part 2] or southern exee peninsula. 99 



Quartz is quite xenoblastic, and is interstitial in its development. 

 Small granules of magnetite are included in hornblende at the 

 junctions of abutting amphibole-grains. Acicular crystals of 

 apatite are present in the felspar. There is also a very small 

 amount of low-refraction felspar present in strand-like threads 

 between grains of plagioclase, and this has the properties of 

 orthoclase. The few grains of pyroxene present associated with 

 felspar have the properties of diopside. The hornblende here is 

 not paramorphic after pyroxene, but has crystallized as such. 



In another type at the same locality, the igneous origin of the 

 rock is at once made apparent by the presence of blastophenocrysts 

 of felspar in a hornblendic finely-granular ground-mass. In such 

 types the blastophenocrysts of felspar have undergone considerable 

 alteration, in which a sericitic mica (? paragonite) and zoisite are 

 prime decomposition -products. The composition (where determin- 

 able) suggests andesine or labradorite. 



The ground-mass felspar is usually less decomposed, and its 

 composition varies from calcic andesine to a labradorite (Ab^An^). 

 The hornblende is usually associated with a small amount of 

 quartz, and the iron-ore is generally magnetite, although ilmenite 

 is also recorded from some of these types. 



A striking amphibolitic type is met with, intercalated in the 

 Flinders gneiss, on the western shores of Sleaford Bay. This is a 

 foliated rock of medium grain, containing lenticles or augen of 

 felspar, sometimes with a little garnet, arranged with their longer 

 axes parallel to the foliation. The whole structure is suggestive 

 of 1 it-par-lit injection of an uniform amphibolite. Under the 

 microscope the constituents are seen to be hornblende, plagioclase, 

 pyroxene, and a little biotite. 



A section through one of these eyes shows it to consist of 

 plagioclase-grains. The texture is characteristically of the pflaster 

 type. These grains prove to be labradorite, with a symmetrical 

 ■extinction in the zone perpendicular to (001) of 33°. The plagio- 

 clase of the amphibolite proper is of approximately the same 

 composition (30'^). 



Pyroxene and hornblende are intergrown. The former occurs 

 as clear grains of diopside of pale greenish tint. The amphibole 

 and pyroxene have evidently crystallized together. 



The proneness of the felspars to decomposition in the amphi- 

 bolites is especially noticeable in the blastophenocrysts of the 

 blastoporphyritic varieties, and this decomposition is usually (as 

 noted above) to a member of the zoisite group and a sericitic 

 mica. The decomposition is doubtless expressed by the equation 



^NaAlSigO.-f 4CaAl2Si,0, + 2 HOH = H.NaAlgCSiO,) 



-f2HCa2Al3Si30i3+2SiO,. 



The accessories accompanying the hornblende and plagioclase in 

 the amphibolites of the Flinders Series are usually magnetite, 

 quartz, apatite, and (occasionally) zircon. Ilmenite has been but 

 rarely observed, and sphene is conspicuous by its absence. Con- 

 sidering the widespread occurrence of titanite in normal amphibolites, 



