556 MR G. W. TYRRELL. 



3. Solvsbergite. 



The volcanic vent of Chieuca is filled with tuff and agglomerate consisting of 

 boulders, pebbles, and comminuted debris of shonkinite, sodalite-syenite, and grey or 

 green compact rocks identified as solvsbergite. It is possible that the latter rock 

 also occurs as small intrusive masses or as lava-flows. The principal rocks of the 

 vent may be described as shonkinite-tufT (I70h), and as solvsbergite-tuff (170c, 170/). 



Many specimens of the solvsbergite were collected (170, 170a, 170e, 1626); and 

 amongst these three varieties may be distinguished. One contains large phenocrysts 

 of soda-orthoclase, oligoclase (AbaAni) diopside with mantles of segirine-augite, and 

 small crystals of segirine-augite, in a dense, felted, trachytic groundmass consisting 

 of minute laths of orthoclase, granules of green pyroxene and magnetite, with some * 

 indeterminate material. A few, small, partially resorbed crystals of barkevikite 

 occur. In the second variety the barkevikite constitutes the only phenocrystic 

 mineral apart from small crystals of segirine-augite, and this type may therefore be 

 called hornblende-solvsbergite. The groundmass here contains abundant segirine 

 and a little nepheline, and the rock is therefore transitional to tinguaite. The 

 barkevikite has suffered considerable resorption ; the crystals are always surrounded 

 by a dense corona of granules of green pyroxene. Frequently the core of barkevikite 

 has entirely disappeared. 



A third type is characterised by a colourless glassy groundmass crowded with 

 acicular green microlites. It carries small and sparse phenocrysts of euhedral 

 barkevikite, segirine-augite, orthoclase, and a little nepheline. In some of the slides 

 are small rounded areas of sodalite. The glassy base is probably highly alkaline, and 

 the rocks may be termed solvsbergite- and tinguaite-vitrophyres. These rocks un- 

 doubtedly represent the hypabyssal or volcanic phases of the syenites described above. 

 The lavaform equivalent of the shonkinite does not appear in the collection, although 

 it probably occurs in the field. 



4. Ouachitite. 



A remarkable rock belonging to the alkaline series occurs as dykes in the sodalite- 

 syenite of Ochilesa (209). In hand specimens it is a dark-green, grey, or brown, 

 compact rock, which effervesces freely with acid, and in which the only identifiable 

 constituent is biotite. It is characterised by an irregular banding due to the alterna- 

 tion of strips differing slightly in texture, colour, or mineral composition. In thin 

 section the bulk of the rock is seen to consist of biotite and a grass-green pyroxene, 

 embedded in a groundmass consisting of allotriomorphic plates of calcite. Anatase 

 derived from ilmenite, and apatite, are important accessories ; whilst soda-orthoclase, 

 and a mineral identified as melilite, occasionally appear in the slides. The biotite 

 forms large plates of a bleached yellow tint, with much darker borders. Large cross- 

 sections ranging up to 5 mms. in length are frequent, as are also good basal sections. 

 It is extensively altered to a peculiar yellow mineral, which is usually granular in 



