17 



I am indebted to Mr. C. T. Tille} 



,B.Sc. 



, for the 



analysis of the rock : — 







SiO, 





.. 68-60 



AI.O3 





.. 14-30 



FeA 





.. 0-79 



FeO 





4-70 



MgO 





.. 1-24 



CaO 





.. 2-88 



Na,0 





.. 3-40 



K,0 





2-28 



H.0+ 





0-87 



h;o- 





0-24 



TiO. 





.. 0-59 



co; 





abs. 



P.O, 





... si. tr. 



Total 



99-89 



Sp. gr. at 20°C ... 2*716 

 The approximate mode, calculated from the analysis, is : — 



Quartz 34*20 



Plagioclase ... ... ... ... 43'67 



Biotite 20-90 



Ilmenite ... ... ... ... 1*23 



In this calculation all the KoO was taken to form biotite, 

 potash felspar being practically absent from the rock. The 

 agreement between the measured and calculated modes is 

 very close. 



No. 2 represents another portion of the rock-mass which 

 is practically free from large crystals. Like the previous 

 specimen it appears fairly evengrained megascopically. It 

 differs from No. 1 only in being not quite so fine grained, and 

 in the presence of potash felspar. This mineral is in relatively 

 large allotriomorphic plates enwrapping all the other minerals, 

 including even quartz at times. There are obscure traces of 

 gridiron twinning, indicating microcline. The plagioclase 

 prisms are about 6 mm. long. 



No. 3 is of somewhat different fabric from Nos. 1 and 2. 

 It is perhaps in the average slightly finer than No. 1, and the 

 grainsize for the individual grains is very variable. Plagio- 

 clase is in -tiny subidiomorphic laths, but also occurs in larger 

 more equidimensional sections. Quartz while mostly granular 

 very often forms irregular micrographic inter^rowths with 

 felspar. Some of the felspar is microcline, but this is very 

 subordinate. The plagioclase is altering zonally to carbonates. 

 The rock is noteworthy as containing a little hornblende 

 intergrown with the biotite. Sphene and ilmenite are as in 



