546 MR G. W. TYRRELL. 



and granulites of Finland and Sweden, which include rocks derived from various 

 porphyries, along with cordierite, andalusite, and scapolite-bearing rocks. # Their 

 characters are supposed to be due to " immersion " in a great granite batholith. It 

 may also be noted that scapolite rocks occur along with the charnockite series in 

 Peninsular India, t 



IV. Granites, Granodiorites, and Associated Rocks. 



These are fine- to coarse-grained rocks of white, pink, or grey colour, and of 

 comparatively uniform texture. In only one specimen is there any tendency for 

 felspar to assume a porphyritic development. Hornblende is macroscopically 

 prominent in some of the granodiorite specimens. 



Under the microscope two quite distinct types may be distinguished. The most 

 abundant group, that of the granites proper, are distinguished by an early generation 

 of euhedral striped felspar approaching albite in composition, and usually much 

 altered with the development of white mica. These crystals are prominent as white 

 opaque areas in hand specimens and slides. Rocks of this type occur on the Benguella 

 Railway at Kunhungamua (138), Portella Coroteva (116), and Caimbango (123), in 

 the district of Quingenge (248, 249, 252), on the Cuiva River (242), and W. of 

 Chingw T ari (23 1). They consist of a coarse hypidiomorphic aggregate of fresh 

 microcline and quartz, enclosing the above-mentioned early crystals of albite. The 

 quartz also occurs as rounded blebs embedded in the microcline. The only ferro- 

 magnesian constituents are biotite, partially altered to chlorite and epidote, and a 

 little magnetite. The quantitative mineral composition of the Quingenge rock, as 

 estimated by the Rosiwal method, is given in Table II. It may stand as a good 

 average sample of the Benguella granites, w r hich may therefore be said to consist of 

 about 50 per cent, of microcline, 10 of albite, 35 of quartz, and 5 per cent, of biotite 

 and iron-ores. The textures are typically granitic and vary in grain-size from coarse 

 (Benguella Railway) to fine (Chingwari and the Cuiva River). 



The second type is a coarse granitoid rock indistinguishable in general appearance 



from those described above. In thin section, however, they are seen to contain no 



albite, the place of which is taken by abundant fresh plagioclase felspar of average 



composition AbyAna (acid andesine). They are further distinguished by the presence 



of hornblende along with biotite (although the latter remains the dominant coloured 



constituent), and by a larger proportion of the ferro-magnesian constituents. These 



rocks correspond to the Continental tonalites and to the American granodiorites. 



They occur at kilometres 135f, 223, and 238 on the Benguella Railway (119, 121, 



123), and at a locality between Quingenge and the Cuiva River (244). The rock from 



kilometres 223 on the Benguella Railway is a fresh typical sample of this type. It 



consists mainly of quartz and oligoclase-andesine in hypidiomorphic relations, with 



* P. Ekkola, " Petrology of the Orijarvi Region in South-western Finland," Bull. Comm. Geol. de Finlande, No. 40, 

 1914, pp. 14, 15, 131. 



+ Holland, op. cit., p. 232. 



