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



caverns on the coast examined by the Challenger naturalists — were collected. This 

 rock, an augite-andesite, is black and massive like a compact basalt ; the fracture is 

 plane. No constituent minerals can be detected either by the naked eye or with a 

 lens, but the microscope shows some microporphyritic sections. Amongst these there 

 are a very few plagioclastic lamellae giving large extinctions, and some sections which, 

 from the absence of polysynthetic twins may be referred to sanidine ; the latter are 

 traversed by two cleavages at right angles, and give straight extinction. The augite 

 of this rock is of a light violet colour, its outlines are irregular, and large crystals 

 seldom occur. Corroded hornblende sections are also found as microporphyritic 

 elements, sometimes twinned according to the ordinary law ; they show the pleochroism 

 — a, yellow ; /3, brown ; 7, brown. This mineral is sometimes quite decomposed, being 

 invaded by augite microliths and magnetite. The ground-mass of the rock resembles 

 that of basalt in some respects ; it contains numerous plagioclastic lamella? and micro- 

 liths of several minerals. Those of augite are almost always twinned, the sections 

 appearing to be divided in two longitudinally ; the summit is terminated by a low dome, 

 and transverse sections appear as irregular, slightly-coloured grains. Hornblende is 

 present in small pleochroic fibrous prisms which might be taken for biotite, but 

 the extinction is oblique. The rock has been slightly altered with formation of 

 delessite. 



An intrusive vein of amphibolic andesite crops out on the floor of Bromley's Cave. 

 It is a black rock with a plane, more or less schistoid, fracture. A very few drawn out 

 vesicles are to be seen, and to the naked eye only some fine needles of hornblende 

 appear, while the lens shows a mass composed of crystalline grains. Microscopical 

 preparations show that microporphyritic crystals of plagioclase, hornblende, augite, and 

 magnetite are embedded in the ground-mass. Under a low power the paste appears 

 brownish and homogeneous, but when more highly magnified it is seen to be made up 

 of an aggregation of plagioclase, augite, and hornblende microliths, the last named 

 being present in greatest number. The crystals of the first generation which produce 

 microporphyritic structure are generally corroded. 



Large sections of plagioclase are sometimes lengthened following the edge PjM, 

 sometimes flattened parallel to M ; this mineral also occurs as grains. The felspar 

 is related to anorthite, the maximum angle of extinction in the zone P : k being about 

 39°; two adjacent hemitropic lamella? gave a maximum extinction of 31°. The structure 

 is usually homogeneous, but when the sections are zonary the centre is more basic than 

 the outer layers. There is nothing remarkable about the large ill-defined sections of 

 augite which are identified by their pale greenish colour, characteristic cleavages, 

 crystallographic outlines, and the angles of extinction. Hornblende is more important, 

 and often appears in irregular corroded grains, although the form is sometimes fusiform, 

 or that of a much-lengthened prism. The sections are almost always twinned according 



