464 Transactions. 



An examination of the rock confirms Captain Hutton's description of 

 it. It consists, in liand-specimens, of whitisli feldspar and crystals of 

 black or dark-green hornblende showing prismatic cleavages rather pro- 

 minently. Occasionally, as is common in all igneous masses, parts of the 

 rock are finer grained than others, and the rock then has a slightly lighter 

 green colour. On exposure to the weather the feldspar is very soon acted 

 upon, disappears to a great extent, and leaves the rock with more or less 

 rounded crystals of hornblende projecting from it, and with an external 

 appearance not unlike the nephrite coating seen, but very rarely, on a 

 troctolite.* The rock is exceedingly tough, and very prone to decom- 

 position, so much so that when a fresh surface has been exposed for about a 

 year a more or less thick scale of decomposed material can be taken off. 

 A fresh surface shows very fine cleavage surfaces of hornblende, and when 

 the feldspar is not so prominent as usual the rock has almost the appear- 

 ance of a pure hornblende rock. The feldspar usually has a faint greenish 

 tinge, probably due to the proxim.ity of the green ferro-magnesian mineral. 

 After weathering has acted on it for a considerable time, as with the rock 

 ' under the alluvial deposits of the claim, the hornblende becomes almost 

 pea-green in colour, studded with white specks of decomposed feldspar. 

 Ultimately a clay, bluish-green in colour, and of the consistency of putty, 

 results. 



Examined mider the microscope in section, the rock may be said to exist 

 in two varieties — {a) one in which hornblende is plentiful to the exclusion 

 of augite, (6) one in which much augite is present. 



The (a) variety consists of plagioclase and hornblende almost wholly. 

 The plagioclase is usually quite fresh, and shows very pronounced poly- 

 synthetic twinning after both the albite and periclinc laws. Karlsbad 

 twinning was also seen in one or two sections. From the extraordinary 

 number of lamellae and their marked development we should infer that 

 the feldspar was very basic. A determination of the feldspar proved this 

 conclusively. In the determination, as usual, a section was selected cut 

 nearly perpendicular to the lamellae. This was to be recognized by the 

 fact that the illumination of the two sets of lamellae was almost equal when 

 the twin-line was parallel to a cross-wire. Several measurements of the 

 extinction-angles were taken, with the result that the angle was found to 

 be about 35° (or 55°). The feldspar, then, is an almost pure anorthite. 

 That this is the case was also proved by an analysis of the rock, in which 

 it was found that, while the amount of lime was very high, the amount of 

 potash was exceedingly low (see later). The rock, therefore, is rather ex- 

 ceptional, for the general rule is for the feldspar to vary between oligoclaso 

 and labradorite. Inclusions are in some cases numerous, consisting of 

 fluid pores and other kinds whose exact nature it was impossible to deter- 

 mine. A striking peculiarity with regard to these feldspars is the presence 

 in some sections of a decided bending of the lamellae. This bending, too, 

 is not altogether restricted to the plagioclase, for a section has been seen in 

 which the augite (or diallage) seems to have been bent. This bending, we 

 know, is a result of dynamic metamorphism, and it is probably here due 

 to pressure subsequent to the solidification of the rock-mass after intrusion. 



Hornblende. — This is uniformly green in colour in small plates, with 

 usually no distinct terminations. Pleochroism is marked, from green to 

 yellow. The extinction-angle varies between 13° and 15° ; the plates are 



* It is seen very conspicuously in ore from the west coast of Otago. 



