R K Walls— The Geology of Portuguese Nyasaland. 203 



the traveller can scarcely put his foot down without crushing 

 several at every step. The result of such a huge population of 

 Crustacea must naturally be to add a big percentage of lime to tlie 

 mud, and when a sample of this mud is dried and then treated with 

 dilute acid it usually gives considerable effervescence. The mud of 

 the mangrove swamps is very fine and plastic to the touch, and 

 quite characteristic. 



But there have been considerable changes of level along this coast, 

 and very often this mud is covered again by the sandy deposits 

 of the shore and the mangrove swamp may disappear locally. 

 A mangrove swamp disappearing in this fashion is shown in PI. VII, 

 Fig. 1.' In fact, it is pretty safe to state that in earlier Tertiary times 

 the mangrove swamps were much inore extensive than they are to- 

 day. Where the whole region has been raised subsequently as in the 

 section shown in PI. VII, Fig. 2, calcareous sandstones of raised 

 beaches are found lying on impervious clay, which is identical with 

 the mud of present day mangrove swamps. This is of iminense 

 economic importance to a country which suffers so nmch from 

 drought. The impervious clay holds up the water in the porous 

 sandstone, and it is to this that the small villages on the coast 

 area ow e their very existence. Water is thus found in the neighbour- 

 hood of Pemba Bay at from 30 to 40 feet below the surface all the 

 year round, and at Ibo at about the same depth. More extensive 

 boring for water in the raised beaches along the coast is absolutely 

 necessarv if the country is ever to support even a black population. 



The Tertiary deposits of the coast area measure less than 20 

 miles across in the south, though they are rather more extensive 

 farther north. Immediately to the \^est of this coastal strip an 

 area of extreme metamorphism is found which extends right across 

 to Lake Nyasa. The rocks of this area appear to belong to the very 

 oldest Palaeozoic formations with very small areas which might 

 be of Carboniferous age. These will be discussed again. The 

 Tertiary rocks of the coast area lie unconformably on these 

 raetamorphic rocks, and though a complete section can nowhere be 

 seen so far as the writer knows, yet the Tertiary deposits would 

 appear to be of no great depth, possibly about 400 or 500 feet at most. 



About 40 miles \\est of Pemba Bay there is a marked north- 

 south ridge, running niore or less parallel with the coast. The ridge 

 is really a steeply folded anticline of hard metamorphic rock- 

 generally speaking, gneiss. The ridge is made more conspicuous by 

 a series of " humps " at intervals along its length. About half-way 

 between this ridge and the coast is a second, but less conspicuous 

 ridge, roughly parallel to the first and very similar hi its construction. 

 In the region west of Pemba Bay, however, this minor ridge is pierced 

 and broken by a later intrusion of granite, and the folded anti- 

 clinal structure of the whole is not so clearly marked. These two 

 anticlinal ridges run north across the Montepuesi River, but are not 

 so conspicuous in the north, where the calcareous sandstones of the 



