T11K MKTA.MORl'HIC ROCKS OF ADEL1E LAND STILLWELL 159 



Cape Pigeon Rocks. 



The hypersthene gneiss from the Cape Pigeon Rocks possesses many of the 

 peculiarities noted in the preceding rocks from Stillwell Island. The specimen, though 

 a little larger than No. 947, is no more than 3^in. long, and reveals the same remarkable 

 variation in mineral content. Four sections cut from different portions of the specimen 

 have been necessary to understand the character of the rock. These will be dealt with 

 separately in order to again illustrate this variation. A rough determination of the 

 specific gravity of the specimen No. 785 gave the value 2-75, and, therefore, its total 

 composition is likely to be very similar to the composition of No. 947 or No. 979. 



No. 785 (1). No. 785 (1) was the first slide cut and examined from the specimen 

 from the Cape Pigeon Rocks. In it there are only scattered fragments of garnet which 

 has been largely replaced by biotite and quartz. A crystal of ilmenite often occupies 

 the central position of the biotite aggregates as before. The larger biotites are some- 

 times bent or crushed, but they often open out into radial sprays set in quartz, which 

 again enter into the fan-like myrmikoidal intergrowths of felspar. In other cases we get 

 aggregates of small biotites with small quartz crystals. Pleochroic haloes are still 

 common. The felspar is often cloudy and in part there is a good deal of sericite. In 

 part the orthoclase is transformed into microcline. The plagioclase has. in most cases, 

 a refractive index less than quartz and a small extinction angle, and is probably an 

 oligoclase andesine. Some of it includes the common blebs of rounded quartz, and it 

 frequently presents a diablastic structure. Along the junction of two felspar crystals 

 we may find one of them bordered with a diablastic zone. In one instance where the 

 diablastic structure has developed in a corner of a crystal, the twin lamellae can be traced 

 from the unaltered part through the diablastic area. Some of the plagioclase is 

 saussuritised, and epidote is found both in sporadic grains and in the finely granular 

 form with the saussurite. Chlorite is more abundant in this slide than in the others. 

 Apatite appears in fairly large crystals, and pyrite and zircon are also accessory. No 

 hypersthene is present ; and on this description alone the rock would have to be named 

 a biotite felspar gneiss. 



No. 785 (2). The slide No. 785 (2) is cut from the opposite end of the specimen, 

 distant 3jin. In general, there is less chlorite, epidote, saussurite, or sericite than in 

 the preceding slide. The garnet rims are well developed, and these, with the presence 

 of hypersthene, indicate the relation of the rock to the hypersthenic gneisses of Stillwell 

 Island. In the hand specimen there is nothing to indicate this variation. The igneous 

 origin of this rock type is further evidenced by the large, well-defined crystals of apatite 

 and zircon. 



The hypersthene possesses a beautiful and intense pleochroism from pink to green. 

 The depth of the pleochroism in hypersthene is usually associated with the iron per- 

 centage ; but when one recalls the pleochroism of the titaniferous augites, it seems 

 probable that the deeply pleochroic nature of these hypersthenes may be partly due to 



