552 MR G. W. TYRRELL. 



Nepheline-syenites appear to be widely distributed in Central Africa. Berg * has 

 described a ncpbeline-sodalite-syenite from Senze do Itombe, and HolmesI" has recently 

 described a nepheline-syenite from the same locality in north-western Loanda, the 

 nortliern province of Angola. This rock contains segirine-augite as the principal ferro- 

 magnesian constituent, but this mineral is frequently bordered by amphiboles which 

 are identified with hastingsite or a mineral of the katoforite-arfvedsonite series. 



2. Shonkinite. 



This remarkable rock occurs in the Chieuca volcanic vent (170&, 170<7), and at 

 Ochilesa (172a, 208b, 208c). It appears to be the dominant type in this volcanic 

 district. It is a fine-grained rock of bluish-grey colour and doleritic appearance, and 

 is besprinkled with numerous small black crystals of hornblende ranging up to \ inch 

 in diameter. 



In thin section the rock is ideally fresh, and consists mainly of orthoclase with 

 interstitial sodalite, and a comparatively large quantity of mafic minerals which 

 include titanaugite, barkevikite, and magnetite. Apatite is an important accessory. 

 The texture of the rock is thoroughly granitoid or hypidiomorphic. Many of the 

 augite and hornblende crystals are larger than those of the orthoclase composing the 

 mass of the rock, but their porphyritic character is not so apparent in thin section 

 as in the hand specimen. The felsic constituents poikilitically enclose numerous 

 small crystals of titanaugite, magnetite, and apatite, which doubtless gives the rock 

 its dark colour. 



The orthoclase forms elongated prisms which are simply twinned, or it builds more 

 equidimensional subhedral crystals which contain numerous, evenly spaced, parallel 

 rows of very minute bubbles, apparently of liquid. These rows are perpendicular to 

 a direction of cleavage within the mineral, and are also parallel to a direction of 

 extinction. A pale-bluish sodalite fills • up many interspaces, and is frequently 

 crowded with minute bubbles and inclusions. The presence of sodalite was confirmed 

 by obtaining a silver chloride precipitate in a nitric acid digest of the rock, after 

 removing apatite by means of a heavy liquid. 



The felsic minerals form a sort of coarse groundmass to numerous, pseudo-porphy- 

 ritic, perfectly euhedral crystals of barkevikite and titanaugite, and also poikilitically 

 enclose a swarm of small grains of the same minerals along with apatite and magnetite. 

 The barkevikite and titanaugite are in roughly equal amount. The former occurs in 

 fine, euhedral, red-brown crystals, occasionally with simple twinning, and narrow 

 darker-tinted borders. In some of the rocks considerable resorption of the hornblende 

 has taken place, the final appearance being of an irregular mass of magnetite mixed 

 with granules of a greenish amphibole, the whole being surrounded by a dense corona 

 of magnetite granules. The pyroxene has occasionally undergone a similar alteration, 

 but to a lesser extent. The augite is of the purplish tint which is held to indicate a 



* Tscherm. min. ydr. Mittk, vol. xxii (1903), p. 359. t Geol. May., dec. vi, vol. ii (1915), p. 323. 



