REPORT ON THE PETROLOGY OF OCEANIC ISLANDS. 167 



microliths of the ground-mass yield extinctions that appear to refer the felspar to 

 labradorite. 



Augite is one of the most constant minerals in these rocks ; it is found in the 

 pyroxenic andesites, and it is also, although subordinate, always present in the 

 amphibolic andesites. The description we are about to give applies to the augite of 

 both types of andesite. This mineral occurs in microporphyritic sections and in 

 microliths in the ground-mass. The augite of the first generation usually takes the 

 shape of grains without definite crystallographic contours ; often these crystalloids are 

 found grouped at one point, where four or five may be seen together. They are 

 traversed by fissures, which sometimes assume the direction of the cleavages ; most 

 frequently their direction is irregular. These fissures are marked out by a black coating, 

 which might be considered as due to incipient decomposition ; the outer outlines are 

 themselves strongly marked in black, the cleavages being less pronounced ; but the 

 most striking feature is the pleochroism which gives a = 7, green ; /S, reddish or flesh 

 coloured. The structure is zonary, and the sections often show twinnings of the ordi- 

 nary type, or twinned groups of two individuals, referable to the type + Pi. Magnetite 

 may be mentioned as a pretty common inclusion in this mineral. In certain cases the 

 augite shows also inclusions of plagioclase, but, on the whole, it is in the interspaces 

 of the large crystals of pyroxene that we can observe these felspathic inclusions. 

 Augite itself sometimes occurs as an inclusion within sections of olivine. We have 

 just remarked that the decomposition of the augite betrays itself by a network of 

 black lines, and by strongly marked outlines ; when the mineral is more weathered a 

 black nucleus is found at its centre. But another kind of decomposition occurs in 

 the microliths of the paste, and in some large microporphyritic grains ; they take on a 

 reddish tint, due to hydroxide of iron, which sometimes makes them almost opaque. 



The small crystals of augite in the ground - mass belong, without doubt, to a 

 second generation. They are prismatic, much better formed, slightly rounded at the 

 extremities, and, in ordinary light, almost colourless or with a greenish tinge. They 

 are not easily distinguishable from the plagioclastic microliths, except that, when decom- 

 posed, they are charged with red ferric oxide. Augite and hornblende are frequently 

 intimately associated in the augitic andesites of Camiguin, especially when the latter 

 mineral shows decided indications of alteration. For instance, a prismatic section of 

 hornblende may be seen terminated at both ends, and edged along the prismatic faces, 

 by greenish microliths arranged parallel to the vertical axis of the crystal they 

 surround. While the yellow amphibolic nucleus extinguishes between crossed nicols 

 at an angle of about 15°, the small crystals of the outer zone sometimes extinguish 

 at 40°, clearly establishing their nature as augite. In other cases no nucleus is found, 

 only some outlines remaining to indicate the previous presence of a hornblende crystal, 

 parallel to the vertical axis of which the small green augite prisms, by which it was 



