135 



by about 5 mm. thick. This mineral is light greasy green 

 in colour, with pleochroism from light bluish-green to darker 

 green. There are abundant very minute inclusions (zircon ?), 

 round which are dark halos. The optical character is almost 

 uniaxial and positive. In vertical sections twinning is seen, 

 giving symmetrical extinctions of oh deg. The flakes lie across 

 the schistosity at irregular angles, and contain much of the 

 groundmass. 



Another very peculiar feature of the rock is the occur- 

 rence of pseudomorphs of light-coloured materials. The 

 original mineral was porphyritic, and slightly larger than the 

 chlorite. It was probably andalusite, as we encounter this 

 mineral very strongly developed a short distance to the west 

 of the present locality. The material composing the pseudo- 

 morphs is a granular mixture of about two-thirds muscovite 

 and one-third quartz. They contain very little biotite 

 and magnetite. In shape they are lozenge-shaped and dis- 

 torted. (Plate ii., fig. 2.) 



Betaveen Callington and Paringa. — The rock in hand 

 specimen is silvery and lustrous, perfectly fissile, and shows 

 abundant small dark spots in a fine groundmass. Round these 

 ''knots" the groundmass exhibits flow structure. 



Under the microscope the rock is found to consist of about 

 equal proportions of fine groundmass and porphyritic aggre- 

 gates. The groundmass is composed chiefly of light-brown 

 biotite, in flakes about '004 mm. in average diameter, and 

 quartz of about the same order of size. A little muscovite 

 and tables of ilmenite are present. 



The aggregates, which in hand specimen appear like 

 porphyritic crystals, are found to vary considerably in char- 

 acter. Some appear as ovoid areas very ill -defined, and are 

 distinguished from the groundmass by their lighter colour. 

 The minerals composing such an aggregate are quartz mus- 

 covite, biotite, ilmenite, and dark chlorite in grains and 

 flakes, with average dimensions of about* '05 mm. The light- 

 coloured constituents are more abundant than the dark, and 

 in the better defined aggregates make up fully 90 per cent, 

 of the material. There is no trace of parallel arrangement 

 of the plates in some of the aggregates, but in most of them 

 the muscovite tables have their large surfaces parallel to 

 the plane of schistosity of the rock. Between crossed nicols 

 these aggregates are much more conspicuous than in ordinary 

 light, owing to the contrast between the polarization colours 

 of quartz and biotite. 



Other aggregates are in the form of fairly well-defined 

 polygonal areas, more or less distorted, in which the minerals 

 are clear muscovite and dark-green chlorite in large flakes. 



