THE TSETSE PROBLEM IN NORTH MOSSURISE. 325 



Boil (PI. X, fig. 2) are stated always to burn earlier than the granite gneiss near 

 them (PI. xiv, fig. 2 ; PI. xv, fig. 1) with its impervious underlying bed, and the 

 sandstone-shale areas of the uplands west of Spungabera definitely do bum 

 considerably earlier than the dolerite beside them. The type of bush also influences 

 the date of burning. High canopy-forming Brachystegia, such as occurs in 

 Mossurise on the upper Buzi, burns effectively much later than the same bush 

 where it is lower and more broken, and some types (as high dense secondary, 

 including Milletia) burn, if at all, late yet tamely. 



It is stated that under the Zulus burning was the subjecfc of regulation for hunting 

 purposes, a late, thorough burn being aimed at and usually achieved. Under the 

 white man everyone burns when he pleases. It thus comes about that the first 

 fires take place when the grass is but half ready to burn and there is relatively little 

 recent leaf-litter on the ground. They attract but little wind to swirl the flames 

 into the tree-tops and the thickets, and they are very small in extent, so that the 

 flies driven from them find ample refuge all round. When later fires take place — 

 at various dates — the areas burned earlier are already becoming fit for the reception 

 of the fly and the latter suffers no inconvenience. A still worse effect of unregulated 

 burning, from the point of view of tsetse control, is the encouragement given by 

 too early burning to the wooding and particularly to the formation of the thickets 

 on which G. brevipalpis rather specially depends (PI. xvii, fig. 2). Late burning 

 on the other hand finds the grass and fallen leaves at their driest and most 

 abundant, and is so extensive as to draw in great wind. It achieves much actual 

 destruction of young growth, and a temporary destruction of much high shade 

 as well, at a hot, dry time that must be relatively critical for the tsetse. 



A failure to burn, again, may be made the means of destroying thicket growth 

 if the next year's burning is late, owdng to the additional amount of inflammable 

 material present. The fire no longer stops at the outskirts. This result actually 

 followed the temporary effort to keep fires from the rubber forests, and I was shown 

 where pieces of these forests (in Umtobi's country — of the type shown in 

 PI. xiii, fig. 2) were completely destroyed in consequence. 



VI. — Distribution of Rocks, Woodland Types, Food Animals and Fly. 

 Geological Formations. 



Geologically, the country investigated is divisible into four clear-cut blocks, 

 two of them east of the Sitatongas, two west. The two to their east are a granite- 

 gneiss and a basalt formation. The two to their west are (a) a sedimentary formation 

 of shale, sandstone and quartzite ; (6) dolerite, breaking through and capping the 

 sedimentary formation, which in the dolerite area is visible only in places. 



The granite-gneiss country consists of lowland plains, flat to undulating.* It 

 is bounded on the west by the Sitatongas, on the north (I believe) by the Mabiti 

 hills across the Lusitu and by their north-easterly continuation, on the south by 

 a fairly well demarcated line a few miles south of the Umvuazi and apparently 



*My rocks, identified by Mr. Mennell as mica granite, gneissose granite and Muscovite 



franite or gneiss, show more or less foliation, and the area appears to be that, east of the 

 itatongas, described by Theile and Wilson as metamorphic. The general character- 

 istics are those of the eranite. 



