CONTRIBUTIONS TO THE GEOLOGY OF BENGUELLA. 529 



The volcanic neck of Chieuca may be approximately of the same age as these 

 dolerites, for some of its rocks are fresh and show no signs of pressure. I was in the 

 field disposed to regard the alkali series of Chieuca and Ochilesa as probably belong- 

 ing to the same age as the alkaline volcanic rocks of British East Africa and of the 

 Atlantic islands and as probably of early Kainozoic age. They are certainly much 

 younger than the Oendolongo Series, though there is no direct stratigraphical evidence 

 to establish this conclusion. 



The Charnockite Series of Ochilesa is probably much older than the alkaline lavas, 

 and it belongs to the underlying foundation of older rocks. The charnockites have 

 been intruded into the gneisses, and their age is probably the same as that of the 

 great series of granites along the railway route from Mount Sahoa to Candumbo. 



Near the charnockites at Ochilesa are considerable occurrences of a sodalite- 

 syenite and shonkinite which doubtless belong to an intrusion of the same age as the 

 Chieuca neck. A sodalite-eleolite-syenite has been described from Zenza do Itombe, 

 in the interior of Loanda (Berg, 1903, p. 359), and a nepheline-basalt from Dombe 

 Grande has been identified by Gomes and recorded by Choffat (1888, p. 29). The 

 latter rock is doubtless Kainozoic, and is one of the basalts of the coast zone of 

 southern Benguella and Mossamedes ; but the eleolite-syenite was probably intrusive 

 into the biotite-gneisses of Loanda, and may be of any post-Eozoic age. M. Pereira 

 de Sousa (1913, pp. 1451-2) has announced the discovery in North- Western Angola 

 of riebeckite-fegyrine-leptynite, nordmarkite, nepheline-syenite, nepheline-phonolite, 

 and tinouaite* 



'&■ 



IV. Tectonic Geology. 



The tectonic geology of Benguella appears to be comparatively simple. The 

 problem of chief importance is the origin of the steps by which this part of West 

 Africa rises from the sea to the high plateaus of the interior. This structure was 

 clearly expressed by Livingstone, the value of whose observations in Angola is 

 usually underrated, as they are judged from the remarks in his Missionary Travels 

 instead of from his technical paper and section in the Journal of the Geographical 

 Society (1855, p. 232). Livingstone's Ideal Section of Angola was an important 

 contribution to its geology and topography. It shows the trough -like form of the 

 valley of Cassange and the abrupt descent from the main plateau in the Golungo- 

 Alto district ; and it truly represents the coast-lands as formed of marine fossiliferous 

 marls, which are modern near the coast and include older beds inland. 



The coast zone between Hanha and Benguella shows the step and plateau 

 structure on a smaller scale. The land rises to the height of 1000 feet by three 

 steps ; and that they are due to parallel step faults appears conclusively proved by 

 the geological evidence. The inflata beds occur at sea-level on the coast at Old 



* A description of these and other alkaline igneous rocks from this area by Mr A. Holmes is now in course of 

 publication (Holmes, 1915). 



