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DC. A. HOLMES ON THE TERTIARY 



[vol. lxxii, 



The Volcanic Area. 



V. The Distribution and Former Extent or the Lavas. 



The volcanic rocks of the district of Mozambique occur along a 

 narrow belt of country which extends from Mokambo Bay to 

 beyond the Sanhuti River, following a line approximately parallel 

 to the general direction of the coast (see map, tig. 1, p. 226). The 

 boundary-fault between the sedimentary coastal belt and the Pre- 

 Cambrian complex roughly bisects the volcanic area, although in 

 the north the lavas are more abundant on the western side of the 

 fault. The lava-flows are everywhere horizontal, and are clearly 

 the result of flssure-eruptions, no traces of pyroclastic ejecta having 

 been anywhere encountered. 



The flows are now restricted to innumerable small outcrops 

 surrounded, according to the locality, by gneiss and granite, by 

 Cretaceous or Tertiary sediments, or by more recent lateritic earths 

 and alluvium. Small dykes and sills, representing the feeders 

 through which the magmas reached the surface, penetrate the 

 underlying rocks, and are exposed in many places between the much 

 dissected lava-flows. On the accompanying sketch-map (fig. 1), it 

 has been possible to express only the broader outlines of the various 

 formations which enter into this complicated patchwork. No 

 pretence is made to detailed accuracy, for the inherent difficulties 

 of pioneer mapping, topographical as well as geological, were 

 considerably increased by widespread sheets of superficial debris, 

 and in places by a dense undergrowth of tropical vegetation. 



Throughout the area the prevailing lavas are basalts of a remark- 

 ably uniform type. All are vesicular, and most of them are coarsely 

 arnygdaloidal. That similar flows of amygdaloid originally extended 

 northwards as far as Fernao Velloso Harbour is made clear by the 

 presence of abundant pebbles of agate and jasper around the Nakalla 

 River and along the western shores of the Harbour. Pebbles of 

 agate and jasper are still being derived from the amygdaloids, and 

 naturally they occur in great abundance on the coastal side of the 

 exposures, particularly along the lower reaches of the Mitikiti, 

 Monapo, Mikati, and Sanhuti Rivers. Similar pebbles, however, 

 also occur to the west of the exposures, especially in the Monapo 

 district, where the flows appear to have been laterally more extensive 

 than elsewhere. The existence of these relics in places (such as 

 Mitikiti) to which the} r could not have been transported by the 

 present drainage, makes it certain that the lavas flooded a strip of 

 country from 5 to 10 miles broad, and at least 50 miles long. 



Beyond Mitikiti's Kraal, a solitary basaltic dyke, well encrusted 

 with laterite, was met with in Maravi's carefully guarded country. 

 Its position, near Nashologoto, is marked on the map. In a line 

 with this dyke, a few miles away to the south, a series of hot 

 springs breaks through the gneissic surface, providing the Mitikiti 

 River with a small but permanent tributary. We encountered the 

 steaming waters in a dense forest during the dry season, when all 



