404 PKOF. P. MAESHALL ON THE [Aug. I906, 



The rock rests upon andesite at the Paper-Mills, and is covered 

 by basalt. In most localities there is no basalt-covering, and this 

 phonolite seems to be the latest lava emitted over the greater part 

 of the district. 



(D) Green phonolite, — In hand-specimens this rock has all the 

 appearance of a tinguaite, but it forms a massive lava-flow 300 feet 

 thick, extending from Elueskin Cliff almost to the summit of Mopanui. 

 ~No section has yet been found that indicates the relation of this 

 rock to others above or below it, although its field-occurrence shows 

 that it is one of the latest products of eruption ; and this conclusion 

 is supported by its apparent relationship to the trachydolerite, which 

 is itself one of the youngest of the rocks of this district. 



11. jSTephelinitoid phonolites. — Prom a large quarry at 

 rTormanby comes a type that contains a good deal of felspar, 

 although its amount is less than that of the nepheline. The 

 rock is fine-grained, and in addition to these minerals contains 

 only Eegirine and a little magnetite. Coarser examples of a 

 similar rock occur at Signal Hill and elsewhere. One of the 

 most interesting rocks of the district occurs at the top of 

 Mopanui. In it very fine-grained nepheline is the most abundant 

 mineral, and with this are occasionally seen minute laths of felspar, 

 apparently sanidine. Crystals of sodalite are not uncommon ; 

 they are often extended in the direction of a dodecahedral axis. 

 The coloured constituents are Eegirine and cossyrite, in about equal 

 proportions. The cossyrite is in small spongy growths, enclosing 

 crystals of nepheline in its meshes : its pleochroism is from pale 

 pink to reddish brown, and its arrangement is very similar to that 

 in the rock from Kamnye River so well figured by Dr. Prior. 1 

 Prof. Eosenbusch was good enough to examine a slice sent to him, 

 and he compares the cossyrite to the development of that mineral 

 in the rock known as apachite. A few bigger crystals of brown 

 augite fringed with Eegirine are seen, and very rarely a small 

 olivine fringed in the same way. 



One of the peaks forming the summit of Mount Cargill is built up 

 of a rock that does not differ very widely from that of Mopanui. 

 There is less cossyrite, more sodalite, and less brown augite, but the 

 fine-grained nephelinitoid base is very similar in the two rocks. 



Chemically, the Mopanui rock — the only one analysed — is 

 characterized by a high percentage of iron. The chlorine was not 

 estimated, although qualitative tests showed that a considerable 

 amount was present. The analysis agrees rather well with the 

 other one quoted from Equatorial East Africa. Mineralogically, the 

 latter rock appears to contain much more felspar, and nepheline is 

 present in small crystals but not in the groundmass. I am unable 

 to make a comparison with those African types which have nepheline 

 in the groundmass, as there is no analysis in the literature at my 

 disposal. 



1 Min. Mag. vol. xiii (1903) pi. v, fig. 4. 



