PHYSICAL GEOGRAPHY 
47 
fact that in a little more than a year a lake which has existed beyond the 
memory of man has suddenly been resolved into a sandy marsh and a broad 
river channel. 
I think I have enumerated all the known permanent lakes of the country, 
though I should not be surprised if travellers who read this book came forward 
and said, “You have forgotten such and such a lake in the Chambezi Valley, or 
the small lakelet between Chilwa and Mlanje, or the great sheet of open water 
on the Upper Tuchila, or such and such a lake in the Luangwa Basin. ’ None 
of these sheets of water, however, as far as is yet known, have any permanent 
existence. They are only the creation of the rainy season floods. Seen at that 
time, of course, their existence is recorded ; in the dry season they would be 
found either not to exist at all or to be confined to a patch of marsh. There 
were lakes at one time, undoubtedly, near the junction of the Ruo and the Shire 
(the Elephant Marsh) and at the junction of the Shire and Zambezi (Morambala 
Marsh) ; but in the course of time the alluvium of the rivers, together, even, 
THE LIKUBULA GORGE, MLANJE 
with a slight upheaval of the ground, or more probably still the deeper cutting 
of the river-channel have turned these former lakes into marshes or vast extents 
of dry alluvial soil. In like manner Nyasa was evidently united not many 
centuries ago with Lake Malombe ; and it may be, also, that Lake Chilwa was 
joined with Lake Chiuta and was then the head waters of the great Lujenda- 
Ruvuma river. Much of the decrease in volume of the great lakes must be 
attributed to a slow and slight process of upheaval which has caused their 
waters to more rapidly drain away ; but the disappearance of these shallow lakes 
along the courses of the rivers is chiefly due to the rivers having in course of 
time cut their channels deeper, so that the lakes which formerly represented 
their overflow have their bottoms now removed even above flood limit. 
The geology of British Central Africa would appear to be relatively simple. 
The commonest formation, perhaps, is a mixture of metamorphic rocks, 
grauwacke , clay-slates, gneiss and schists. This prevails . over much of the 
country lying between the west of Lake Nyasa and the Luapula River, on the 
Nyasa-Tanganyika plateau, in parts of the Shire Highlands, and north of the 
Zambezi. The valleys of the great and sluggish rivers, however, (the Shire, 
