DOLERITES AND KHYOLITES OF NATAL AND ZULULAND, 155 



about 16°. The glassy base, wliicli is dense Avith globulites, 

 contains also long needle-like microliths of augite, with high 

 extinction angles, and small prismatic felspars generally 

 showing albite twinning and nearly straight extinction. Of 

 somewhat similar character as regards phenocrysts but with 

 holo-crystalline instead of glassy base are specimens (447- 

 450) from the Mkuzi River. One of these (447) is a red and 

 white speckled rock showing to the naked eye a few small 

 phenocrysts of felspar. These phenocrysts, which are mostly 

 of plagioclase showing albite twinning" with symmetrical 

 extinctions as high as 18°, together with a few small, pale- 

 green augites, occur in a ground-mass consisting of quartz and 

 felspar, the quartz being either in micropegmatitic intergrowth 

 with the felspar or forming a paste round small felspar prisms. 

 A chemical analysis of the glassy rock (402) from Indula- 

 wane Hill gave the following result under I : — 



I. II. III. 



100-02 100-00 99-76 



Under II is given the molecular percentage composition, 

 and under III the analysis of a Toscanite from Monte Bracci- 

 ano, Italy .'I 



• ' Jonrn. Geol. Chicago," v (1897), p. 49. 



