124 



•enced by the scliistosity of the rock. The scapolite is per- 

 fectly colourless, and forms a base in which the actinolite is 

 scattered. It is optically continuous over wide areas, whose 

 boundaries are extremely irregular. [The diablastic structure 

 of Grubenmann.] The refractive index is notably greater 

 than that of Canada balsam, and the double refraction is 

 quite strong. The mineral is uniaxial and negative. [Xot in- 

 frequently the cleavage is very sharply marked.] Biotite pleo- 

 ■chroic in red-brown to light-yellow tints occurs locally in the 

 rock, forming in little rounded or irregular flakes. [Pale- 

 brown sphene occurs in small fusiform grains.] Fine iron- 

 ore is scattered through the rock, particularly in the actino- 

 lite." 



There are to be noted, however, certain striking differ- 

 ences. The most important feature, and one which confirms 

 the conclusion drawn from macroscopic evidence — that the 

 Inglewood scapolite-amphibolite is an alteration of the Ingle- 

 wood intrusion — is the occurrence of a considerable amount 

 of microcline. This microcline also forms rather large poikilo- 

 blastic areas, including actinolite and scapolite. The occur- 

 rence of this potash felspar may be explained thus : — Scapo- 

 lite is an alteration prodiict of plagioclase felspars, as is the 

 actinolite of pyroxene. Microcline is the altered form of ortho- 

 clase. The Blakiston rock is then to be regarded as the altera- 

 tion of the dioritic form of the intrusion, i.e., that form 

 which is poor in potash felspar. The Inglewood amphibolite 

 is an alteration of a rock more nearly allied to monzonite, in 

 which potash felspar occurs in considerable amount. Such 

 extremes are found well represented in the main intrusion. 



Another feature not found in the Blakiston rock is the 

 occurrence of areas of diopside in colourless but very diisty 

 grains in optical continuity. It has just commenced to be 

 uralitized, and this process is preceded, as is often the case 

 in these rocks, by the formation of much dusty material not 

 unlike kaolin. Sections perpendicular to an optic axis show 

 that the axial angle is not far from 90°. 



Along the western boundary of the intrusion occur rocks 

 at first sight easily mistaken for schists. Two typical speci- 

 mens are here described, the first occurring in the gorge of 

 the Torrens and the second on the main road from Teatree 

 Gully to Inglewood, just below the Recreation-ground. They 

 are dark-green rocks, containing a great deal of quartz and 

 felspar, but with bands of sericite and black mica giving the 

 rock a schistose appearance. The graiusize is about 1 mm. 



The Torrens Gorge rock is syenite, consisting chiefly of 

 moire orthoclase, occasionally passing into microcline : a 

 little albite also occurs irregularly, often a little strained. 



