PETROLOGICAL NOTES ON FURTHER ROCK SPECIMENS. 313 



In crossing the islet areas are found consisting of more strongly foliated gneisses, and the 

 trend of the foliation is a little west and north. Irregular bands of black gneiss, with 

 dyke form, exist here as at Cape Denison, and some are full of fine garnet." (pp. 145-6.) 

 Mention is then made of 



(a) Garnet gneisses (No. 917), p. 151 ; 



(b) Acid hypersthene gneisses (Nos. 979, 947), p. 155; 



(c) Metamorphosed basic dyke series (Nos. 951, 942, 935, 953, 952), p. 171. 



The present contribution is devoted to a further description of these groups with 

 mention of specimens not dealt with by Dr. Stillwell, but which show variations from 

 the main types and important differences in detail. 



The specimens dealt with are Nos. 59, 699, 972, 973, 974, 976, 977, 978, 980. 



It is found that Nos. 699 and 980 fall in Group (a), the Garnet Gneisses, and Nos. 

 591, 972, 973, 974, 976, 977 and 978 are all members of the metamorphosed basic dyke 

 series, Group (c). 



2. PETROGRAPHY. 



(a) The Garnet Gneisses. 



As mentioned above the additional members to this group of gneisses which occur 

 both at Cape Pigeon Rocks and Stillwell Island are Nos. 699 and 980. 



No. 699. Garnet-Felspar-Quartz-Gneiss. 



This rock consists of alternate bands of biotite and quartz-felspar matter. Through 

 the rock, but concentrated to a greater extent in the vicinity of the biotite, are clusters 

 of deep red garnet crystals, some of which assume a high degree of crystalline form. 



In some respects this rock resembles Nos. 777 and 917 described by Stillwell. 

 The former was collected at Garnet Point and the latter at Stillwell Island. There are 

 large porphyroblasts of garnet, and although they are to some extent penetrated by 

 biotite, they are not skeletal like those of No. 777. The mica developed is of two kinds. 

 One is biotite which is found as a greenish-yellow fibrous mass forming an aureole around 

 the garnet, and as more compact masses pleochroic in dark and light browns. The other 

 mica is sericite which has developed during the alteration of potassic felspar. 



The areas not so immediately associated with the garnet consist of quartz, 

 plagioclase, orthoclase, microcline, microcline-microperthite, sericite, calcite and 

 accessory apatite. 



The quartz is frequently undulose and contains inclusions of fluorite arranged in 

 long parallel lines. The fact that the fluorite shows abnormal birefringence is evidence 

 that the mineral has suffered internal tension. The quartz and orthoclase frequently 

 occur in a myrmekitic intergrowth ; other varieties of intergrowth occur, of which the 



