746 ON A^ HOBNBLENDE-BIOTITE EOCK PKOM NEW ZEALAND. 



was cut parallel to the base. The crystal consisted of a single 

 twin on one half, and several twin laminae on the other half, and 

 the boundaries of the laminae were so sharp, although the section 

 was not very thin, that it was evident they had been cut nearly at 

 right angles, or, in other words, that the section was nearly parallel 

 to the basal pinacoid. I therefore tried the simple half with con- 

 vergent polarized light, and found a very distinct optic axis, with 

 revolving band, on the circumference of the field, thus confirming 

 the previous determination of the green crystals as hornblende. 



Some of these crystals show traces of schillerization in one direction, 

 which I take to be a face of the prism. I saw no inclusions in them. 

 There are no other essential constituents of the rock but hornblende 

 and biotite. Occasionally an actinolitic structure is seen, but not 

 commonly. The mineral which shows aggregate polarization is 

 either crushed hornblende or some altered form of it ; it is identical 

 in colour with, and shows the same dichroism as, the hornblende 

 crystals. In one case I saw a small quantity of calcite in a crack. 



I suppose that this rock will come under Dana's name of horn- 

 blendite ; but I think it objectionable to take the name of a mineral 

 and apply it to a rock, especially when that rock consists of two 

 minerals in nearly equal proportions. There is in the Canterbury 

 Museum a very similar-looking rock from Wet-Jacket Arm, Breaksea 

 Sound ; but I have not been able to examine it microscopically. 



Discussion. 



The President remarked upon the rare occurrence of such rocks 

 as the one described, and regretted that no specimen of the rock 

 could be exhibited to the meeting. 



