A STUDY OF LAKE BASINS 



405 



of the district and the constituted authorities, is a lake more than 

 twenty miles across and came into existence during the great 

 earthquake of the lower Mississippi valley in 1811. 



Crater lakes. The craters of volcanic mountains are natural 

 basins in which surface waters are certain to be collected, provided 

 only the supply is sufficient and seepage into the loose materials is 



FIG. 436. View of lake in Poas Crater in Costa Rica, a volcanic crater more 

 than half a mile across and with walls 800 feet deep. At intervals there is an 

 ejection of steam mixed with mud and ash after the manner of a geyser (after 

 H. Pittier). 



not excessive. Some craters, still visibly more or less active, are 

 occupied by lakes (Fig. 436). 



In the larger number of cases in which craters become occupied 

 by lakes, the evidence of continued activity is lacking, and it would 

 appear in such cases that the lava of the chimney had consolidated 

 into a volcanic plug, closing the bottom of the crater. Notable 

 groups of crater lakes are the Caldera of the Roman Campagna 

 (Fig. 437) and the so-called maare of the Eifel about the Lower 

 Rhine. Crater lakes are easy to recognize by their circular plan, 



