MAGNETITE GARNET ROCKS COULSON. 



JST 



No. 149. This rock shows a pronounced gneissic banding. It is a medium 

 grained rock and flakes readily owing to the amount of mica. Quartz, garnet, felspar, 

 mica, and magnetite can be recognised and the mineral proportions are given in Table 1. 

 Magnetite with intergrown ilmenite forms 23 per cent, of the rock. The ilmenitc is 

 recognised l>y its alteration to leucoxene. Sphene is doubtfully present. Garnet, 

 magnetite, and also apatite contain inclusions of one another. Some garnets are 

 idioblastic and others show evidence of resorption. Quartz, plagioclase, and orthoclase 

 are present in approximately equal proportions. The plagioclase, is mostly labradorite, 

 and, like the orthoclase, is slightly kaolinised. Quartz contains numerous opaque 

 inclusions of iron oxide. 



The biotite is interesting. It is a yellowish-brown variety occurring in large 

 sporadic flakes which contain radioactive inclusions surrounded by beautiful pleochroic 

 halos (Plate II, fig. 4). All the inclusions are of considerable dimensions relative to the 

 halos which surround them and their longer and shorter axes have been measured. 

 The radii of the halos in the following table are the average of two or three measurements 

 of the distance from the edge of the inclusion to the outer edge of the halo. A 4 cm. 

 ( x 45) objective lens was used in conjunction with a Beck micrometer eye-piece and the 

 measurements given below are in millimetres. 



TABLE IV. 



In the measurements made by Joly* a correction is made for the nucleus on the 

 assumption that the nuclei are zircon and that they are sensibly spherical. In this 

 rork, however, the nuclei are too large and such corrections would not be valid. The 

 halos are remarkable even in colour, but in two cases there is an appreciable difference 

 in colour, which, however, is too slight for accurate measurement. The inclusions 

 show high order polarisation colours and appear to have straight extinction, thus 

 suggesting zircon. 



" Genera of Pleochroic Htloi." 

 16640 B 



J. Joly. Loud. Roy. 800., Phi). Trans.. Seriei A, vol. 217. pp. 61-79. 1W17. 



