Art. XLIII.—Notes on a West Coast Dolerite. By W. A. MacLeod, B.Sc. [Read before the Auckland Institute, 15th August, 1898.] Plate XLVI. This is a rock forming a large dyke running parallel to the strike of the lodes now being prospected by the Anglo-Continental Syndicate on Victoria Range, Westland. Microscopically it is a dense, dark-coloured, basaltic-looking rock, and from its appearance might be any one of several of the more basic rocks. The fracture is on the whole even, though slightly inclined to be rough and hackley. Specific gravity, 2.88. The following is a. chemical analysis:— SiO2 54.23 Al2O3 15.22 Fe2O3 2.84 FeO 9.47 CaO 8.56 MgO 2.93 K2O 0.92 Na2O 5.80 Ignition loss 1.00 100.97 The percentage of SiO2 in the above is slightly high, and on the other hand that of the MgO is a little-low, probably, due to the rather small proportion of augite present. Under the microscope the constituent minerals are augite and plagioclase feldspar, a little base of a micro-crystalline nature being present, but, as the rock is almost holocrystal line in nature, I have termed it a dolerite, and not a basalt. The augites in ordinary light are colourless, or of a faint violet-brown tint, in which latter case pleochroism is just visible. The crystalline form is generally rudely developed, though sometimes well shown. The prismatic cleavage is only moderately developed, and many of the crystals show an irregularly fractured surface. In size they vary from 0.05 mm. to 1 mm., though larger crystals may be found in other sections. The polarisation colours are brilliant, and resemble those of olivine. The angle of extinction C: c = 41° (approximately). The feldspars occur abundantly in laths and needles, varying in. size-from about 0.05 mm. to 0.5 mm: By ordinary light