ON LAKE BASINS. 
587 
water that remains on the surface after the ordinary drainage 
has carried off all that it is capable of doing. These lakes or 
pools are larger or smaller according to the rate of evaporation^ 
and they occasionally disappear altogether, leaving no mark. 
We may then see the bottom of the basin — the bed of the 
pool, and the depression in which the water was contained 
is often quite undistinguishable by the eye. Sometimes, 
indeed, as in limestone countries, the water sinks into the earth 
by crevices in the rock, and only exists as a lake when these 
are choked. Such a condition often results in malaria, owing 
to rapid evaporation through decayed vegetable matter. 
A third class of lakes is illustrated in the plate, by the Lago 
di Bolsena, in Tuscany, and the Laacher See, near Bonn on 
the Rhine. There are round dark deep pools of clear and cold 
water, and are quite removed from the marine basins on the one 
hand, and from ordinary ponds or lakes on the other. If the 
water were emptied they would be like cups. They are the 
craters of old volcanoes. Bolsena is upwards of twenty-six 
miles in circumference, and is as remarkable for the volcanic 
rocks that surround it, as for the terrible malaria that rises 
from its banks. The Laacher See is much smaller, but appa- 
rently of the same nature. Of such lakes, there are also many, 
but they are confined to those districts where volcanic erup- 
tions and disturbances have been observed in recent times, or 
where the rocks are such as to make it certain that they have 
been active at no distant period. 
It is evident, then, that a large number of the lakes of the 
world offer nothing in their form or structure, or the circum- 
stances of their existence, to justify a doubt as to their origin. 
They are the result of natural inequalities of the surface, 
inevitable when we consider how all land surfaces have been 
formed and modified. They represent such of these inequali- 
ties as have received and retained water. 
But a large class remains. Mountain countries and parts of 
the world where there are or have been in recent geological 
times considerable changes of level caused by forces acting 
from below, present mregularities far more abrupt' and 
irregular than the plains. One may travel from the Arctic 
circle to the shores of the Black Sea, without seeing any ridge 
or any tract of land rising a few hundred feet above the 
general level. In this wide area, which ranges from the 
Atlantic to the Pacific for a distance of at least fifteen hundred 
miles of latitude, there are no high lands. Here and there, in- 
deed — frequently in the north, more rarely in the south, there 
are depressions of a few hundred feet or less below the general 
surface. Some of these are full of water ; some contain a little 
water at the bottom ; and some are dry, but water may generally 
