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



Under the microscope these scales show two optical axes of a very small angle ; the 

 sections where the lamellae are seen superposed are pleochroic, showing a yellow tint 

 for the rays parallel to the scales, and a brownish one for those perpendicular to them. 

 Irregular cracks appear on the scales parallel to OP. In general this mineral is much 

 altered. It is associated in the same rock with pretty large fragments of augite and 

 olivine ; the former are cracked and of a greenish colour. These different minerals are 

 grouped in an irregular manner and mingled with microscopic lapilli. The mica often 

 forms small groups. This heterogeneous assemblage of minerals leaves the impression 

 that the rock is of clastic origin. 



A reddish brown spongy lava of basaltic nature containing zeolites is nearly allied 

 to the tufa of which a short description has just been given. This lava, like the tufa, 

 contains black mica and augite ; the latter mineral is granular ; more rarely its sections 

 possess regular outlines with traces of twinning. This is almost the only microporphy- 

 ritic mineral. The alteration of the rock is seen by the little lamellae of biotite which 

 take a reddish colour from the deposit of ferric oxide. These micaceous sections are 

 pleochroic, and present, with regard to their physical properties, some analogy with 

 those of the preceding rock. The ground-mass is formed of a vitreous base, in which 

 there are to be observed numerous plagioclastic lamellae of rather small size, entirely 

 transformed into zeolitic matter. With these plagioclases are associated small crystals 

 of augite. In certain cavities between the crystals just mentioned a layer of greenish 

 substance lias been deposited ; it is more or less mammillated on the surface, resembling 

 delessite, and is probably derived from the decomposition of a bisilicate. Amongst 

 the minerals of secondary origin may be mentioned zeolitic masses which fill the small 

 cavities of the rock with fine fibro-radiated needles. These zeolites are often covered 

 with or accompanied by a deposit of ferric oxide. The tufts of zeolites are formed of 

 small very elongated prisms with straight extinctions, often thinning at their point of 

 insertion, and thickening towards the summit which advances to the opening of the 

 little drusy cavity. This summit is often terminated by an obtuse pyramid, or else it 

 has the pinacoid OP. The sharpness of these little prisms, their aspect, their localisation, 

 and their clearly marked character of rhombic minerals, can leave no doubt as to their 

 identification with the minerals of the zeolite group, and they might, from their 

 crystallographic characters, be placed with natrolite or brevicite. 



Finally, we have to mention a blackish rock, studded with more or less circular 

 zeolitic points and with large crystals of augite. Under the microscope its aspect 

 resembles kersantite in a striking manner ; lamellae of plagioclase, associated with a 

 mineral which might be taken for biotite, are seen. But in observing the extinctions of 

 these brown lamellar sections it is perceived that they do not extinguish parallel to the 

 length, but at an angle which attains on an average 10°. This mineral must therefore 

 be hornblende ; hexagonal transverse sections are seen. In the very elongated sections, 



