“Crush-Tuffs” formed of Basic Glass
This is a remarkable group of compacted tuff-like rocks which as hand-specimens would be generally regarded as pitchstone-tuffs. Their detrital origin is, however, often very doubtful. They are composed of fragments of basic glass, carrying plagioclase phenocrysts, with the interspaces occupied by palagonite and by the finer debris of the glass and felspar. The larger glass fragments, which vary in different rocks from 1 or 2 to 4 or 5 mm. in size, have been crushed in situ, the broken portions often remaining more or less in position. These fragments are invested by palagonite and have eroded borders, as shown in the figure on page [342]. The glass is bottle-green, non-vacuolar, fuses readily, and only at times displays incipient crystallisation. The explanation of the origin of these rocks is attempted in [Chapter XXIV.] They contain neither carbonate of lime nor organic remains. The most typical example is present in a bed underlying a pitchstone-agglomerate near Narengali (see page [149]). It is not uncommon to find evidence of crushing in the glassy matrix of a pitchstone-agglomerate or of rubbly pitchstone, as in the Va Lili Ridge (142), the Korotini Bluff (157), and Mount Soloa Levu (313); and here also palagonite has been produced around the crushed fragments.