Chap. XV. ISLANDS IN THE ZAMBESI. 409 



aries on board and proceeded to the Zambesi. It was in 

 high flood. We have always spoken of this river as if at 

 its lowest, for fear lest we should convey an exaggerated 

 impression of its capabilities for navigation. Instead of 

 from five to fifteen feet, it was now from fifteen to thirty 

 feet, or more, deep. All the sandbanks and many of the 

 islands had disappeared, and before us rolled a river capable, 

 as one of our naval friends thought, of carrying a gunboat. 

 Some of the sandy islands are annually swept away, and 

 the quantities of sand carried down are prodigious. 



The process by which a delta, extending eighty or one 

 hundred miles from the sea, has been formed may be seen 

 going on at the present day — the coarser particles of sand 

 are driven out into the ocean, just in the same way as we 

 see they are over banks in the beds of torrents. The finer 

 portions are caught by the returning tide, and, accumu- 

 lating by successive ebbs and flows, become, with the 

 decaying vegetation, arrested by the mangrove roots. 

 The influence of the tide in bringing back the finer 

 particles gives the sea near the mouth of the Zambesi a 

 clean and sandy bottom. This process has been going on 

 for ages, and as the delta has enlarged eastwards, the river 

 has always kept a channel for itself behind. Wherever we 

 see an island all sand, or with only one layer of mud in it, 

 we know it is one of recent formation, and that it may be 

 swept away at any time by a flood; while those islands 

 which are all of mud are the more ancient, having in fact 

 existed ever since the time when the ebbing and flowing 

 tides originally formed them as parts of the delta. This 

 mud resists the action of the river wonderfully. It is a 

 kind of clay on which the eroding power of water has little 

 effect. Were maps made, showing which banks and which 

 islands are liable to erosion, it would go far to settle where 

 the annual change of the channel would take place ; and, 

 were a few stakes driven in year by year to guide the water 



