vit RIVERS AND LAKES 313 
4. Those due to inequalities of elevation 
or depression. | 
5. Lakes in craters of extinct volcanoes, 
for instance, Lake Avernus. | 
6. Those caused by subsidence due to the 
removal of underlying soluble rocks, such as 
some of the Cheshire Meres. 
7. Loop lakes in deserted river courses, of 
which there are many along the course of the 
Rhine. 
8. Those due to rockfalls, landslips, or lava 
currents, damming up the course of a river. 
9. Those caused by the advance of a gla- 
cier across a lateral valley, such as the Mer- 
gelen See, or the ancient lake whose margins 
form the celebrated “ Parallel Roads of Glen 
Roy.” 
As regards the first class we find here and 
there on the earth’s surface districts sprinkled 
with innumerable shallow lakes of all sizes, 
down to mere pools. Such, for instance, occur 
in the district of Le Doubs between the 
Rhone and the Sadne, that of La Sologne 
near Orleans, in parts of North America, and 
in Finland. Such lakes are, as a rule, quite 
