
420 STRUCTURAL AND FIELD GEOLOGY 
(explosion craters) produced by paroxysmal outbursts. The 
Maars of the Eifel, and the numerous crater-lakes of Auvergne 
and Central Italy, are well-known types. Other volcanic 
lakes occupy what may be termed barrier basins, and owe 
their origin to the obstruction of the drainage by lava or 
fragmental ejecta.’ The Lac d’Aydat of Alweronemaor 
example, is confined by a barrier of lava. 
(c) Dissolution Basins.—These may be shortly defined 
as depressions of the surface caused by the gradual removal 
of underlying soluble rock. They are the result, in short, of 
the chemical and mechanical action of underground water. 
Such depressions are of common occurrence in regions where 
massive limestones occupy the surface, and are caused by 
the falling-in of subterranean galleries, tunnels, caves, etc. 
Owing to the highly fissured character of the limestone, these 
depressions seldom contain lakes. Now and again, however, 
after very heavy rain which the underground channels are 
unable to dispose of at once, temporary lakes come into 
existence. Even permanent lakes are occasionally present in 
such regions. These may sometimes be the result of crustal 
movements which have brought the fissured bed-rock under 
the influence of the subterranean water-level. In most cases, 
however, such lakes probably owe their origin to the closing 
of the underground outlets by the accumulation of red earth 
and débris. Occasionally,'in glaciated limestone regions, the 
depressions have been rendered water-tight by the deposition 
of morainic materials. Dissolution basins not infrequently 
occur in places where the bed-rocks, although themselves 
of a more or less impermeable character, are yet underlaid 
at a greater or less depth by soluble material such as rock-salt | 
or gypsum. The gradual removal of these by underground 
water eventually brings about slow subsidence or sudden 
collapse of the surface. 
(2) Alluvial Basins.—Owing to irregular accumulation 
of sediment, shallow depressions are of common occurrence 
in deltas and other broad fluviatile and estuarine flats. 
These during floods may become lakes, temporary or per- 
manent, as the case may be. The deserted “loops” of rivers; 
and the pools and “creeks” which occupy the deeper hollows 
of dried-up river-courses, come under the same head. Again, 

