Notes on the Area of Intrusive Rocks at Dargo, 129- 



over each other, appear to reraain permanently obscured : 

 and even when further examined by the delicate tints of a 

 quartz-plate, I could not detect any apparent change. In 

 this ground-mass are numerous sections of minute crystals 

 of a colourless mica, some of which are clearly foliated, and, 

 being lath-shaped, are sections parallel to the main axis of 

 the crystal. These latter sections polarise with a little 

 colour, and are, I think, referable to an alkali mica, as are 

 also the silvery micaceous scales, which can be made out 

 on the cleavages by the pocket-lens. In addition to these, 

 there are very numerous small gi-anules and irregularly 

 bounded plates of iron ore, which become translucent with 

 yellowish to reddish tints. In places they coalesce into 

 masses, and are then opaque. Some of these are probably 

 hydrated iron ores, but most are referable to the hematite,, 

 which can be made out in the hand specimens. Finally, 

 there are numerous scattered dots and laro^er flakes of an 

 opaque black material — graphite. No grains of quartz are 

 determinable. 



In order to gain some information as to these various 

 minerals, I treated the thin slice with concentrated 

 hydrochloric acid, with occasional warming. The only effect 

 observable was the slow solution of the iron ores, the other 

 minerals not being appreciably affected. As the fragile 

 nature of the thin slices did not admit of igniting them upon 

 platinum foil, I had recourse to the less satisfactory pro- 

 ceeding of testing the powdered rock. 



A portion of this treated with hot hydrochloric acid for 

 some time was rapidly acted upon, the acid being strongly 

 and characteristically coloured by iron. The solution, 

 examined qualitatively, proved to contain much iron and 

 magnesia aud a little potassa. The residue was partly 

 flocculent and partly a blackish powder. The former being 

 got rid of by careful decantation, the latter was dried, and 

 strongly ignited upon platinum foil, and rapidly lost its 

 black colour, and on cooling was of a greyish white. When 

 wetted with a little water I found it to be largely composed 

 of minute scales of a silvery mica. The inference may there- 

 fore be drawn that this rock is composed of a magnesian 

 silicate, having a micaceous structure, and apparently 

 optically uniaxial; a mineral resembling muscovite, hema- 

 tite, and probably also limonite, and finally graphite. 



I also made a quantitative analysis of this rock, the- 

 results of which I subjoin : — 



K 



