122 PRINCIPLES OF STRATIGRAPHY 



proportionately deep for their size. Though mostly due to irregu- 

 larity of deposition, many owe their chief characteis to the caving- 

 in of the sand upon the melting of an included ice block, e^. Land- 

 slip basins are rare and small, forming in irregularities or depres- 

 sions in the land-slide surface, eo. Hollows between dunes are com- 

 mon, but these, as a rule, hold no water, owing to the porousness of 

 the sands. Examples of lakes held up by sand dunes are, however, 

 abundant in Cherry county and elsewhere in the sand-dune region 

 of Nebraska, f. Finally, artificial structures, such as built-up (not 

 excavated or dammed) reservoirs, belong here. 



C. Destructional basins. 



1. Volcanic destructional. Of this type the most characteristic 

 are the pit craters, or the more or less circular hollows produced 

 by volcanic explosions, without the building up of a crater, or by 

 subsidence. The Maare of the Eifel and elsewhere in Germany are 

 typical examples, the largest of this type being the Laacher See,- 

 near Coblenz, with a diameter of a mile and a half and a depth of 

 about 200 feet. Other examples are found in the Auvergne. Where 

 the surrounding region is strewn with the pyroclastic ejectamenta, 

 the basin may be regarded as due to explosion, as in the case of the 

 Maare of the Eifel in general. In other cases subsidence after the 

 removal of rock matter below may be the cause of the depression. 

 Lake Balaton (Flatten See) in Hungary is regarded as of this 

 type. It is 50 miles long, 3 to 10 miles wide and 40 feet deep. 



2. Chemical basins of the destructional type include (a) solu- 

 tion basins, where limestone or gypsum has been removed by 

 solution ; some of the lakes in Florida may have originated in this 

 way. Sink-holes, due to caving-in of the surface over caverns, also 

 belong here. These are common in limestone regions. Lakes due to 

 solution of underground salt deposits and to the caving-in of the 

 surface are also of this type. An example of a recently formed 

 lake of this class is the intensely saline Lake Illyes or Medve Lake, 

 near Szovata, Hungary. Temporary solution lakes on glacier sur- 

 faces must also be included in this group, (b) Decomposition and 

 disintegration Jiollozvs. These are of little significance, since they 

 can be cleared out only by wind, ice, or other agency. 



3. Fluviatile destructional basins are never of great size, for 

 rivers cannot excavate large hollows, their chief work resulting in 

 free drainage channels. Pot-holes, however, and the depressions 

 gouged out by retreating waterfalls, are rock basins of this type. 

 Pot-holes are mostly small, though exceptional cases have a diameter 



