REPORT ON THE PETROLOGY OF OCEANIC ISLANDS. 11 



fibres lying parallel to the axis c. The felspars belong to two types ; one of these is 

 lamellar, the other occurs in short prisms. The latter, generally, have less numerous 

 plagioclastic lamellae than the former, and the angles of extinction are large. These 

 plagioclase sections generally show a large individual, in which are one or two 

 hemitropic lamellae, the thickness of which is very small compared to the size of the 

 section. Some crystals of albite and of anorthite have the same peculiarity, and in 

 this case the extinctions seem to indicate that the felspar may be anorthite. The 

 lamellar felspar, on the other hand, judging by the extinctions, seems rather to be 

 labradorite. These plagioclases do not kaolinise ; when altered they appear of a 

 milky colour and slightly granular. With polarised light they remain dark or 

 assume a very faint bluish tint. Perhaps this modification is a transition to a 

 zeolitic substance, the nature of which it is difficult, if not impossible, to ascertain. 

 The augite has the ordinary characters of that mineral in doleritic basalts. The 

 grains are generally wedged into the triangular space formed by the inter-crossing 

 of the lamellae of plagioclase. When decomposed, its violet colour is weakened. 

 The vitreous base, rather distinct patches of which are found around the augitic 

 microliths, sometimes forms a narrow and colourless zone, surrounded in turn by an 

 isotropic rim of a light brownish colour, filled with a blackish globulitic granulation. 

 The existence of these zones may be explained if we bear in mind that when the augite 

 crystallised the surrounding parts of the magma gave up their metallic pigment to the 

 crystal that was being formed, and so the first zone was necessarily discoloured. 

 The darker external vitreous zone may be considered as a residuum of crystallisation 

 richer in metallic oxides ; these have often become isolated, assuming the globulitic 

 form. As we have already stated, this rock belongs to the felspathic dolerites with a 

 vitreous base. 



Below Casa Blanca a brownish rock was collected ; it is earthy, with an altered 

 appearance, has an irregular fracture, is fine grained, and contains tabular crystals of 

 sanidine measuring 3 to 5 mm. Microscopic sections show a ground-mass composed of 

 lamellae of tridymite with a faint yellow colour. Bather large sections of felspar and 

 augite can be distinguished in it ; this latter mineral is frequent in small sections 

 embedded in a tridymite mass. Two kinds of felspar are to be seen ; some lamellae 

 have small extinctions like those of oligoclase, which is known to occur in the older 

 rocks containing orthoclase and quartz. The other felspathic sections are those of a 

 monoclinic felspar ; they have irregular and indistinct outlines, and never show poly- 

 synthetic striation, but they are twinned and composed of two individuals. The outlines 

 of these sections and their extinction show that this felspar is twinned according to the 

 law of Manebach ; these sections show, like sanidine twinned according to the Carlsbad 

 law, two halves joined together, but, whereas in a Carlsbad twin, the direction of 



