THE PETROLOGY OF SOUTH GEORGIA, 



831 



altered crystals. The volumetric proportions of phenocrysts to one another and 

 to the groundmass have been estimated by the Rosiwal method with the result 

 given below ; — 



Orthoclase 12 "8 



Oligoclase 

 Hornblende 

 Biotite . 

 Groundmass 



127 

 6*3 

 3'0 



65'2 



The groundmass consists mainly of quartz intermingled with sericitised orthoclase. 



Epidote, chlorite, apatite, sphene, and a little iron-ore are also found. The epidote 



and chlorite are undoubtedly derived from the alteration of small scraps of biotite. 



The proportions of the various constituents of the groundmass are roughly estimated 



as follows : — 



50 



V^J LlCH \JLi ..... 



Orthoclase (sericitised) 



40 



Epidote and chlorite 



6 



Sphene ..... 



2 



Apatite ..... 



1 



Iron-ore ..... 



1 



Combining ■ the two sets of figures we get the following as the approximate 

 mineral composition or mode of the rock : — 



Quartz ...... 



32-6 



Orthoclase ..... 



38*9 



Oligoclase (Ab 5 An 3 ) 



Hornblende ..... 



127 

 6'3 



Biotite, with epidote and chlorite . 

 Sphene ...... 



Apatite ...... 



Iron-ore ...... 



6'9 



1'3 



•6 



7 



The rock is therefore a granite-porphyry. It well illustrates the fallacy of the 

 prevalent practice of basing the identification of a porphyritic rock largely on its 

 phenocrysts. Considering the latter alone, the rock would be regarded as a 

 monzonite-porphyry. 



IV. Distribution and Affinities of the Rocks of South Georgia. 



Assuming that Mr Ferguson's collection is representative, and that the number 

 of specimens of each type roughly corresponds to the bulk-development of that rock, 

 it is possible to make some general remarks as to the distribution of the rock- 



TRANS. ROY. SOC. ED1N., VOL. L, PART IV (NO. 25). 118 



