340 Transactions. — Geology. 



Bay and Cape Colville, the presence of these led me to the 

 discovery of the rock in situ. An examination of the neigh- 

 bourhood showed that the boulders were shed from a massive 

 dyke descending from the western flanks of Moehau. The 

 dyke itself forms a bold precipitous spur or ridge, terminating 

 somewhat abruptly before it reaches the sea, about half a 

 mile north of Waitoitoi. It obviously owes its present ex- 

 posure to the denudation of the Palaeozoic slaty shales through 

 which it has been erupted, and which flank it on both sides. 



Under the microscope the essential mineral constituents- 

 are seen to be plagioclase feldspar, hornblende, and biotite,. 

 with quartz, apatite, chlorite, epidote, augite, and magnetite 

 as associates. It is completely holocrystalline. 



The feldspars are generally well developed and mostly 

 idiomorphic. They occur principally as broad tabular cry- 

 stals, many of them ranging from 4 mm. to 6 mm. long. By 

 transmitted light they appear fresh, glassy, and transparent, 

 except a few phenocrysts, which are slightly clouded with 

 dust-like enclosures of a dark-grey colour, resembling minute 

 microlites. The majority of the feldspar plates exhibit fine 

 polysynthetic twinning on what is known as the " albite 

 plan," but there are examples in each slide of combined 

 twinning according to the albite and pericline plans. The 

 polarisation colours are very bright, especially in thick plates. 

 In one slide a phenocryst shows a zonal shell-structure, being 

 apparently built up of plates of gradually diminishing size 

 towards the centre. The successive layers exhibit different 

 colours in polarised light, and some of them are clouded by 

 lines of microlites zonally arranged. The finely developed 

 twinning laminae, brilliant interference colours, and large 

 extinction angles all point to a feldspar of a basic character, 

 probably calcareous labradorite or bytownite. 



The hornblende occurs in irregular aggregates and im- 

 perfectly developed crystals, ranging from 001 mm. to 9 mm. 

 long. The colour in transmitted light is pale - brown to 

 greenish-brown. The pleochroism is very pronounced, and of 

 the usual character in hornblende — i.e., the a ray, nearly 

 parallel to the clinodiagonal axis, is light-yellow ; the /3 ray,, 

 coincident with the orthodiagonal, yellowish -green ; and the 

 r ray bluish-green. The absorption is r > (3 > a. The basal 

 pinacoids are the most strongly pleochroic. and show the 

 parallelogram formed by the prismatic cleavage-cracks in a 

 very marked degree. The extinction angle is about 16 degrees. 

 Some of the hornblendes show a narrow opaque border of 

 magnetite, due to corrosion and resorption of edges before con- 

 solidation of the magma, but this feature is not common and 

 never conspicuous. In one slide a hornblende phenocryst has 

 a black border and a clear centre. Twins are not uncommon. 



