202 R. R. Walls — The Geology of Portuguese Nyasaland. 



uninhabited for the greater part of the year on account of the 

 drought. The whole of the interior is covered by a dense dry bush 

 with elephant-grass and bamboo growing to an average height of 

 12 or 15 feet, and many larger trees yielding good and valuable 

 timber. It is seldom possible to see 200 yards ahead. Geological 

 surveying in such a country is naturally very difficult. Eock out- 

 crops are comparatively few, often hidden away in the bash, and 

 owing to the lack of accurate maps only the most general survey 

 work can be attempted. 



The coast-line of Portuguese Nyasaland is low, sandy, and shelving, 

 and good harbours are few, with the outstanding exception of 

 Pemba Bay, which will be discussed more fully later on. Behind and 

 among these great sandy stretches, sedimentary rocks of Tertiary 

 or perhaps Cretaceous age appear. These sedimentaries are 

 invariably calcareous sandstones or shelly and coralliferous lime- 

 stones. Everywhere they have the appearance of consolidated 

 shallov/ sea deposits, and in many places they are thrown up to form 

 raised beaches between 200 and 300 feet high. An oolitic structure is 

 very common among these calcareous sandstones, evidently due to 

 percolating water which dissolves the lime and recrystallizes it 

 round the sand-grains. These latter, although not of great extent, 

 are of considerable economic importance, furnishing the only 

 suitable building stone in the country. Coral abounds on the coast, 

 and is burned by the natives for lime. Coral rock forms a number of 

 islands lying off the Montepuesi River, of which the largest is Ibo, 

 containing the town of that name, one of the earliest settlements of 

 the Portuguese in East Africa and still practically the only town 

 worthy of the name in the whole territory. 



Conspicuous features on the coast are the mangrove swamps, 

 which are particularly extensive at or near the mouths of the 

 numerous rivers, even though, as a general rule, these rivers do not 

 contain runnmg water for more than two months in the year. 

 Thus all the islands off the mouth of the Montepuesi River are joined 

 together and to the mainland by mangrove swamps, and the western 

 shore of Pemba Bay is bordered by mangrove swamps, which are 

 rapidly encroaching on the bay. The study of these mangrove 

 swamps is exceedingly instructive to the geologist. On the seaward 

 edge of such swamps the trees grow directly from shelly sand in clear 

 water. But the traveller does not penetrate far into the swamp until 

 a fine, green, slimy mud is encountered, which makes progress slow 

 and even dangerous. If this is uncovered at low water it is oily 

 and evil-smelling, and a heavy, oppressive air fills the whole .swamp. 

 Stagnant pools of rotting vegetation occur here and there, and 

 over these the will-o'-the-wisp lights play at night. Sometimes the 

 mud is so black with carbonaceous matter that it seems as if the 

 traveller were walking through a quagmire of coal-dust. Crabs of 

 all colours and sizes abound everywhere, crawling over the mud 

 and running up and down the trees. There are millions of crabs, and 



