410 Transactinnn, 



birefringence. This is derived from the decomposition of the 

 feldspar — an intergrown mass of small plagioclase lathes and 

 fine needles, irregular masses and grains of bright-green eegerine, 

 showing brilliant birefringence. The feldspar is partially decom- 

 posed to calcite, and in places to calcite and kaolin, and is 

 twinned polysynthetically on the albite law. It is of the variety 

 oligoclase. No evident flow structure is present. Magnetite 

 occurs in small rounded grains throughout. Though nepheline 

 does not occur in the rock, it seems probable that when the rock 

 was fresher it was present in small crystals ; therefore the name 

 " trachytoid phonolite " has been given to it. 



No. 2 (Plate X, fig. 4) occurs in the valley just above the mine- 

 mouth. It is not to be seen in situ, but its course is marked by a 

 line of surface boulders extending as shown on the plan. It seems 

 to have formed a dyke along this line. It is holocrystalline and 

 porphyritic. The phenocrysts consist of large square and rectan- 

 gular plates of orthoclase and large columnar crystals of oligoclase 

 very finely twinned (maximum extinction on albite plane = 10°). 

 Calcite frequently occurs in granular masses, often taking the 

 orientation, both externally and internally, of the finely twinned 

 oligoclase, a fact apparent between crossed nicols. The ground- 

 mass consists of an aggregate of plagioclase laths and needles 

 and small irregular masses of green segerine. The feldspars 

 show a pronounced trachytic structure, and are twinned on the 

 albite law. Treatment of the powdered rock with hydrochloric- 

 acid, on evaporation on a microscope-slide, shows under the 

 high power small cubes of sodium-chloride, thus proving the 

 existence of nepheline in very small crystals. On account of this 

 the name " trachytoid phonolite " has been given to the rock. 



Basalt. (Plate X, fig. 3.) 



As mentioned under " General Geology," this rock forms 

 the top of the mountain, and apparently extends down for an 

 unknown depth, filling an old volcanic pipe. The rock in hand- 

 specimens is seen to be very undecomposed ; it is coarsely 

 crystalline, and plKUiocrysts of feldspar and augite can be dis- 

 tinguished, the latter often of large size. Olivine is also dis- 

 tinguishable, but in far less quantity than distinguishes the 

 basalt family as a whole. Under the microscope the rock is 

 seen to be composed chiefly of mass of feldspar laths twinned on 

 the albite law. They are all plagioclase, with a maximum ex- 

 tinction of 32£°, thus being a variety of labradorite of medium 

 composition. These laths show, a general flow structure, and 

 wrap round numerous small hypidiomorphic crystals of augite. 

 These laths are of a clear pale variety, showing the typical 

 non-pleochroic and highly birefringent characters of that mineral. 



