PART XI 



PETROLOGICAL NOTES ON SOME OF THE 

 ERRATICS COLLECTED AT CAPE ROYDS 



(With Two Plates and One Figure in the Text) 

 BY 



Dr. W. G. WOOLNOUGH, D.Sc, F.G.S. 



Professor of Geology, University of West Australia 



566. Pegmatite 



Macroscopic Character s. — Light coloured, somewhat, mottled, and slightly 

 foliated. Texture varies from rather fine to extremely coarse in different parts of the same 

 specimen, which is evidently part of an aplitic vein in a granite mass. The rock 

 consists essentially of grey quartz and white felspar. The latter mineral occurs in 

 crystals up to 40 mm. in diameter, poikilitically enclosing quartz grains. There is 

 little biotite in flakes up to 5 mm. in diameter. 



Micro sc opic Characters. — The fabric is allotriomorphic granular, the 

 average grainsize being about - 6 mm. 



The minerals are orthoclase, microcline, quartz, oligoclase, and garnet, in order 

 of abundance. 



Orthoclase shows quite considerable strain effects, where these attain a maximum 

 a decided moiree structure is produced. There is a good deal of granophyric inter- 

 growth with quartz round the borders of the larger grains. In addition there is 

 frequently a poikilitic enclosure of quartz grains. 



Distinct microcline is present with " Gitter-struktur " well developed, with increasing 

 fineness of the lamellae a moiree structure is produced so that the microcline and 

 orthoclase grade insensibly into one another. 



Quartz shows very pronounced evidence of strain. In some of the grains this 

 amounts merely to production of cloudy (undulose) extinction, but many of the grains 

 are completely shattered. 



Oligoclase is subidiomorphic in places though it never has definite crystal boun- 

 daries. Considerable strain effects are noticeable, including local suppression of albite 

 twinning, and development of that after the pericline law. 



All the felspars are slightly kaolinised ; the oligoclase less so, and more locally 

 than the potash felspars. 



Garnet is fairly abundant in small, faintly pink trapezohedra. 



As an accessory mineral a very small amount of greenish mica (bleached biotite) 

 may be noted. This rock may stand as an example of a very common type amongst 

 the erratics. 



1874. Pegmatite 



Macroscopic Characters. — A medium to coarse grained rock, consisting of 

 quartz, white felspar, and occasional very large flakes of dark-reddish biotite. 



n 169 2 a 



