﻿222 



DR. A. HOLMES ON THE TERTIARY 



[vol. Ixxii, 



11. The Tertiary Volcanic Eocks of the District of 

 Mozambique. By Arthur Holmes, D.Sc.(Lond.), D.LC, 

 A.R.C.S., F.G-.S. l (Read June 28th, 1916.) 



[Plates XX & XXI.] 



Contents. 



Introduction. Page 



I. History of Exploration 222 



II. General Features 223 



III. The Coastal Belt 224 



IV. Bibliography 225 



The Volcanic Area. 



V. The Distribution and Former Extent of the Lava3 . . . 228 



VI. Period of Eruption 22& 



VII. Order of Eruption, and Classification 231 



Petrographic Descriptions and Analyses. 

 Rocks of Group A. 



VIII. Solvsbergite 233 



IX. ^girine-Trachyte 235 



X. Trachytoid-Phonolite 236 



XL Tephritic Pumice 23& 



XII. Basalt 241 



XIII. Picrite-Basalt 244 



Rocks of Group B. 



XIV. Basalts : Sills and Dykes 246 



XV. Basalts : Lava-Flows 250 



XVI. Hornblende-Andesite 256 



XVII. Pyroxene-Andesite 259 



Relationships and Origin of the Volcanic Rocks. 



XVIII. Distribution of Similar Lavas in Eastern Africa 260 



XIX. Variation-Diagrams 264 



XX. Origin of the Lavas 271 



XXI. The Radioactivity of the Lavas, and its Bearing on 



their Origin 275 



Introduction. 



I. History of Exploration. 



The Portuguese East-African District of Mozambique was, until 

 recently, one of the least known of the East-African coastlands. 

 Livingstone explored the regions on the south-west (1865) and 

 north (1874), but Mozambique itself he did not touch. Sadebeck 

 (1879) was the first to give a geological description of the 

 sedimentary rocks that fringe the coastal plain of the mainland 

 north and south of Mozambique 1 Island. This work was followed 

 up by Choffat (1903) in an important pala?ontologieal memoir on 

 the Cretaceous formations of Conducia Bay. Exploration beyond 

 the coastal zone was initiated by Lieut. H. E. O'Neill, who made 

 a number of successful journeys westwards across the mainland 

 during the years 1881-1884. In his papers (1882, 1884, 1885) he 



1 The name Mozambique has been used indiscriminately for (a) Portu- 

 guese East Africa, (b) the district lying between the Luli and Ligonia 

 Rivers, (c) the Island of Mozambique, and (d) the city of Mozambique. 



