526 THE FRESH-WATER LOCHS OF SCOTLAND 
drainage areas may, then, be traced to the fact that they are situated in 
those regions of the earth's surface where the prevailing winds blow 
from colder to warmer latitudes, and from off land and not directly 
from off the ocean. The distribution of salt lakes may consequently 
be said to depend more on meteorological than on topographical or 
geological phenomena. These meteorological conditions have also a 
very marked influence on vegetable, animal, and human life, as well as 
on the geological strata now in process of formation. 
Should the climate in the neighbourhood of these inland drainage 
areas change and the rainfall become more abundant, the salt lakes 
would slowly increase in size, and would ultimately find an outlet 
by means of a river to the ocean at the lowest part of the rim of 
the basin. What was once a small salt lake would gradually become 
a large fresh- water lake, pouring its waters directly into the ocean 
through a river. It is most probable that this has frequently occurred 
in the past history of the earth, but traces of the change have been 
wholly obliterated, or are now difficult to detect. Numerous instances 
of the contrary process, where large fresh-water lakes have been con- 
verted in recent geological times into salt lakes, are to be observed 
in many inland drainage areas. Instances of this nature will be 
pointed out in the following pages. In some desert regions both 
the river channels and the lakes are completely filled up with blown 
sand. The artesian wells along the course of the Oued Rhir in the 
Sahara often throw up fresh-water fishes and crustaceans, thus 
indicating a buried river. The lakes of inland drainage areas may 
be, as previously stated, either fresh or salt. The higher lakes, having 
an outflowing river, remain fresh and drinkable, while bhe salts which 
are leached out of the surrounding land all accumulate in the lowest 
lake of the series, the salts in solution varying both in quantity and 
composition in each locality. 
In the following pages we shall in the first instance refer to the 
inland drainage areas of the northern hemisphere, and then to those 
of the southern hemisphere. 
Northern The largest inland drainage area is that of central Eural-Asia, 
Hemisphere, ^j^i^]^ occupies, according to Murray, 4,785,000 English square miles, 
Eural-Asia. and stretches from about long. 35° to 125° E., and from lat. 25° to 
60° N. (see fig. 63). It includes the lakes of the Aralo-Caspian 
depression, the lakes of the Gobi Desert, Lake Hamun in the Seistan 
depression between Afghanistan and Persia, Lake Urmi on the Persian 
plateau, and Lake Van in Eastern Anatolia. 
The greater part of Central Asia is occupied by two high 
plateaus : a western one extending in a south-eastern direction from 
the Black Sea to the valley of the Indus, and an eastern one stretch- 
ing from the Himalayas to the north-eastern extremity of Asia. 
