part 2] OF SOUTHERN EYllE PENINSULA. 103 



the felspars have been recrvstallized, and the iiiicropegmatite is 

 represented by the quartz and orthoclase-grains. 



This border recrystallization, with a more or less unchanged core 

 of igneous rock, is the result of the thermal metamorphism of the 

 invading granites, and the nature of the original rock from which 

 the amphibolite is derived supports the suggestion that has already 

 been noted in the similarity of the analysed amphibolite to the 

 quartz-gabbros and dolerites. 



Some further remarks will be made in connexion with the origin 

 of the amphibolites in general. 



G a r n e t - A m p h i b o 1 i t e s. 



Amphibolites containing garnet as a constituent have been met 

 with as inclusions in the Sleaford gneisses, and a closely similar 

 type is found at Kirton Point, Port Lincoln, The specimens here 

 were obtained from a large block on the shore not in situ ; but 

 obvioush^ the source is near at hand, although a search did not 

 discover it. These garnet-amphibolites are foliated dark rocks in 

 which there are conspicuous grains of red garnet. 



Under the microscope the constituents are seen to be hornblende,, 

 garnet, jjyroxene, plagioclase, quartz, and sphene, Accessorily 

 present are magnetite and pyrites. 



As the nuclei of numerous pleochroic haloes, minute inclusions- 

 of two types occur : one a colourless mineral of high refractive 

 index, the other being grains of a yellowish strongly-refringent 

 mineral, which is probably rutile. The colourless mineral is^ 

 probably epidote ; but its minute size prevents a definite deter- 

 mination. Numerous spindle-shaped grains of titanite associated 

 with hornblende appear not to be radioactive, as they are never 

 surrounded by pleochroic haloes. Hornblende is the most abundant 

 mineral in the slides, and occurs in plates measuring up to '75 mm. 

 in length. 



The pleochroic scheme is as follows : 



X = yellow, Y = yellowish-green, Z = brownish-green. 



Garnet porphyroblasts occur measuring in greatest diameter 

 up to 1| mm. The shaj^e is extremely irregular, but the crj^stals 

 never show anomalous birefringence. They contain poikiloblasti- 

 cally grains of quartz and often grains of hornblende and plagio- 

 clase. 



The irregular outline of the garnet and its much-fractured state 

 are indicative of movement in the rock since the genesis of garnet. 

 There is evidence here also of an initiation of degradation of the 

 garnet yielding hornblende and plagioclase. The crystals are often 

 surrounded by a thin ring of colourless material, of which a pro- 

 minent constituent is plagioclase. This gives place externally to 

 little crystals of hornblende. Again, the garnet may be sur- 

 rounded by a quasi-diablastic intergrowth of hornblende and 

 plagioclase, and this may develop in the form of bays into the 

 original garnet-crystal. 



