METEOROLOGY AND GEOLOGY :ni 



Mr. (!. 'P. Trior C()iitril)ut((l ti) the Mineralogical Society (ni .Mar(-li 2.")tli, l!i():i, a 

 liajier on the jietrology of J^)ritish East Africa, the result of exaiiiiiiatioiis of tlie 

 €ollectioii of rock specimens made by Professor J. W. Oregory during his expedition 

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

 Protectorate made recently l>y Sir Harry Johnston. Descriptions were given of the 

 three main groups of rocks — viz., the basement Archa>an gneisses and schists, the 

 PaliEozoic shales and sandstones, and the Tertiary volcanic rocks. The gneisses and 

 schists are associated ■with dykes both of acid ])egmatites and of Vjasic diabasis and 

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

 and Ceylon. Of the PaUeozoic 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 Hospital Hill .series of the Transvaal and with 

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

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

 consist mainly of soda-rich i)honolitic rocks which have resulted doubtle.ss from a 

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

 and of ^Nlount 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-amphiboles (cossyrite, catophorite, arfvedsonite), as well as of soda-pyroxenes 

 and by the absence of sphene and noseau. They form a remarkable example of a 

 rock series showing a gradation in composition from basic i)honolites, containing 

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

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

 finally to acid riebeckite-rhyolites containing much quartz. The later erui)tive rocks 

 from Mount Elgon and the western side of the Great Pift Valley i)resent some 

 points of distinction A\ith 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 presence in them of titanic acid in large amount, 

 in the form of perofskite in the more basic nephelinites, and as sphene in the ithonolites, 

 Avhich are of the more ordinary tyjie without soda-ami)liiboles. Most of the specimens 

 from :Mount Elgon and the neighbourhood consist of nejihelinites, but in some of them 

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

 replaced by melilite. Examples of rnelanite-nepheline rocks allied to borolanite, 

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

 of nephelinite from the neighbourhood of Ptuwenzori containing much ])erofskite 

 •suggested the contemporaneity of the eruptive rocks of :Mount Elgon and of the volcanic 

 redon at the foot of Mount Kuwenzori. 



