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



seen to be traversed by black lines, with shaded borders, showing a certain parallelism. 

 In other cases, when the section is turned round between crossed nicols, shadows are 

 seen sweeping across. The difference between this appearance and that of undulating 

 extinction does not appear at first, but, as we have just said, pressure cannot be called 

 in, in this case. These phenomena are never seen in sections which show hemitropic 

 striae with great sharpness, nor in sections parallel to M. Sections of an intermediate 

 zone, approaching M, show this peculiar extinction ; on the other hand, when the 

 sections are more in the zone P : k, the parallel black lines with shadowy borders appear. 

 These observations lead us to conclude that this extinction is due to the fine lamellation 

 of this plagioclase, the sections of which, cut more or less obliquely to the plane of 

 twinning, must in polarised light show these undulations, or these traces of albitic 

 lamellae with indistinct borders. Olivine is somewhat uncommon in this basalt ; it 

 usually appears in grains, but occasionally the sections present crystallographic outlines. 

 Amongst the latter there is one form which is hexagonal, with two parallel sides longer 

 than the others. In ordinary light it appears quite homogeneous, but in polarised light 

 it is seen to be divided into halves by a straight line perpendicular to the longer sides. 

 The two halves in certain positions between crossed nicols show sharply different 

 colours, although these are not very intense on account of the section being cut 

 perpendicular to an optical axis. In convergent light this axis is shown to have the 

 same position for the two halves, and to be eccentric. Everything indicates, however, 

 that the plane of the optical axes is perpendicular to the direction indicated in the 

 section by the trace of oo P } which corresponds to the longer sides of the hexagon. The 

 shorter sides should be traces of flattened domes. This section shows two cleavages : 

 one parallel to the base, the other parallel to a pinacoid of the prismatic zone, and 

 perpendicular to the former. More or less irregular fractures may also be observed 

 parallel to the short sides of the hexagon, indicating a less distinct cleavage following 

 the faces of flattened domes. The sections of olivine, sometimes little altered, are 

 crowded with inclusions of magnetite. 



The augite presents no noteworthy peculiarity, except that the crystals are often 

 grouped. They are sometimes twinned in the ordinary way, or intercrossed with con- 

 siderable regularity, although not clearly enough to show a law of twinning. 



The ground-mass is chiefly composed of microliths of augite and plagioclase — the 

 latter lamellar and giving great extinctions — and of a vitreous base, which surrounds 

 all the minerals of the rock. 



Another specimen from the same place closely resembles that just described, except 

 that the colour is greyish, and rather large crystals of augite are visible to the naked 

 eye. The microscope shows that it also is a felspathic basalt. 



Finally, rocks from the same locality have a scoriaceous structure ; they are black, 

 and contain somewhat large vesicles. The ground-mass appears compact and fine- 



