( 537 ) 



XIV. — A Contribution to the Petrography of Benguella, based on a Rock Collection 

 made by Professor J. W. Gregory. By G. W. Tyrrell, A.R.C.Sc, F.G.S., 

 Lecturer in Geology, Glasgow University. Communicated by Professor 

 J. W. Gregory, F.R.S. (With One Plate.) 



(MS. received June 2, 1915. Read June 28, 1915. Issued separately August 11, 1916.) 



Very little is known of the petrography of the Portuguese West African colony 

 of Angola, and of its three provinces that of Benguella is probably least well known 

 in this respect. Granites, gneisses, schists, limestones, and red sandstones have been 

 recognised by the earlier observers of the geology of the region, and detailed descrip- 

 tions of rocks from Angola, and especially of its northern province, Loanda, have been 

 given by Berg # and Holmes, t From Benguella itself J. P. Gomes J has described 

 the collection of R. P. Leconte, which contained granites, amphibolite, adinole with 

 epidote veins, various schists, quartzites, diabase-porphyrite, and basic eruptives. 

 Senhor J. P. de Nasciamento§ in 1912 recorded granite, diorite, basalt, and quartzite 

 on the Benguella plateau. 



During his recent journey in Benguella, Professor Gregory collected about a 

 hundred and fifty specimens of igneous rocks or their metamorphic derivatives, and 

 these offer a most interesting and varied study. This paper is concerned only with 

 their petrographic character ; for their geological relations the reader is referred to 

 the preceding paper by Professor Gregory. 



For descriptive purposes the classification given below has been found to be most 

 natural and convenient. The order in which the rocks are arranged also closely 

 follows their sequence in time : — 



I. Basement gneisses and schists. 

 II. Charnockite series. 



III. A series of hornfelsed porphyries and other hornfels. 



IV. Granites, granodiorites, and associated rocks. 

 V. Rhyolite (dellenite). 



VI. An alkaline series, including nepheline-sodalite-syenite and other syenites, 



shonkinite, solvsbergite, and ouachitite. 

 VII. Basic intrusions. 



I am much indebted to A. Scott, M.A., B.Sc, for two excellent chemical analyses 

 of rhyolite and shonkinite respectively. 



* Berg, G., " Gesteine von Angola, Sao Thome, und St Helena," Tscher. min. u. petr. Mitth., xxii (1903), 

 pp. 357-362. 



t Holmes, A., " Contribution to the Petrology of North-western Angola," Geol. Mag., dec. vi, vol. ii, pp. 228-232 ; 

 267-272 ; 322-328 ; 366-370, 1915. 



+ Gomes J. P., " Echantillons de roches recueillis entre Benguella et Catoco," Comm. Dir. Trab. Geol. Portugal, 

 vol. iii, fasc. 11, pp. 239-243, 1898. 



§ Nasciamento, J. P. de, and Mattos, A. A. de, A Colonisacao de Angola, Lisbon, 163 pp., 1912. 

 TRANS. ROY. SOC. EDIN., VOL. LI, PART III (NO. 14). 78 



