137 



similar to that last described, but lacks the diablastic struc- 

 ture. In this rock the structure is porphyroblastic. 



The constituent minerals are abundant light green 

 actinolite, and felspar, sphene, vesuvianite, magnetite, and 

 decomposition products. 



Light green actinolite makes up about 75 per cent, of 

 the rock. It occurs in idioblasts, about '2 mm. in diameter, 

 with strong suggestions of crystallographic boundaries in the 

 vertical zone. The pleochroism is weak from colourless to 

 light green, extinction on (010) is 19 deg., and the optic 

 axial angle is not far from a right angle. 



About 23 per cent, of the rock is felspar. This is 

 mostly in untwinned rounded grains, about 02 mm. in diam- 

 eter. An occasional fragment of plagioclase of larger 

 dimensions occurs. The untwinned felspar is probably near 

 oligoclase in composition, as its refractive index is scarcely 

 to be distinguished from that of balsam. The mineral is 

 not quartz, as it is biaxial and decomposed. I have not been 

 able to recognize any quartz at all in the rock. The borders 

 of the felspar grains show much alteration into an opaque 

 greyish fibrous mass. Under higher powers this is resolved 

 into fine fibres with weak double refraction strongly sug- 

 gestive of zoisite. 



Sphene in dark orange-brown irregular grains makes up 

 the bulk of the remaining 20 per cent, of the rock. 



A few grains of vesuvianite occur of light brownish colour, 

 the colour being somewhat irregular in its distribution. They 

 show no trace of crystal form. 



A very few grains of magnetite, slightly decomposed to 

 haematite, occur. (Plate ii., fig. 4.) 



DESCRIPTION OF THE PLATES. 



Plate I.^ — Geological sketch map of the area described in 

 the paper. Geological traverses have not been run, so that boun- 

 daries are only sketched to show approximate positions. No 

 attempt has been made to differentiate the various mica schists. 



Plate II., fig. 1. — Orthoclase felspar in pegmatite, Rocky 

 Gully, Murray Bridge, showing inclusions (flakes of muscovite) 

 arranged parrallel to crystal planes of the host. x60 diameters. 



Plate II., fig. 2. — Porphyroblastic chlorite schist, near Cal- 

 lington, showing large crystals of clinochlore arranged across 

 the schistosity of the rock, and light pseudomorphs parallel to 

 the schistosity. xl6 diameters. 



Plate II., fig. 3. — Porphyroblastic mica schist, near Calling- 

 ton, showing general structure of rock, x 16 diameters. 



Plate II., fig. 4. — Amphibolite, Blakiston, showing poikilo- 

 blastic structure. x86 diameters. 



