CHAPTER XXII 



basic glasses and volcanic agglomerates 



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 2'6i to 277, 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"6i and 

 exhibiting large porphyritic crystals (6 to 8 mm.) of plagio- 

 clase. 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 



