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



indicate that the volcano has been affected by a movement of elevation since its 

 formation. 



In the specimens which we have examined we have not found any of the rocks of 

 ancient type mentioned by Professor Doelter in the passage above alluded to. All those 

 collected by the Challenger belong to recent volcanic rocks, which we shall now 

 describe ; they come from localities not far from Porto Grande. 



We shall first describe the specimens from the dykes, which traverse sheets of lava. 

 They are basalts presenting the microscopic characters of that lithological type. One 

 of the specimens is a dolerite ; under the microscope sections of olivine of small size 

 and lamellae of felspar are seen enclosing grains and crystals of augite. The sections of 

 plagioclase show the characters of the felspar of the basaltic rocks. The same may be 

 said of the augite. Generally the latter mineral is in sections with irregular outlines, in 

 other cases it is seen in the form of intercrossed groups. The augitic sections 

 showing these groupings in our preparations were not cut so as to allow of estimating 

 exactly the angle at which the twinned crystals were joined, or of determining the 

 law of twinning ; but their aspect resembled sufficiently that of the twin of augite 

 according to an acute pyramid, which has been observed macroscopically by Vrba, and 

 of which Professor Becke has indicated the presence in microscopical specimens. These 

 augitic sections are also twinned, following the ordinary law parallel to oo Poo . 

 The olivine is not microporphyritic ; it is seen in small sections often lozenge-shaped, 

 with a centre of the same form of which the sides are parallel to the outlines of the 

 section. It is often yellow by decomposition ; and disposed in the mass of the rock so 

 as to contribute to its doleritic structure. Small scales of biotite are occasionally seen ; 

 sections of magnetite, on the other hand, are numerous. Finally, a small quantity of a 

 yellowish fibrous matter is found amongst these minerals, which, it appears very 

 probable, was originally a vitreous substance. 



Another basaltic rock, forming a dyke and covered with zeolitic incrustations in 

 which may be observed isolated crystals of chabasite, must, like the last mentioned, be 

 referred to the felspathic basalts ; it contains crystals of augite visible to the naked 

 eye. In the ground-mass, formed of a colourless base with microliths of augite and 

 felspar, crystals of the same minerals but of a larger size are seen associated with 

 olivine and magnetite. The augite is generally perfectly crystallised ; the pleochroism 

 and absorption are — 



y and ft purplish > a pale yellow. 



With respect to the plagioclase, the extinctions on the face M are negative and 

 about 27° ; for two adjacent hemitropic lamellae symmetrical extinctions are seen 

 with the maximum value 34°, which brings this felspar very near to anorthite. These 

 plagioclases have often crystallised according to the albite law, and at the same time 

 show the Carlsbad twinning. The presence of the latter twin may even be recognised 



