ch. xxii BASIC GLASSES 



313 



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 con- 

 nection, 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 - 6i 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 277 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. 



