158 THE VOYAGE OF H.M.S. CHALLENGER. 



to explore this crater. They could only descend it to a depth of 60 feet, for the 

 suffocating acid vapour which enshrouded them, and the difficulties of the ground, 

 compelled them to return. They found deposits of sulphur in the crevices, and saw 

 everywhere rocks profoundly modified in structure by the action of vapours exhaled 

 from the volcano. The rocks about to be described were collected from the summit of 

 this cone. 



The rocks of Ternate belong to the augite-andesites, but in some cases, from the 

 presence of olivine, they ought to be classed amongst the basalts. We shall first 

 describe the andesitic lavas. 



The most characteristic specimens are slightly scoriaceous, and of a dark colour ; 

 the naked eye and the lens only show some vitreous or white points which are crystals 

 of plagioclase. Microscopically the rock is vesicular; the matrix, chiefly formed of 

 vitreous matter, is devitrified here and there by spherulites, and numerous plagioclase 

 microliths are scattered through the brownish class. 



The large sections of plagioclase are zonary, and full of vitreous inclusions ; they 

 exhibit at the same time the twins of the albite and pericline law. Sections, where 

 the lamellae are twinned following the albite and the pericline laws, appear clearly 

 defined and intercrossing each other at right angles (also parallel to k), and give 

 symmetrical extinctions of from 20° to 16°. These values show that we are dealing 

 with a plagioclastic mixture which approaches labradorite. 



Most of the augite sections are twinned polysynthetically. The lamellae, often 

 resembling those of plagioclase, are sometimes very numerous and closely packed, 

 giving some sections of this mineral a fibrous appearance. The central part of the augite 

 is often the most lamellated. Twinned lamellae are sometimes noticed in the form of 

 two triangles meeting at the apex, and thus resembling the well-known clepsydra 

 structure which occurs in this species. A rather long augite crystal cut nearly parallel 

 to oo P co showed these lamellae closely packed in bundles at the centre, but spreading 

 out by the addition of more lamellae towards the extremities of the section. They thus 

 present an appearance like a sheaf bound tightly in the middle, and show considerable 

 analogy to the internal structure of augite just referred to. The pleochroism = 7 greenish, 

 @ yellowish. This pyroxene has a great angle of extinction ; the hemitropic lamellae 

 intercalated in the principal individual extinguish at 50°, and the large crystal itself 

 at 44°. The cleavage is not well marked, doubtless because the slices cut off this 

 somewhat scoriaceous rock are not so thin as those obtained by polishing a more 

 compact mass. Magnetite, presentiug no noteworthy peculiarity, is also an essential 

 constituent of this andesite. 



Some other specimens, which must also be classed with the andesites, resemble that 

 just described very closely in their microscopic characters, only the ground-mass is 

 darker, more iridescent, and less vesicular. There are some minor differences also 



