Bay and Cape Colville, the presence of these led me to the discovery of the rock in situ. An examination of the neighbourhood 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 exposure to the denudation of the Palæozoic 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 crystals, 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 laminæ, 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 imperfectly developed crystals, ranging from 0.01 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 α, ray, nearly parallel to the clinodiagonal axis, is light-yellow; the β ray, coincident with the orthodiagonal, yellowish-green; and the r ray bluish-green. The absorption is r > β > α. 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 consolidation 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.