MacLkod. — On a West Coast Dolerlte. 



487 



Akt. 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-Con- 

 tinental Syndicate on Victoria Range, Westland. 



Microscopically it is a dense, dark-coloured, basaltic-look- 

 ing rock, and from its appearance might be any one of several 

 of the moi-e 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 : — 



SiO, .. ... ... ... 54-23 



AlA 



Fe-A 



Feb 



CaO 



MgO 



KoO 

 Na.,0 

 Ignition loss 



100-97 



The percentage of SiOa 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 1mm.. though larger crystals may be found in other sec- 

 tions. The polarisation colours are brilliant, and resemble 

 those of olivine. The angle of extinction C : c = 41° (approxi- 

 mately). 



The feldspars occur abundantly in laths and needles, vary- 

 ing in size from about 0-05 mm. to 0-5 mm. By ordinary light 



