part 2] OF SOUTHER]^ EYEE PElSriNSIILA. 107 



seating in such cases a transition from the true ampliibolites 

 wanting in the pyroxene minerals. Some of the hornblende is 

 obviously derived from the pyroxene, as all stages can be seen. 

 In these cases, it usually presents the distinctly fibrous appearance 

 of uralite. Much of the hornblende, however, is original, and 

 whatever its ultimate origin, it has crystallized in situ as such, 

 and is often intergrown with the pyroxene. It has the properties 

 of the normal amphibole of the ampliibolites. 



The pleochroic scheme is 



X=pale greenish-yellow, Y= brownish-green, Z=green, ZAc = 16°. 

 and the intensit}^ scheme is Z>Y>X. 



The plagioclase occurs often in rounded grains — some showing 

 twinning after the albite law, with occasional bands after the 

 pericline type. Twinning may, however, be absent, and it is then 

 (in the absence of cleavage) difficult to distinguish from quartz. 

 Its composition is comparatively constant, and is within the limits 

 of calcic andesine to labradorite. Inclusions of little jmsms of 

 apatite may be a marked feature. 



In some of the rocks, notably those collected from the southern 

 end of Boston Island, considerable strain is indicated in the 

 plagioclase by a pronounced undulose extinction, and bent twinning 

 lamellae. As a result of stress, a secondary twin-lamellation is 

 induced locally in originally untwinned grains. An alteration of 

 the felspar with development of sericite (? paragonite) -flakes is 

 sometimes observed. 



Of the minerals which less commonly enter as constituents, 

 quartz and garnet are noteworthy. Quartz is present in a number 

 of these rocks collected from the south of Sleaford Bay, and is 

 usually here an interstitial constituent. 



When the garnet is present, the rock bears a strong resemblance 

 to the garnet-amphibolites, the greater amount of the pyroxenic 

 constituent affording the only noteworthy distinction. 



Biotite miay enter as an important constituent of some niem.bers. 

 Often a distinct foliated appearance is given to the thin section by 

 the development of flakes elongated uniformly. 



Other examples show that some of the biotite is of secondary 

 origin, and the association of the mica centreing around grains of 

 mao;netite adds further confirmation to this view. 



The pleochroic scheme in all cases is 



X=pale straw- yellow, Y=:Z — deep red-brown (almost opaque). 



YII. The Origix op the Amphibolites. 



The petrography of the amphibolites described in the foregoing 

 pages is essentially that of metamorphosed igneous rocks. This is 

 indicated not only by their mineralogical composition, but also by 

 their textures ; for, despite the degree of metamorphism which 

 these rocks have endured, palimpsest features are in many of them 

 still recognizable. This is the case in such as show the blasto- 

 porphyritic or blastophitic texture. In other cases the textures 



Q. J. a. S. No. 306. K 



