REPORT ON THE PETROLOGY OF OCEANIC ISLANDS. 5 



lamellar, their extinction taking place under very small angles, the crystals frequently 

 showing twins without repetition ; in short, all seems to indicate that their index of 

 refraction is higher than that of the tabular felspar. The ground-mass reacts like 

 annealed glass between crossed nicols, the tints being hazy. It is uncertain whether 

 these phenomena are to be ascribed to contraction or to pressure which may have acted 

 on the isotropic substance and caused its devitrification. This rock ought to be classed 

 with the pyroxenic andesites containing sanidine, a type related to the trachytic series. 

 Another rock from the Canadas is bluish grey, with an irregular fracture ; it contains 

 small vesicles with a homogeneous aspect, speckled with black granules. Examination 

 of microscopic slides shows that the rock is a felspathic basalt. The plagioclases, 

 of which numerous sections are seen, have very large lamellae, and their extinctions 

 are those of labradorite ; the augite and the olivine have generally rounded outlines, 

 the latter mineral being decomposed. With these minerals are associated grains and 

 crystals of magnetite, which are rather numerous, and very elongated and truncated 

 prisms of apatite. The ground-mass is formed of a vitreous matter, which is under- 

 going alteration, as shown by the phenomena of chromatic polarisation it exhibits. 



Lastly, a porphyritic lava was collected in the Canadas. This rock is black, 

 massive, finely grained, scoriaceous, has an irregular fracture, and contains porphyritic 

 felspar crystals. Microscopic examination shows that it has a vitreous base, with 

 very distinct traces of fluidal structure. Crystals of augite, felspar, hornblende, and 

 black mica stand out of the ground-mass, and give the rock a microporphyritic 

 structure. The felspar crystals are twinned according to the albite law, these 

 plagioclastic lamellae being embedded in two principal individuals twinned according 

 to the Carlsbad law. The separation between the two individuals is clearly 

 marked only on a portion of the length of the section ; in several 

 places there is an irregular interpenetration of the two halves. 

 Interposition of biotite scales in the plagioclases can occasionally 

 be ascertained. The same mineral is found as inclusion in augite, 

 as shown in the adjoining figure (fig. 3). The crystals of augite are 

 distinctly prismatic and of light greenish tint, hardly pleochroic. 

 Hornblende is not abundant; its sections are often irregular, some- 

 times they are aggregated, or form groups. They might be taken for 

 twinned crystals, but there is only a simple juxtaposition ; indeed, = °' p 



the lines of cleavage of adjoining sections never follow the direction ' Canadas." 



they would do in the case- of true twin crystals. The rays parallel ' ^^lameuS parallel to 

 to a are of a pale yellow colour, those parallel to /3 are reddish. 

 The most characteristic mineral of the rock, and the most widely distributed, is without 

 doubt biotite ; sections cut parallel to the base are very frequently observed ; these 

 lamellae are very pleochroic, the rays vibrating perpendicular to a being of a pale yellow 



