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VOLCANIC ROCKS OTP MOZAMBIQUE. 



277 



there were a considerable relief of the pressure appropriate to such 

 depths. In this connexion, the boundary-fault along which the 

 lavas are aligned is of particular interest, for it seems to mark a 

 zone along which pressure actually was relieved to an extent and 

 depth sufficient to promote fusion. 



The boundary-fault system is probably a consequence of isostatic 

 readjustment between the uplifted mainland and the sunken 

 region of Mozambique Channel. Long denudation of the crystal- 

 line plateau should, on the hypothesis of isostasy, naturally lead 

 to its uplift, and that such a movement has taken place is demon- 

 strated by the presence of a coastal belt of Cretaceous and 

 Tertiary sediments, and by the existence of still more recent raised 

 beaches at various levels along the coast (see fig. 5, p. 230). More- 

 over, the heavy denudation of the crystalline rocks implies the 

 removal of an important section of the ' radioactive layer.' This 

 process therefore leads, like uplift, to the cooling and contraction 

 of the lithosphere, and thus additional tension-stresses are developed 

 that can only find relief in fissures' and faults. Doubtless the 

 actual formation of fissures was helped by regional movements of a 

 tensional character, associated in a complementary way with the 

 compressive movements of Tertiary mountain-building elsewhere. 



In conclusion, I wish to place on record the unfailing courtesy 

 and hospitality with which we Avere everywhere received by the 

 Portuguese officials in Mozambique, who were always ready to 

 help us in every way that lay in their power. For permission to 

 publish the geological observations made during the expedition, 

 I owe my thanks to the Directors of the Memba Minerals Ltd. ; 

 in particular, I have to thank them for placing at my disposal a 

 large collection of rock-specimens, including those collected in the 

 Sanhuti-River district, and many of those collected by the staff 

 of the Company during the 1910 season. From my friends and 

 former colleagues Messrs. E. J. Starey, E. J. Wayland, and D. A. 

 Wray, I have received notes and sketches referring to the districts 

 in which they worked, and I desire to express my indebtedness to 

 them for help thus afforded. In connexion witli the investigation 

 of the rocks in the laboratory, I wish to thank Prof. W. W. Watts 

 for granting every possible facility for work at the Imperial 

 College of Science & Technology; Prof, the Hon. P. J. Strutt 

 for continuing to lend me his apparatus for determining radium 

 in rocks and minerals ; Prof. C. Gr. Cullis and Dr. J. W. Evans for 

 examining many of my sections, and offering valuable suggestions ; 

 and Dr. Gr. T. Prior and Lieut. W. Campbell- Smith for looking 

 through my sections, and allowing me to compare the Mozambique 

 lavas with specimens of similar rocks in the Natural History 

 Museum from British East Africa, Abyssinia, Nyasaland, and 

 other African localities. In making the analyses published in this 

 paper I received considerable help from Dr. H. F. Harwood, 

 Lieut. J. L. Harris, M.C., the late Lieut. Gr. Kirby, Lieut. A. 

 Mclntyre, and Mr. E. Spencer, and for their assistance in an 



Q. J G. S. No. 287. x 



