REPORT ON THE PETROLOGY OF OCEANIC ISLANDS. 



79 



this felspar is traversed by few hemitropic lamellae. The augite sometimes contains 

 granules of olivine, magnetite, and apatite as inclusions. 



The beds formed by this altered felspathic basalt are overlaid by a basaltic tufa. 

 The transition is effected through rocks that are richer in glassy materials, but belong, 

 nevertheless, to the same lithological type. The tufa covering the sheet in question is 

 formed of fragments in which the vitreous element predominates ; they appear, under 

 the microscope, to consist of a vesicular yellowish or brownish glass, passing occasionally 

 into the hydrated, reddish, resinoid product of decomposition of certain basic volcanic 

 glasses. The crystals that separate out from these vitreous fragments belong chiefly 

 to greenish pleochroic augite, and are generally irregular in contour. The prepara- 

 tions show, besides, sections of the same mineral and of plagioclase of smaller size, with 

 clean cut outlines embedded in the glassy matrix, and belonging to a secondary period of 

 consolidation. Olivine and magnetite are relatively rare. Frequently the large crystals 

 of augite and plagioclase are partly lined or entirely surrounded by a vitreous substance 

 more opaque and blacker than the glass that forms the ground-mass. 



This tufa is overlaid in its turn by a rock of the same kind, but of a coarser grain. 

 It consists of lapilli, 2 to 3 centimetres in diameter, and 

 is full of augite crystals visible to the naked eye. There 

 also occur in it fragmentary crystals of olivine, which 

 show their clastic origin very clearly under the micro- 

 scope. The same remark applies also to some of the augites 

 in this tufa. As is shown by fig. 15, the sections of these 

 clastic minerals exhibit certain outlines which represent 

 the crystallographic contours. These traces of faces are 

 distinct and straight (a), and are bordered by black glass of varying thickness ; but 

 wherever this section shows fractures, this coating of black glass is absent. This 

 furnishes evidence that the crystals in question were once 

 entirely embedded in a dark or almost opaque glassy 

 magma, from which they were projected as loose material; 

 they must have been partially crushed, and wherever 

 fracture occurred the glass was carried away, while where 

 they remained unbroken the vitreous mass protected the 

 faces of the crystal. The augite of the tufa we are 

 describing has a great tendency to form twin-crystals as 

 polysynthetic as those of some plagioclases. These lamellar 

 individuals, intercalated in the principal crystal, are ex- 

 tremely distinct and remarkably regular ; when large 

 enough, they betray their presence by sections with reentrant angles (see fig. 16) 

 formed by the alternating faces of two adjacent individuals. Sometimes, too, the 



Fig. 15.— Tufa of Tristan da Canha. 



Clastic grain of olivine crystal, certain 

 outlines (a) exhibiting crystallographic 

 contours bordered by black glass. 



Fig. 16.— Tufa of Tristan da Cunha. 



Polysynthetic twinning of augite ; re- 

 entrant angles at the upper part of 

 the section by the alternation of the 

 faces of two adjacent individuals. 



