Shkewsbuky. — On the Auckland Volcanoes. 373 



aggregates ; sometimes, as in the lava-streams of Lake Taka- 

 puna and in the blocks of basalt in the tuff at Northcote, 

 reaching ^in. or more in diameter. These crystals are some- 

 what granular an-!l irregular in shape, having no doubt been 

 rounded since their formation by the solvent action of the 

 magma before the lava actually flowed at the surface. We 

 must here, however, distinguish between the macro-porphyritic 

 and the micro-porphyritic olivine, for the latter often shows 

 very perfect and regular crystalline form. Except at the 

 surface, where it has been exposed to weathering, the olivine 

 is perfectly fresh and unaltered, unless the rock is very porous ; 

 very often, indeed, the olivine crystals stand out almost un- 

 changed by the corrosive influences of the atmosphere on the 

 very surface of the lava-streams. 



The augite is also porphyritic in the Lake Takapuna and 

 Northcote basalt, but not, so far as I am aware, elsewhere ; 

 nor have I observed either of the other constituents occurring 

 porphyritically. 



In describing the microscopic structure of this basalt it will 

 be convenient, I think, to divide it into three classes, accord- 

 ing to the depth at which it solidified and the relative arrange- 

 ment of its constituents. The classification being one of degree 

 rather than of kind, there is no hard-and-fast line between the 

 three classes — they shade imperceptibly into one another — but 

 it will serve to define the three main varieties in the microscopic 

 structure of the rock, from which the numerous intermediate 

 forms and gradations are derived Since a description of a large 

 number of sections would entail repetition, and become tedious, 

 I will merely describe the three sections of which I have ap- 

 pended sketches, and which illustrate respectively the three 

 classes into which this basalt is divided. These classes are, — 



1. Basalt which has cooled comparatively rapidly at or 

 near the surface. — The rocks of this class are more or less 

 glassy, usually dark-coloured, and finely porous or scoriaceous 

 to a varying degree. The abundance of magnetite, especially 

 in the darker varieties, renders the micro-sections very opaque 

 till they are brought down very thin, when it is seen to be 

 matted together, as it were, with the feldspar microliths, 

 minute augite grains, and glassy matter. Eock-section 70 (PI. 

 XXXV.) is typical of this class : the olivine is both macro- and 

 microporphyritic, surrounded by a base composed of small 

 feldspar microliths, minute augite grains (colourless and almost 

 indistinguishable except with polarised light), a large quantity 

 of magnetite as grains and fine dust, and glassy matter. It is 

 very pale yellowish-green, almost colourless, and is exceedingly 

 fresh-looking, being unaltered even at the margin and along 

 the cracks. It contains numerous inclusions of magnetite and 

 glassy base, showing that some of the magnetite began to 



