1915- No. 4' REPORT ON ROCK-SPECIMENS. 5 



the only potash-felspar. The frequent occurrence of bent and broken 

 twinning-lamellæ of microcline demonstrates the effect of stress. 



P/agiocI(ise. The plagioclase-felspar was determined by the angle of 

 extinction a in sections -t- the axis o, and calculation of the percentage 

 of anorthite-silicate according to Beckes tables ^ 



In the common type the plagioclase shows a constant composition, 

 the different determinations varying between An2^ and Anao- The deter- 

 minations were verified by comparison of the réfringences of the plagio- 

 clase with those of quartz in different positions. According to the deter- 

 minations the plagioclase in the common type is oligoclasc. In one slice 

 with prevailing orthoclase, the very sparing plagioclase was determined as 

 albite. In the aplitic granite analysed, the plagioclase is oligoclase An^o- 



Quartz, usually abundant, commonly as cracked grains with undulatory 

 extinction. This is certainly due to the initial effect of stress. 



Biotite, partly altered into chlorite, shows strong absorption: i and y 

 dark brown, a straw-yellow. Probably the ironcontent is high. Inclusions 

 of small zircons, surrounded of pleochroic halos are common. 



Muscovite is shown in all slices, but more sparingly than biotite. 



Apatite is sparingly met with as small grains included in and between 

 the felspars. 



Zircon is mentioned above as inclusions in biotite. 



Iron-ore (magnetite and pyrite) occur very sparingly. 



Titanite is only shown in one slice. 



The microscopical examination has proved that the granite-specimens 

 from Mt. Betty, though very like each other in external appearance, form 

 a series of rocks with rather great differences of mineral composition, 

 passing from a pure potash type with only traces of plagioclase to plagio- 

 clase-granite with oligoclase Ab3 An , as the sole felspar. 



The ordinary medium-grained granite from Mt. Betty is rather similar 

 to the granite from Scott's Xunataks. The latter has been analysed and 

 will be described later on, and hence I will reserve the further description 

 of the ordinary type from Mt. Betty. 



Of the samples from Mt. Betty I have obtained analyses of a white 

 aplitic granite and the plagioclase-granite. 



The Aplitic granite from Mt. Betty attracts special notice by its pure 

 white colour. The sample of this rock (the biggest in the collection) shows 

 on both sides a coating of rather coarse scales of muscovite. Undoubtedh* 

 this sample is derived from a small intrusive sheet in the gneiss, about 



1 Denkschriften d. K. .-Xk. d. Wiss. Math. Nat. Kl. B. LXXV i. Vienna 191 3. 



