538 METAMORPHOSED SEDIMENTS IN BRITISH EAST AFRICA. [Dec. 1913. 



are rare accessory minerals, and the rock contains also occasional 

 grains of dirty epidote or sphene. 



A rock with the same associates and from the same stream 

 consists principally of scapolite, diopside, and brownish-red garnet 

 with accessory calcite, quartz, hornblende, plagioclase, sphene, 

 apatite, and zircon. The water-clear plagioclase belongs to the 

 acid end of the series. The apparently corrosive action of the 

 quartz and plagioclase on the hornblende is noteworthy, the last- 

 named mineral giving finger-shaped cross-sections, which, it is of 

 interest to see, are peripherally converted into glaucophane. 

 This mineral has a pleochroism varying from yellowish-brown to 

 ultramarine. 



From the bed of an important tributary to the Turoka, entering 

 below the streams above-mentioned, comes a compact gneiss con- 

 taining epidote in quantity. The mineral forms irregular grains, 

 and is occasionally idiomorphic. The scheme of pleochroism is, 

 A, colourless, or very pale yellow ; B, colourless ; C, pale greenish- 

 yellow. Sphene is a common accessory mineral, and is earlier in 

 formation than the epidote. Quartz is abundant, and forms large 

 grains containing many inclusions ; apatite and zircon are rare. 

 The acid plagioclase is often zoned, microcline is common, and 

 corrodes the earlier plagioclase. 



Three slides of kyanite-schist were cut from specimens re- 

 presenting three successive beds only a few inches apart, situated 

 on the bank of one of the headwater streams of the Turoka. 

 The uppermost is a kyanite-muscovite-biotite-quartz-schist with 

 subsidiary microcline or microperthite. Quartz greatly pre- 

 dominates, the grains are elongated parallel to the predominant 

 foliation, one grain polarizing over large and irregular areas, giving 

 the impression of the slow rupture of viscous substance. 



The underlying rock has a rather confused crystallization : micro- 

 perthite is conspicuous, but there is no simply-twinned plagioclase. 

 The quartz is dusty and full of inclusions, and the kyanite forms 

 large irregular plates ; a red-brown mica is conspicuous. White 

 mica and zircon are accessory minerals. 



In the third section, muscovite and biotite are subordinate, the 

 felspar has an irregular wavy extinction, suggesting very strongly 

 fine microperthite ; and, as before, there is no simple plagioclase. 

 Locally, the kyanite is altering into a fine aggregate of hydrous 

 white mica. 



Elsewhere in the same stream-bed, kyanite-gneisses, still richer 

 in the specific mineral, occur. A thin section of one shows the rock 

 to consist almost entirely of kyanite and quartz, a few crystals of 

 zircon and flakes of white mica being almost the only accessory 

 minerals. The quartz shows slight signs of crush. The kyanite 

 in these slides appeared to me in some respects abnormal, and I 

 am much indebted to Mr. T. Crook, of the Imperial Institute, for 

 checking my determination. 



