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IMt. A. HOLMES OjS t THE TERTIARY 



[vol. lxxii, 



biscuit-coloured glass or of innumerable blebs that suggest a 

 frozen emulsion of felspar and glass. Large pyroxenes are also 

 wholly or partly included by the larger felspars, and apatite, 

 magnetite, and serpentine are accessory inclusions. The glass- 

 filled felspars are surrounded by a clear mantle of oligoclase. which 

 may represent a final period of growth simultaneous with that of 

 the ground-mass. The average refractive index of the outer borders 

 is approximately 1/545 ; that of the laths in the ground-mass 

 appears to be a little less. 



Hy per s then e and augite occur in somewhat rounded forms, 

 having rarely retained their idiomorphic outlines. Hypersthene, 

 which is beautifully pleochroic, frequently acts as a nucleus around 

 which augite crystals are built. Both pyroxenes contain magnetite, 

 serpentine, and glass as inclusions. Peripheral wisps of biotite 

 occur around many of the rhombic pyroxenes, and also disseminated 

 sparsely through; the ground-mass. In several cases the change 

 from hypersthene to biotite is gradual and continuous, but against 

 felspars the biotite always terminates sharply. Serpentine is- 

 not abundant, and is present as pseudomorphs after idiomorphic 

 phenocrysts of olivine, and as confused aggregates in the ground- 

 mass. It is of the golden-yellow variety found in the basalts. 

 Like hypersthene, it is bordered with wisps of biotite which have 

 clearly grown at the expense either of the serpentine or of the 

 olivine which preceded it. The potash-bearing molecules of the 

 magma seem to have reacted with the ferromagnesian silicates 

 already precipitated — hypersthene, olivine, or serpentine — in such 

 a way as to produce biotite. 1 



The order of crystallization is as follows : — apatite and magnetite, 

 olivine, hypersthene, augite, andesine, oligoclase, biotite, and, 

 finally, interstitial glass. 



Relationships and Origin of the Volcanic Rocks. 



XVIII. Distribution of Similar Lavas in 

 Eastern Africa. 



The outstanding feature of the volcanic district of Mozambique 

 is the development of two strongly-contrasted series of rocks — 

 one mainly composed of amygdaloidal basalts and including ande- 

 sites, the other including alkali-trachytes and phonolites. 



Amygdaloidal basalts similar to those of Mozambique are found 

 in many localities of South and Central Africa, but nowhere else 

 south of Mozambique are they known to be of Tertiary age. 

 At Sinjal, near the border of Nyasaland and Portuguese East 

 Africa, Dr. A. R. Andrew and Mr. T. E. Gr. Bailey 2 found basalts, 

 associated with andesites and rhyolites, which they refer to the close 

 of the Karoo Epoch. Dr. Prior has kindly allowed me to see the 

 specimens from this locality (now deposited in the Natural History 



1 N. L. Bowen, Journ. Geol. Chicago, Supplem. to vol. xxiii (1915) p. 46. 

 - Q. J. G. S. toI. lxvi (1910) p. 220. 



