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



one of those first-generation minerals which determine the microporphyritic structure. 

 This mineral occurs in rather large sections with sharp crystallographic outlines ; 

 sometimes the form is hexagonal ; two of the sides belong to the vertical zone, and 

 are perpendicular to the plane of the optical axes. Others form an angle nearly of 

 77°, these being thus traces of the face Poo (d). These sections show cleavages per- 

 pendicular and parallel to the vertical axis, and a third rather indistinct cleavage 

 parallel to d. This olivine has a light greenish colour, but is transformed into a red 

 hematite-like matter along the cleavage planes and fractures, and on the edges of the 

 sections. It may also be penetrated by a network of dendritic oxide of iron. This 

 formation of hematite may be connected with the accumulation of grains of magnetite 

 on the edges of the olivine. This mineral has been subjected to corrosion and 

 dislocation, and is often enclosed in augite. The large zonary crystals of plagioclase 

 have been deformed by mechanical strain, and exhibit undulating extinction. They 

 are much lengthened and lamellar, being twinned according to the Carlsbad, albite, 

 and pericline laws. Extinction takes place at a large angle, sections more or less 

 parallel to M extinguishing at 43° ; the plagioclase is thus to be grouped with anorthite. 

 Augite is the third microporphyritic element, but its sections show few noteworthy 

 peculiarities ; they are feebly pleochroic, the differences in absorption being scarcely 

 perceptible. Sometimes these sections are twinned and exhibit a zonary structure, the 

 inner part approaching to violet in tint, while the outer layers remain almost colour- 

 less. This augite is filled with vitreous inclusions, magnetite, and sometimes patches 

 of olivine. In the vesicles are seen groups of small acicular crystals, probably some 

 zeolite. 



Judging by the specimens at our disposal, doleritic basalts are not common in 

 Inaccessible, only one instance of a dolerite occurring in the collection, and its 

 characters appear most plainly when the rock is examined microscopically. To the 

 naked eye it is scoriaceous, with large vesicles ; the ground mass is bluish grey, 

 speckled with irregular white spots of altered felspar. The microscope shows that all 

 the minerals are approximately of equal size. In this rock also olivine has crystallised 

 first, and it exhibits several of the peculiarities already described, being coloured 

 yellowish by alteration, and often surrounded by a zone of delessite. Lamellae of 

 felspar, somewhat drawn out, surround grains of augite. Sometimes these two 

 minerals are oriented with their axes parallel, at other times they cross each other at 

 various angles ; both belong to a secondary stage of consolidation. 



Another rock, resembling in structure the dolerite just described, differs from it by 

 the absence of olivine and the presence of a base, in which the minerals giving the rock 

 a doleritic structure are embedded. The base, which is devitrified by trichites, 

 surrounds crystals of augite, appearing to play an unimportant part, and large zonary 

 sections of plagioclase. These felspar sections are more basic at the centre than in the 



