Art. XXX.—Note on the Boulders in the Port Hills, Nelson.

By Captain F. W. Hutton, F.G.S.

[Read before the Philosophical Institute of Canterbury, 4th November, 1891.]

A good many years ago I pointed out that the Arrow rock, at the entrance of Nelson Harbour, was composed of a conglomerate of large boulders, and that boulders of the same nature were also found in the sandstones forming the northern part of the Port Hills.³⁶⁵ These boulders are rounded, go up to 3ft. or more in diameter, and are composed of a granitoid rock which I took to be syenite. Last July, when in Nelson, I collected a fragment of one of these boulders for microscopical examination, and find that it is a biotite diorite. There is a small quantity of quartz, but it is quite subordinate to the feldspars, which are chiefly plagioclase, which has suffered but little decomposition. The ferro-magnesian constituents are biotite and brown hornblende, the former being the more abundant. There is also a little magnetite.

From this description it will be seen that these boulders differ materially from the, syenite of the boulder-bank, in which the orthoclase is more abundant than the plagioclase, and the hornblende much more abundant that the biotite.^† I do not know any rock in the district from which these boulders could have come, but probably it will be found near Motueka or Separation Point.