28 LAKES OF NORTH AMERICA. 



Nevada, and on swampy area,s near Sandusky, Ohio. When these gas 

 eruptions occur, the soft mud is sometimes thrown to a distance of one or 

 two hundred feet, and conical depressions are formed which in some of the 

 instances observed, are twenty feet or more in depth. The caving in of 

 the banks' holes sometimes leads to the formation of pools fifty or sixty 

 feet in diameter. The circular ponds frequently to be seen in swampy 

 regions, when not due to encroaching vegetation, probably, in many 

 instances, originated in the manner here noted. 



The generation of gases, principally carbureted hydrogen, in the soft 

 mud of the Mississippi delta, causes elevations known as " mud lumps," 

 which in some instances are twenty-five feet high. Inequalities pro- 

 duced in this manner might easily lead to the obstruction of cbainage 

 and the formation of lakes, but no instance of such an occurrence seems 

 to have been reported. 



It has frequently been observed that cattle on visiting swampy places 

 carry away considerable quantities of mud, adhering to their feet and 

 matted in their hair. In arid countries where drinking places are usually 

 small and widely scattered, they are visited by cattle and other animals in 

 large numbers and a marked enlargement of the water holes is produced 

 in the manner just stated. This process was more important when 

 the plains of North America were densely inhabited by bisons. Many 

 perennial pools and still more numerous depressions that are water-filled 

 only during rainy seasons, are known as " buffalo- wallows," and are 

 believed to owe their origin to a great extent to the carrying away of 

 mud entangled in the thick hair of the animals after which they are 

 named. 



In the Appalachians there are several water holes, usually on the 

 crests of ridges, that are called "bear-wallows," and are said to have 

 been formed by bears that sought moist places in which to cool them- 

 selves during hot weather. As is well known, swine have a similar 

 habit. 



Basins clue to movements in tlie earth's crust. — Great changes in 

 the earth's crust have produced continents and ocean-basins, while smaller 

 movements on land areas have resulted in the formation of mountains and 

 valleys. During the growth of mountains it sometimes happens that the 

 region between different systems or between two or more ranges, becomes 

 enclosed so as to form a basin. This process has been in action in various 

 localities since land first appeared, and during the course of geological eras 



