METEOllOLOGY AND GEOLOGY 311 



Mr. (t. T. Prior contributed to the Mineralogical Society on March 2'Jth, 1902, a 

 ]ia])er on the petrology of British East Africa, the result of examinations of the 

 collection of rock specimens made by Professor J. ^V. Gregory during his expedition 

 to Mount Kenya and Lake Baringo in 1892-3, and of collections from the Uganda 

 Protectorate made recently by 8ir Harry Johnston. Descriptions -were given of the 

 three main groups of rocks — viz., the basement Architan gneisses and schists, the 

 Paheozoic shales and sandstones, and the Tertiaiy volcanic rocks. The gneisses and 

 schists are associated -with dykes both of acid pegmatites and of basic diabasis and 

 epidiorites, and also with granulitic rocks analogous to the Charnockite series of India 

 and Ceylon. Of the Paheozoic Karagwe series a collection of ferruginous shales 

 and siliceous schists from I'nyoro was described. These rocks present striking 

 similaiities with those of Hatch's Hosjutal Hill series of the Transvaal and with 

 rocks from tlie Ingwenya Berg, Swaziland, and a correlation between the Karagwe 

 series and the Cape system of the Transvaal was suggested. The volcanic rocks 

 <;onsist mainly of soda-rich ]ihonolitic rocks which have resulted doubtless from a 

 neyiheline-syenite magma. The lavas from the volcanoes of the Great Rift Valley 

 and of Blount Kenya and the region between are characterised, like those of the 

 Canary Islands and the Azores, by the prevalence of anorthoclase, by the large amount 

 of soda-amphil)oles (cossj'rite, cato})horite, arfvedsouite), as well as of soda-pyroxenes 

 and by the absence of sphene and noseau. They form a remarkable exami)le of a 

 rock series showing a gradation in compo.sition from ba.sic ](honolites, containing 

 nepheline both in large phenocrysts and in the ground-mass, through phonolitic 

 trachytes containing no recognisable nepheline, to ])honolitic quartz-trachytes, and 

 finally to acid riebeckite-rhyolites containing much quortz. The later eruptive rocks 

 from Mount Elgon and the w^estern side of the Great Kift Valley |iresent some 

 points of distinction with the earlier erupted rocks. They are generally of a more 

 basic character like those of Kilimanjaro as compared with those of Mount Kenya. 

 Another point of distinction is the ])resence in them of titanic acid in large amount, 

 in the form of jierofskite in the more basic nephelinites, and as sphene in the phonolites, 

 which are of the more ordinary tyjie without soda-amphiboles. Most of the specimens 

 from Blount Elgon and the neighbourhood consist of nejjhelinites, but in some of them 

 the nepheline, both as phenocrysts and in the ground-mass, is partially or wholly 

 replaced by melilite. Examples of melanite-nepheliiie rocks allied to borolanite, 

 and of monchiquite dyke-rocks from Mount Elgon, were also described. A specimen 

 of nejihelinite from the neighbourhood of Piuwenzori containing much perofskite 

 suggested the contemporaneity of the eruptive rocks of ^klount Elgon and of the volcanic 

 region at the foot of Mount PuweiiZDri. 



