REPORT ON THE PETROLOGY OF OCEANIC ISLANDS. 67 



The sections of olivine are much altered on the edges ; they are sometimes trans- 

 formed into red hematite, and trichites penetrate them in every direction. That the 

 trichites are of secondary formation is made evident by the fact that they are 

 developed in the interstices between fissures, and sometimes follow the curves marked 

 out by the latter. 



Augite is often lamellated as it appears in some diabases. The lamella? are pro- 

 duced by the repetition of twinned individuals interposed parallel to the pinacoid 

 oo P oo . The nature of this mineral confirms our determination of the rock. The 

 absence of cleavage in these pyroxene sections is striking — they are rarely furrowed 

 by the regular fractures so common in this species, but this peculiarity may be due 

 to the unusual thickness of the microscopic preparation submitted to examination. 



Another specimen of a similar rock contains a very basic plagioclase, as in the 

 preceding case, and also greenish augite, but there is no olivine, its place being taken 

 by some rare sections of a rhombic mineral. These might be mistaken for olivine by 

 ordinary and by parallel polarised light. The sections are colourless, but brilliantly 

 coloured in polarised light ; they stand out in high relief, the outlines being blunted 

 and the surface shagreened. They are, however, distinguishable from olivine by the 

 presence of extremely fine black linear inclusions, running parallel to each other and 

 to the length of the sections, and sometimes assuming the form of negative crystals. 

 Extinction takes place parallel and perpendicular to these inclusions and to the traces 

 of faces of the zone of the prisms. In convergent light it becomes apparent that this 

 mineral should be classed with the rhombic pyroxenes, such as enstatite. The deter- 

 mination as enstatite is confirmed by the use of the condenser, which enables one to 

 distinguish an eccentric optical axis so situated as to show that the plane of the axes is 

 parallel to oo P <x> . 



Veins and Siliceous Infiltrations. 



In his geological description of Ascension, Darwin ' calls attention to the numerous 

 veins of siliceous material which cut through the rocks of the " Crater of an old volcano." 

 These veins he described as white, composed of a material with low specific gravity and 

 conchoidal fracture. The colour sometimes becomes reddish ; in other cases it is 

 yellowish white and the fracture angular, while a whitish powder fills the cavities. 

 Both varieties occur as amorphous masses in the altered trachyte, or as wide irregular 

 veins coloured red and running vertically or in a tortuous manner. This rock, which 

 resembles sandstone in appearance, is nothing but an altered trachyte. Jasper of an 

 ochreous colour is found in large masses, and occasionally in the form of veins enclosed 

 in altered trachyte, or in scoriaceous basalt. The cavities of the latter rock are lined 



1 DarwiD, Geol. Obs., p. 45. 



