Vol. 70.] OF THE COUNTRY AROUND IIUNTLY. 281 



and coarse-grained varieties are seen to consist mainly o£ minute 

 grains of quartz and small flakes of a contact-biotite. With these 

 are associated bigger grains of quartz and felspar, the biggest 

 showing plagioclase stria tion and microcline cross-hatching, while a 

 micropegmatite of quartz and orthoclase also occurs. According to 

 Dr. Flett, this is a porphyritic igneous rock, much contact-altered. 



(b) The Hornblende-Andesine Rock. 



It has been previously noted that the most frequent result of the 

 alteration of the norite by the acid dykes and pegmatites was the 

 production of a dioritic type. In the quarry immediately east of 

 Ladysmith, a different rock has been obtained. Here numerous 

 veins penetrate the norite-pegmatites ; they are connected with 

 the Avochie Granite intrusion, and are composed of quartz, ortho- 

 clase, andesine (/x = l'554), and biotite. In width they attain a 

 maximum of 8 or 10 inches ; the edges are much richer in biotite 

 than the centre, pointing to the fact that the norite was much 

 colder, at the time of their intrusion. The norite at this place 

 is quite normal, somewhat like that of Battlehill, but slightly 

 coarser. 



At some distance from the pegmatites, the norite is seen to 

 contain large black crystals of some mineral, which, as the vein is 

 approached, increase until a border of almost black rock is formed. 

 As seen in section, the first indications of the change are the 

 intergrowth of pyroxene and hornblende — in reality, the conversion 

 of the hypersthene into green hornblende. There is also present 

 much magnetite and biotite, besides bytownite-felspar. So far, the 

 amphibole is confined to single grains occupying approximately 

 the positions of the original hypersthene. As the vein is 

 approached, these separate hornblendes coalesce and now possess 

 the same optical orientation ; while the greenish hornblende gives 

 place to a mineral of a rich brown hue. The grains of felspar, 

 somewhat shrunk in size, are enclosed pcecilitically in this crystal. 

 Outside the crystal, hypersthene-grains occur in various stages of 

 transformation. 



In the final stages close to the vein, the main portion of the 

 rock is composed of brown hornblende, in large plates pcecilitically 

 enclosing felspar. Biotite is met with sparingly, and acts in the 

 same way. Greenish hornblende passing into brown also occurs, but 

 in a much less compact condition. As compared with the original 

 norite, there is an increase in the amount of ferromagnesian 

 minerals and a decrease in that of felspar. In addition to a 

 diminution in the amount of felspar, there has been a diminution 

 in its basicity. The mean refractive index of the felspar (^u — 1"551) 

 now indicates andesine, whereas previously it was bytownite. 

 From the felspar and hypersthene, green hornblende was first 

 formed, while magnetite was deposited abundantly. Later, 

 magnetite became less abundant, and the brown variety of horn- 

 blende appeared. 



u2 



