VIII 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 Sa6ne, that of La Sologne 

 near Orleans, in parts of North America, and 

 in Finland. Such lakes are, as a rule, quite 



