622 
THE FRESH-WATER LOCHS OF SCOTLAND 
Lake Rukwa, 2560 feet above sea-level, is a huge swamp formed 
by the collection of local waters. The waters of the lake are salt, and 
it seems liable to great variations in area and depth, as accounts of 
its size vary greatly. Its principal affluent is the River Saisi, which 
rises in the north of British Central Africa, but it has no outlet. 
Lake Shirwa (or Chelwa), south of Lake Nyasa, is a large oval 
body of water, 1946 feet above sea-level, about 50 miles in length and 
15 to 16 miles in average breadth, lying in a flat central depression 
of extensive lacustrine plains which were at one time portions of 
the floor of the lake. It is very shallow and has no outlet, though 
the rise of a few feet would cause it to drain through the Ruo into 
the Shire and Zambesi. The fauna of this lake appears to have been 
at one time identical with that of Lake Nyasa, but owing probably 
to the rising of the ground, which has separated Lake Shirwa from 
Lake Nyasa and has finally resulted in Shirwa having no outflow, and 
hence becoming salt, the old Nyasa fauna has been killed out, 
except in the curious fresh-water oases which are still maintained 
at the mouths of the permanent rivers flowing into the lake. 
NoRTPi The drainage system of the northern part of North America shows 
America. former maturity of a region that has been recently glaciated. 
The effect of the invasion of the ice-sheet — which advanced from the 
north in the Pleistocene period in a south and south-westward direction 
— is visible as far as the northern half of the Mississippi valley. Ten 
or more important bodies of water lie in a curve from Lake Ontario 
to Great Bear Lake, and the lakes lying between that series and Hudson 
Bay, as well as those situated south and west of the lake-belt, are 
essentially depressions on new land areas ; but, while the one region 
shows the destructive action of the ice, the other exemplifies its con- 
structive action. The soil that the Eastern Canadian" Highlands 
possessed in pre-glacial times has been stripped away, leaving a bare 
unweathered surface on which the ice has eroded numerous rock-basins. 
This area is covered with many lakes which lie in the hollows ; the 
rivers draining them have not yet cut down their drainage slopes, and 
are interrupted by falls and rapids. On the other hand, the surface 
of the region farther south is heavily sheeted with glacial drift, so 
that for tens of miles not a ledge of rock is to be seen. Glacial deposits 
are so varied in character, and so irregularly laid down, that they 
abound in depressions that become filled with water, and it is chiefly 
in this way that the numerous small lakes of New England are to be 
explained. 
Geological evidence seems to point to the fact that previous to 
the glacial epoch North America had been above the sea for a long 
period at a greater elevation than at present, especially to the north. 
