HORN EXPRDTTION — PETROLOGY. 87 



Tlie porpliyritic felspars have liigli extinction angles, l)etvveen labradorite and 

 anorthite, and contain numerous inclusions of two kinds: — (1) A line micro- 

 seopic dust, Plate T., Fig. 3, and Plate lY., Fig. 1/', occupying the centre of each 

 crystal, the outer shell being usually free from it. This dust appears to be 

 composed of globulites, some of which consist of magnetite, while others are 

 transparent ; and, (2) very fine elongated granules of augite, Plate IV., Fig la, 

 arranged in the centre of the crystal in a layer parallel to 010. Occasionally 

 some granules of augite similar to those in the groundmass of the rock are 

 included. The external boundaries of the crystals are moi'e or less infringed ])y 

 granular augite and hypersthene. 



The porphyritic augite, Plate T., Fig. .3, is pale green in coloui', similar to the 

 granular crystals, but showing often a fibrous structure parallel to the vertical axis, 

 somewhat suggestive of diallage. The extinction angle is, however, as lai'ge as 

 42°. The crystals, as a rule, show jilaty metallic inclusions, Plate IV., Fig. .5^?, 

 lying parallel to 100, similar to those occuring in some diallage, as well as small 

 granules of magnetite. These ciystals usually exhiliit altei-ation into green 

 hornblende, Plate IV., Fig. ^ut, the uralite (?) .so formed being present in consider- 

 able amount. These .secondary hornlilendes are compact, and do not exhibit the 

 usual fibrous structure of uialite. 



The granular hypersthene exhibits a somewhat slight pleochroism, brownish- 

 pink and green being the predom inating colours. 



The granular felspar appears to have l)een the last mineral to crystallise out, 

 and occui's in small clear iri'egular grains,^ lamellar twinning being occasionally 

 visil)le. All the minei-als are singulai-ly fresh and clear'. The specimen was 

 obtained from a dyke intruding gneiss on the northern slope of the Hart Ivange, 

 and in close pi'oximity to the large coai'se-grained granite dyke, on which the 

 Oolgarna Mica Claim is situated. 



Diorite. No. 32. Pl.ate I., Fig. 4. 



The greater part of this rock is composed of typical green hornblende, which 

 is very clear and fresh, and thoi'oughly allotriomorphic. A certain amount of 

 comminuted material of a pale greyish-green colour is present. This has a high 

 index of refi'action and sti'ong double refraction, and proljably represents the 

 dl'ln-is of some augite grains. These patches of powdered augite are almost 

 invariably surrounded by grains of hornblende, which seem to pass indefinitely 

 int<j the jiowdered matei'ial. This is readily obser\cd by the gradual loss of the 



