R. R. Walls — TJie Geology of Portuguese Nyasaland. 211 



which formerly carved out huge valleys for themselves across the 

 country do not exist now except for perhaps two months in the 

 year. The Rovuma and the Lurio, though they do persist throughout 

 the year, shrink up to narrow and absolutely insignificant streams. 



Fig. 2. — Sketch-map of Pemba Bay. 



The mangrove swamps are isolated and in process of being buried 

 up in sand. They are poor remnants of the great swamps which 

 formerly stretched up and down the coast and left their traces in 

 the soft greenish-grey mud which now underlies the sand of the 



