﻿262 



BE. A. HOLMES ON THE TERTIARY 



[vol. lxxii, 



Melilite-basalt also occurs among the alkali rocks of the Great 

 Rift Valley, where it appears to stand towards them in a comple- 

 mentary relation, as a mafic end-product of differentiation specially 

 rich in lime and magnesia. In South Africa the 4 alkaline 

 affinities ' of melilite-basalt and kimberlite have been noted, 1 and 

 it is interesting to find that the amygdale minerals of these rocks 

 include zeolites and calcite, but never chalcedony or quartz. 2 



Although the lavas of Mozambique, and of the greater part of 

 Africa south of that latitude, are predominantly amygdaloidal 

 basalts characterized by siliceous and lime-zeolite amygdales, the 

 lavas known in the north are not commonly of this type, except 

 in Abyssinia. The majorit} r of the lavas belong to an alkali series 

 (accompanied by mafic complementary types), to which those of 

 Mozambique bear a close resemblance. 3 The distribution of the 

 various types has been well summarized by Prof. J. P. Iddings in 

 his ' Igneous Rocks,' vol. ii (1913) p. 576 et s^./with references 

 to the literature, and therefore the details need not be repeated 

 here. It will be sufficient to draw attention, as Rosiwal did 

 many years ago, 4 to the frequent association of a calc-alkali 

 series (including amygdaloidal basalts, andesites, and rhyolites) 

 with a more strongly-developed alkali series. 



North of the Songwe River in German East Africa, Bernhardt 

 found basalts, andesites, and trachytes, while south of the river, in 

 the extreme north of Nyasaland, A. R. Andrew & T. E. G. Bailey 

 discovered tuffs containing trachyte and phonolite. 5 In Masai- 

 land, along the Uganda Railway towards Victoria Nyanza, in the 

 Kavirondo country 6 adjoining the Nyanza, in the islands of Lake 

 Rudolf, in Kordofan, and especially in Abyssinia and Somaliland, 7 

 the two series are well represented. Wherever the succession 

 has been noted 8 the alkali series, often accompanied by normal 

 basalts, is the later. In the islands of Lake Rudolf, in Italian 

 Somaliland, and in Abyssinia, the amygdale minerals of the earlier 

 series are chiefly quartz, chalcedony, and opal, accompanied by 

 heulandite and other lime-zeolites. Blanford draws attention to 

 the striking similarity between the amygdaloids of Abyssinia and 

 those of Southern Arabia and the Deccan. In each of the two 

 last-named regions an alkali series is also present. 9 No detailed 

 petrographic account of the Deccan traps has yet been published, 



1 P. A. Wagner, ' The Diamond- Fields of South Africa ' 1914, p. 106. 



2 A. W. Eogers & A. L. Du Toit, ' The Geology of Cape Colony' 1909, p. 348. 



3 Especially in the presence of such soda-amphiboles as katoforite and 

 cossyrite. 



4 Denkschr. K. Akad. Wissensch. Wien, vol. lviii (1891) p. 465. 



5 Q. J. G. S. vol. lxv (1910) p. 222. 



6 F. Oswald, Q. J. G. S. vol. lxx (1914) p. 140 et seq. 



7 W. T. Blanford, ' Observations on the Geology & Zoology of Abyssinia ' 

 London, 1870, p. 180. 



8 As, for example, in Abyssinia, Lake Rudolf, and the Kavirondo country. 



9 See J. W. Evans, Q. J. G. S. vol. lvii (1901) p. 38. In Kathiawar the 

 nepheline-bearing rocks appear to be intrusive into the Deccan traps, and 

 therefore of later age. 



