96 JDK. A. HOLMES ON THE PRE-CAMB1UAX [vol. lxxiv, 



of the surrounding plateau in their immediate neighbourhood. It 

 is considered probable that the landscape may have been developed 

 ■during the more arid conditions of a former period. 



I wish here to repeat the acknowledgments made at the con- 

 clusion of my former contribution ('The Tertiary Volcanic Eocks 

 of the District of Mozambique ' Q. J. G. S. vol. lxxii, 1916-17, 

 p. 227) to the Portuguese officials in Mozambique, to the Directors 

 of the Memba Minerals Ltd., and to my former colleagues in 

 the field, Mr. E. J. Wayland and Mr. D. A. Wray in particular. 

 The greater part of the laboratory investigation of the rocks was 

 carried out at the Imperial College of Science & Technology 

 during the years 1912-14, the long subsequent delay in completing 

 the paper being due to the pressure of more immediately useful 

 work. I wish to thank Prof. W. W. Watts for placing at my 

 disposal the resources of the Geological Department, for en- 

 couragement and critical discussion, and for reading the completed 

 manuscript. Part of the radioactive work was carried out during 

 the years 1912-13 in the laboratory and under the guidance of 

 Prof, the Hon. E,. J. Strutt, to whom I owe especial thanks, 

 not only for invaluable instruction and assistance, but also for 

 continuing to lend me his apparatus for determining small 

 quantities of radium in rocks and minerals. The rest of the 

 radioactive work was done with the aid of a Ro}^al Society Grant 

 awarded in 1914, for which I make grateful acknowledgment. 

 Prof. C. G. Cullis, Dr. J. W. Evans, and Mr. H. G. Smith, 

 my colleagues at the Imperial College, have from time to time 

 examined and discussed with me series of rocks and sections, and 

 for their active interest and willing assistance I wish to express my 

 indebtedness. Finally, I owe my thanks to Mr. G. S. Sweeting, 

 who, as on previous occasions, has skilfully prepared the photo- 

 micrographs with which the paper is illustrated. 



EXPLANATION OF PLATES VIII-XI. 

 Plate VIII. 



Fig. 1. Sediment-gneiss (186), 4 miles north of Mitikiti, in a tributary of the 

 Mitikiti River, Mozambique. Chief minerals : quartz, felspar, and 

 biotite. X 20. (See p. 44.) 



2. Quartz-magnetite-granulite (182), found as a pebble in the IMonapo 



gravels between Mitikiti and Murimatigri. Chief minerals : quartz 

 and magnetite. X 20. (See p. 44.) 



3. Quartz-mica-schist (172), north of Murimatigri. Chief minerals : 



quartz, felspar, and biotite. X 20. (See p. 45.) 



4. Hornblende-schist (176), Mukumburi Valley, west of Memba. Chief 



minerals: hornblende, oligoclase, quartz, epidote, and sphene. X 20. 

 (See p. 45.) 



5. Forsterite-marble (221), Ampwihi Crossing. Chief minerals : calcite, 



partly -serpentinized forsterite, and chondrodite. X 20. (See p. 48.) 

 ■6. Quartz -garnet- scapolite rock (193), Ampwihi Crossing. Chief minerals : 

 garnet, scapolite, quartz, calcite, sphene, and ilmenite. X 20. (See 

 p. 49.) 



