210 R. R. Walls — 2Vie Geology of Portuguese Nyasaland. 



area of just over 20 square miles, of which, perhaps, one-third is 

 sufficiently deep for ocean-going vessels to anchor in. (See Sketch- 

 map.) The eastern arm of the bay is a raised beach, nearly 300 feet 

 high, thrown up on the hump of an anticlinal fold parallel to the two 

 north-south folds farther inland. The " core " of the hump is no- 

 where exposed, so it is impossible to say whether it is folded gneiss 

 or a granite intrusion. The bulk of the two first-mentioned north- 

 south ridges is folded gneiss, but in the nearer ridge due west of 

 Pemba Bay there is much intrusive granite. The " core ", however, 

 whatever its substance may be, appears to be of the humped whale- 

 back formation which is so common over the entire country. The 

 superimposed Tertiary rocks have not folded into the metamorphic 

 complex, but have cracked into fault-lines, which are shown in the 

 sketch-map. The main fault-line is north-south, but there are 

 several minor faults veering round to the west, and so giving rise 

 to the tongue of land which projects into tlie bay. The deep part 

 of the bay apparently occupies, therefore, an original syncline 

 in the metamorphic complex. It is deepest close to the fault-line on 

 the eastern margin and shallows gradually to the west, where 

 mangrove swamps are rapidly encroaching. Rivers flowing into 

 the south and west bring down vast quantities of s'and in their short 

 seasons, which also helj) to silt up the bay on these sides. The 

 raised portion is composed for the most part of soft unconsolidated 

 mud, identical with the mud of mangrove swamps. This is protected 

 by a thin cap of calcareous sandstone, which has been recrystallized 

 by percolating water and is very hard. The mud, however, is rapidly 

 falling away, and the sandstone cap breaks up into boulders forming 

 screes down the steep face of the cliff. One of the minor fault-lines 

 (marked B on sketch) is shown in Plate VII, Fig. 2. The figare in the 

 foreground is standing on the same geological horizon — the calcareous 

 sandstone cap — which appears again on the summit of the cliff in 

 the background. The cliff face is the fault-line, composed almost 

 entirely of soft mud, the sandstone cap being only between 30 and 

 40 feet thick. A well has been bored through the sandstone cap 

 on the top of the hill to the underlying mud for water, and two wells 

 have also been bored on the foreshore, not shown in the photograph, 

 and water is again found held by the underlying mud. Water 

 escapes along the junction line of the sandstone cap and mud on 

 the cliff face shown in the photograph, and attempts have been 

 made to secure this for a Avater supply. The storage tanks and pipe 

 line, however, bemg set in the loose mud of the cliff face, fell away as 

 the cliff crumbled. Except for the thin sandstone cajt and occasional 

 coral rock the boundary walls of Pemba Bay are composed of very 

 loose unconsolidated material, which show a decided lack of 

 structural stability and clearly indicate formation in very recent 

 geological times. 



Another most important natural feature must be considered, viz. 

 the drying-up of the whole country within recent times. The rivers 



