THE METAMORPHIC ROCKS OF ADELIB LAND. STILLWELL. 129 



In section the rock (No. 794) has a granoblastic structure, which is modified by 

 subsequent cataclastic structures. The average absolute grain size is approximately 

 0-30mm. The mineral composition has been determined by the Rosiwal method to be 



Felspar 42-5 



Pyroxene 45-5 



Hornblende 3-3 



Iron ore 84 



Biotite 0-3 



The rock (Plate IV., fig. 1) is therefore essentially an aggregate of felspar and pyroxene 

 grains, which are of approximately equal dimensions. Apatite is also present as a 

 minute accessory. 



The greater proportion of the felspar is untwinned. When lamellar twinning is 

 found it is irregular, patchy, and often bent. There is often undulose extinction, so 

 that the determination of the felspar by the use of extinction angles is not satisfactory. 

 The refractive index of most grains is in the neighbourhood of 1551 (nitrobenzol), in 

 some cases above and in others below. In a few cases the refractive index is below 

 1-542 (nelkenol). These observations are explained by the presence of two plagioclase 

 felspars. Sections of the plagioclase with higher refractive index may be found which 

 possess two good cleavages, and are therefore considered to be normal to (001) and 

 (010), and an extinction angle of 30 was measured. Hence we consider this plagioclase 

 to be a calcic andesine. The second plagioclase with the lower refractive index is in 

 much smaller quantity, and is considered to approach albite in composition. One 

 fragment with fine lamellation was noticed to have a refractive index less than Canada 

 balsam. The felspar crystals are often fractured, and show cataclasis. Mortar structure 

 is common between two felspar crystals. 



The pyroxene includes both orthorhombic and monoclinic forms. Hypersthene is 

 readily detected in the thicker sections by the characteristic pink to green pleochroism. 

 The form of the grains is granular. The colour of the augite is pale green, and it is on 

 the whole fresh. It shows cataclasis like the felspar, but not so conspicuously. Strain 

 polarisation may be found, and some crystals are fractured and show mortar structure. 

 The crush zones may develop through a crystal. Many of the pyroxene crystals possess 

 a border of finely pulverised pyroxene produced during the stress action. This crush 

 border may pass out gradually into crushed felspar, and then an apparent transition 

 from augite to felspar appears. Very often fine streams of granulated pyroxene tail 

 out into the felspar and appear as a set of linear inclusions. Ilmenite often forms rims 

 and borders to the pyroxene crystals, and the appearance suggests in itself that the 

 iron content of the primary substance had been thrown out during the development of 

 augite in the first metamorphism. In rare cases the pyroxene is dusty with small 

 ilmenite inclusions. 



Scriee A, Vol. in.. Part 1 I 



