green biotite, which in turn comes from the biotite. The biotite 

 and chlorite appear under one head in the quantitative statement, as it is not always 

 possible to assert the line of demarcation. Sphene is relatively abundant, and some 

 grains are comparable in size with the hornblende, while others are* minute inclusions 

 in the hornblende. Most grains are anhedral, and only a few possess the characteristic 

 wedge-shaped outline. Pleochroism is strong in thick sections, and many of the 

 crystals possess a magnetite nucleus. As almost all the magnetite occurs in this manner, 

 the sphene-magnetite individuals were treated together in the Rosiwal measurement. 

 In some cases the rim of sphene is made up of a number of sphene grains with different 

 optical orientation. Some of the iron ore is ilmenite, as it is associated with its whitish 

 alteration product, leucoxene, but when the leucoxene recrystallises as sphene, the 

 ilmenite may change to magnetite. Occasionally there is a reddish-brown mineral 

 which is taken to be rutile. Epidote appears in colourless or honey-yellow pleochroic 

 grains. It is sometimes included in the hornblende, sometimes interlaminated with 

 biotite, and sometimes found as individual grains surrounded by the felspar 

 decomposition products. Pyrite in small scattered cubes and apatite are present. 



The rock is thus seen to correspond very closely with the description of a typical 

 amphibolite of the dyke series. The same type of hornblende, the same saussuritised 

 felspar, and the same clear felspar, and also the same peculiarities of the mica are found 

 in both cases. The quantitative expression of the mineral composition is now valuable 

 for comparison with the mineral compositions of the amphibolite dykes in Table I. 

 The composition of No. 9 is quoted in Table I. for this purpose. The strong similarity 

 towards types like Nos. 631 and 635 becomes obvious, and as the texture of No. 9 is 

 approximately massive, any error due to the schistosity is very small. The felspar 

 content is the smallest of the series, but only by a very small amount ; but the 

 hornblende percentage is the same as a normal amphibolite, and so also is the mica. 

 The mineral composition is therefore quantitatively as well as qualitatively essentially 

 the same as a normal amphibolite with dominant hornblende produced in the 

 metamorphism of a dolerite dyke. The chemical composition must also be the same. 

 Specimen No. 9 is therefore identical in kind with the examples of the undoubted dykes 

 series, and the conclusion is unavoidable that it is part of the same series. The apparent 

 difference is due to the fact that secondary crystallisation has proceeded under more 

 favorable circumstances and larger crystals have been formed. This larger granularity 

 signifies nothing in primary origin. 



