Davis.] 
322 
[January 18 , 
sometimes be again divided into varieties distinguished from 
their fellows by some slight change of cause, and into hybrids 
or basins of complex origin. We shall note that certain species 
are much more prolific than others; for example, drift-lakes, 
which probably outnumber those from all other causes combined. 
On the other hand, some species include lakes altogether insig- 
nificant in number or size, as glacial-lakes, held by barriers of ice, 
but these deserve attention as in times lately past they were of 
much importance. Ponds must not be neglected, for they gain 
from their common occurrence a value not warranted by their 
small area ; and from being frequently met in the field of our obser- 
vation they give many opportunities for analysis and classifica- 
tion. Size and variety are therefore to be considered only 
secondarily; difference in origin is the prime object of study. 
A. 1. Great Basins. When the growth of confluent or neigh- 
boring mountain-chains encloses a less disturbed area within 
their circuit so as to leave no low drainage exit to the sea, the 
basin thus formed will be taken possession of by a lake of a size 
proportionate to the rainfall of the region and the area drained 
by its rivers. This may be called a Great Basin Lake. 
If plenty of water be furnished, it will rise to the lowest point 
of the surrounding rim and there overflow, so attaining its 
greatest volume. Certain ones, as the Great Lakes of Central 
Africa which probably belong here, are the only examples of 
this species now to be found where the water supply is sufficient 
to cause overflow and keep the lakes fresh : the Nyanzas are 
held permanently up to their outlets, so far as explorers have yet 
reported, by the heavy equatorial rains, but Tanganyika is on the 
verge of becoming salt; its waters are faintly brackish, and its 
outlet, the Lukuga, that flowed westward into an upper arm of 
the Congo, when seen by Cameron in 1874, 1 was found by Stanley 
to be only a series of stagnant, reedy pools. W e need not how- 
ever adopt Stanley’s catastrophic theory to explain the change. 2 
In this stage Great Basin Lakes are deep and generally have 
well marked shores, but they are now rare : the basins are 
1 Cameron, Across Africa, London, 1877, 302. 
2 Stanley, Through the Dark Continent, n, 45, 31, 50. Thomson, To the Central 
African Lakes and back, 1881, n, 62, 71. 
