﻿part 3] 



VOLCANIC ROCKS OF MOZAMBIQUE. 



223 



gives a very complete and faithful account of the natives and their 

 customs ; but, beyond the suggestion that the prevailing rocks 

 are gneisses and granites, he makes no reference to the geology of 

 the territory. In 1887, Mr. J. T. Last made a journey from 

 Blantyre to the Namuli Peaks, which had for long been considered 

 to be of volcanic origin. This view he disproved (Last, 1887), 

 describing the summit as ' an almost perpendicular mass of white 

 stone.' I have since learned from Mr. Last that he referred in 

 this expression to the grey granite and gneiss which builds up the 

 Namuli Peaks, as it does the other mountainous masses of the 

 country. 



Thus, until a few years ago, Mozambique, with the exception of 

 part of the Cretaceous coastal fringe, was practically a terra 

 incognita to geologists. In 1910, a prospecting and surveying 

 expedition was organized by the Memba Minerals Ltd., under the 

 leadership of Mr. E. W. E. Barton. The coastal belt was investi- 

 gated during the first season with Memba as the base of operations ; 

 but, owing to the hostility of the natives, little progress was 

 made inland. Early in 1911, a fresh party was formed with head- 

 quarters at Mosuril, and Mr. E. J. Wayland, Mr. D. A. Wra} r , and 

 myself, were appointed as geologists to the expedition. A short 

 account of the geographical work accomplished, together with the 

 map made during the two seasons' work, has already been published 

 (Reid, 1913), and in two papers introductory to the present more 

 detailed treatment Mr. Wray and myself have described the 

 geography of the countrv and the outstanding features of the 

 geology (Holmes & Wray, 1912 & 1913 ; see also Wray, 1915). 



The present paper deals with the Tertiary volcanic rocks of 

 Mozambique, and will be followed by one describing the Pre- 

 Cambrian rocks. 



II. General Features. 



With the exception of a coastal zone of variable width, formed 

 by a narrow belt of Cretaceous and Tertiary sediments and flanked 

 on the west by later Tertiary volcanic rocks, the whole territory 

 consists of a complex of gneisses and other foliated rocks with 

 granitic and other intrusions belonging to at least two different 

 periods. Although we proceeded inland for 250 miles, as far west 

 as longitude 37° E., we met with no signs of unmetamorphosed 

 sedimentary rocks, once the coastal zone had been left behind. 

 The oldest rocks lying on the basal complex are of Lower Cretaceous 

 age, and consequently there is no close stratigraphical evidence for 

 the age of the complex. 



The petrological details and architecture of the complex suggest 

 for the greater part of it a Pre- Cambrian age, and in a subsequent 

 paper an attempt will be made to show that this view is justified : 

 for a direct measurement of the age of the gneisses, and that of 

 one of the later granites, by the radioactive method, proves that an 

 approximate correlation can be made between the older rocks of 

 Mozambique and those of other countries far removed from the 

 tropics. 1 



1 A. Holmes, Proc. Geol. Assoc. vol. xxvi (1915) p. 305. 



