Basic Pitchstone and Basic Glass
It is not possible to draw a sharp distinction between the pitchstone and the purely vitreous condition of these glasses. The following remarks will therefore apply to both.
Regarded as components of the pitchstone-tuffs and palagonite-tuffs these rocks have a very extensive distribution in the island; but in the massive state they are hardly ever to be found, whilst in the form of agglomerates they are only frequent in certain localities, as in the cliffs of the Korotini Bluff, in the vicinity of Mbale-mbale, on the slopes of Soloa Levu, and in the dividing ridge between the Mbua and Lekutu plains. On rare occasions they are to be found in a rubbly condition, as in the upper part of a basaltic flow described on p. [92], or they may form veins in a more crystalline basaltic rock as at Vatulele Bay. Their specific gravity ranges from 2·61 to 2·77, and they fuse readily before the blow-pipe, the melting beginning in the ordinary flame. Since they are not dissolved under any condition in HCl, they would be referred to the old hyalomelane group of basic glasses.
One of the most interesting of these rocks occurs on the slopes of Soloa Levu. As displayed on the south-west slope, it presents itself as a brownish-black rock with a specific gravity of 2·61 and exhibiting large porphyritic crystals (6 to 8 mm.) of plagioclase. It is generally compact, but it is in places a little vesicular, the minute cavities being often filled with a zeolite. The mode of occurrence of this pitchstone-porphyry is described on p. [104]. In the slide the plagioclase phenocrysts give lamellar extinctions (21°-27°) of andesine labradorite, and have regular outlines, with but few inclusions of the glassy magma. There are also a few small phenocrysts of augite with dark rounded borders and showing in some cases lamellar twinning. The groundmass is a brown rather turbid glass in which dark points of devitrification occur. It is traversed by cracks that also penetrate the felspar phenocrysts. These cracks are filled with a feebly refractive material like palagonite; and there are traces of the early stage of the palagonitic change in one or two places. This is of importance, because on the north-west side of the hill occurs the same rock, in which the basic glass has been converted into a reddish-brown almost opaque palagonite; but in this case the porphyritic crystals of plagioclase are more affected by the magma, being rounded and extensively penetrated schiller-fashion by this material; whilst the augite phenocrysts are somewhat similarly affected. The altered glass is also vacuolar, the cavities being filled with a zeolite. There is an indication of some degree of crushing in the fracture of some of the felspar phenocrysts in situ. There appears to be a connection, as shown on p. [342], between the crushing of a basic glass and the formation of palagonite. It is noteworthy that with this change the specific gravity drops from 2·61 in the comparatively fresh rock to 2·14 in the palagonitised hydrated condition.
As another example of these basic pitchstones I will take that forming an agglomerate near Mbale-mbale. It has a specific gravity of 2·77 and displays phenocrysts of plagioclase, olivine, and augite. The first-named, which give the lamellar extinction of acid labradorite, (22°-28°), are fresh-looking and only affected to a small extent by the magma. Those of olivine and augite are in much the same condition. The glass of the groundmass is rather turbid and displays numerous dark patches of incipient crystallisation, which in some cases prove to be composed of brush-like crystallites around a clear H-shaped nucleus, and in other cases have a more prismatic form.
A vitreous rock having some of the characters of a variolite is found near Narengali (see page [150]). It, however, has the low specific gravity of 2·43 and is not readily fusible with the blow-pipe. It displays an imperfect spheroidal structure on a small scale, being made up of nodules, the largest having the size of a filbert. In the slide it appears as a grey glass made up of sheaf-like aggregates of fibre-like crystallites, apparently of felspar, with minute skeleton prisms of pyroxene in parallel arrangement, and is traversed by perlitic cracks.