REPORT ON THE PETROLOGY OF OCEANIC ISLANDS. 147 



grained, but is sometimes altered on the surface, assuming a reddish colour, and is 

 impregnated with limonite. Microscopic examination shows that, like the other rocks, 

 it is a felspathic basalt. Eather large sections of augite and olivine predominate in the 

 ground-mass, which also contains small microliths of plagioclase and of augite, with 

 magnetite, and a vitreous base. The felspars do not attain the dimensions of porphy- 

 ritic elements, and this rock presents few noteworthy peculiarities, except those due to 

 the alteration of olivine. The sections of this mineral are, as a rule, partly filled with 

 trichites ; the spots not yet occupied by this secondary product appear clear and 

 bmpid, but in polarised light these apparently unaltered portions hardly show the 

 colours of chromatic polarisation. We remark also that not only is the mineral full of 

 trichites, but that while its external form remained unchanged, it was permeated by a 

 secondary product, part of the original substance being removed. The mineral which 

 has formed in the interior of the sections appears as groups of prismatic crystals, 

 the summits directed towards the centre and the bases attached to the outer margins. 

 These microliths are arranged in parallel bundles, and appear at first sight to be felspar, 

 especially considering that we can detect in the same rock small plagioclases of undoubted 

 secondary origin filling up cracks. Still it seems impossible to reconcile this inter- 

 pretation with the crystalline forms and with the absence of polysynthetic twins, no 

 traces of which are to be found in the prisms included in the olivine sections. The 

 microliths in question present flattened angles at the summit, which may even appear 

 like a terminal pinacoid. From this form, and the fact that extinction takes place 

 almost parallel to the length, the microliths resemble certain zeolites, such as desmine 

 and natrolite. They cannot be ascribed to the zeolites, however, for their outlines 

 stand out too clearly, and the polarisation colours are identical, we may say, with those 

 of the felspar microliths of the ground-mass. They might be identified perhaps with 

 pilite. Olivine often forms in this rock very elongated crystals, which have sometimes 

 been broken by movements of the magma. 



A rock which is also scoriaceous, but contains better developed crystalline con- 

 stituents, approximates in its texture to dolerite. Microscopic examination shows 

 certain details in the structure of the plagioclase crystals which are worth noting. The 

 sections not showing polysynthetic lamellae are never perfectly homogeneous. They 

 are speckled with more or less rectangular points, all of which extinguish simultaneously, 

 and are similarly oriented. These inclusions are not isolated, as they seem, but must be 

 united by a layer of slight thickness extending under the plane of the section. This is 

 proved by the examination of sections of the zone P : k, in which polysynthetic twins 

 appear. The polysynthetic lamella? are not continuous, but interrupted at a certain 

 distance, and the space left free is filled by the principal individual. Thus a section 

 parallel to M ought to show these lamellae in the form of quadratic inclusions ; they 

 ought to present different extinctions from the felspathic mass formed of the principal 



